Bibliography - Stephen M Griffies

1. Drake, H F., Adele K Morrison, Stephen M Griffies, Jorge L Sarmiento, W Weijer, and A R Gray, January 2018: Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions. Geophysical Research Letters, 45(2), DOI:10.1002/2017GL076045 .
   [ Abstract ]

   In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus inter-basin merging of upwelling CDW may be overestimated by coarse resolution models, potentially affecting the skill of centennial scale climate change projections.

2. O'Rourke, Amanda K., B K Arbic, and Stephen M Griffies, March 2018: Frequency-domain analysis of atmospherically forced versus intrinsic ocean surface kinetic energy variability in GFDL’s CM2-O model hierarchy. Journal of Climate, 31(5), DOI:10.1175/JCLI-D-17-0024.1 .
   [ Abstract ]

   Low-frequency variability at the ocean surface can be excited by both atmospheric forcing, such as in exchanges of heat and momentum, and by the intrinsic nonlinear transfer of energy between mesoscale ocean eddies. Recent studies have shown that nonlinear eddy interactions can excite an energy transfer from high to low frequencies analogous to the transfer of energy from high to low wavenumbers (small to large spatial scales) in quasi-twodimensional turbulence. As the spatial inverse cascade is driven by oceanic eddies, the process of energy exchange across frequencies may be sensitive to ocean model resolution. Here a cross-spectrum diagnostic is applied to GFDLs CM2-O hierarchy of fully coupled ocean-atmosphere models to address the transfer of ocean surface kinetic energy between high and low frequencies. The cross-spectral diagnostic allows for a comparison of the relative contributions of coupled atmospheric forcing through wind stress and the intrinsic advection to low-frequency ocean surface kinetic energy. Diagnostics of energy flux and transfer within the frequency domain are compared between three coupled models with horizontal ocean resolutions of 1, 1/4th, and 1/10th degree to address the importance of resolving eddies in the driving of energy to low frequencies in coupled models.

3. Roberts, M, P L Vidale, C A Senior, H T Hewitt, C Bates, S Berthou, P Chang, J H Christensen, S Danilov, M-E Demory, and Stephen M Griffies, et al., in press: The benefits of global high-resolution for climate simulation: process-understanding and the enabling of stakeholder decisions at the regional scale. Bulletin of the American Meteorological Society. DOI:10.1175/BAMS-D-15-00320.1. June 2018.
   [ Abstract ]

   A perspective on current and future capabilities in global high-resolution climate simulation for assessing climate risks over next few decades, including advances in process representation and analysis, justifying the emergence of dedicated, coordinated experimental protocols. The timescales of the Paris Climate Agreement indicate urgent action is required on climate policies over the next few decades, in order to avoid the worst risks posed by climate change. On these relatively short timescales the combined effect of climate variability and change are both key drivers of extreme events, with decadal timescales also important for infrastructure planning. Hence, in order to assess climate risk on such timescales, we require climate models to be able to represent key aspects of both internally driven climate variability, as well as the response to changing forcings. In this paper we argue that we now have the modelling capability to address these requirements - specifically with global models having horizontal resolutions considerably enhanced from those typically used in previous IPCC and CMIP exercises. The improved representation of weather and climate processes in such models underpins our enhanced confidence in predictions and projections, as well as providing improved forcing to regional models, which are better able to represent local-scale extremes (such as convective precipitation). We choose the global water cycle as an illustrative example, because it is governed by a chain of processes for which there is growing evidence of the benefits of higher resolution. At the same time it comprises key processes involved in many of the expected future climate extremes (e.g. flooding, drought, tropical and mid-latitude storms).

4. van Sebille, E, and Stephen M Griffies, et al., January 2018: Lagrangian ocean analysis: fundamentals and practices. Ocean Modelling, 121, DOI:10.1016/j.ocemod.2017.11.008 .
   [ Abstract ]

   Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.

5. Dufour, C O., Adele K Morrison, Stephen M Griffies, I Frenger, Hannah Zanowski, and Michael Winton, October 2017: Preconditioning of the Weddell Sea polynya by the ocean mesoscale and dense water overflows. Journal of Climate, 30(19), DOI:10.1175/JCLI-D-16-0586.1 .
   [ Abstract ]

   TheWeddell Sea polynya is a large opening in the open-ocean sea ice cover associated with intense deep convection in the ocean. A necessary condition to form and maintain a polynya is the presence of a strong subsurface heat reservoir. This study investigates the processes that control the stratification and hence the build-up of the subsurface heat reservoir in theWeddell Sea. To do so, a climate model run for 200 years under preindustrial forcing with two eddying resolutions in the ocean (0.25° CM2.5 and 0.10° CM2.6) is investigated. Over the course of the simulation, CM2.6 develops two polynyas in the Weddell Sea, while CM2.5 exhibits quasi-continuous deep convection, but no polynyas, exemplifying that deep convection is not a sufficient condition for a polynya to occur. CM2.5 features a weaker subsurface heat reservoir than CM2.6 due to weak stratification associated with episodes of gravitational instability and enhanced vertical mixing of heat, resulting in an erosion of the reservoir. In contrast, in CM2.6, the water column is more stably stratified, allowing the subsurface heat reservoir to build up. The enhanced stratification in CM2.6 arises from its refined horizontal grid spacing and resolution of topography which allows, in particular, a better representation of the restratifying effect by transient mesoscale eddies and of the overflows of dense waters along the continental slope.

6. Goddard, P, C O Dufour, Jianjun Yin, Stephen M Griffies, and Michael Winton, October 2017: CO2-Induced Ocean Warming of the Antarctic Continental Shelf in an Eddying Global Climate Model. Journal of Geophysical Research, 122(10), DOI:10.1002/2017JC012849 .
   [ Abstract ]

   Ocean warming near the Antarctic ice shelves has critical implications for future ice sheet mass loss and global sea level rise. A global climate model with an eddying ocean is used to quantify the mechanisms contributing to ocean warming on the Antarctic continental shelf in an idealized 2xCO2 experiment. The results indicate that relatively large warm anomalies occur both in the upper 100 m and at depths above the shelf floor, which are controlled by different mechanisms. The near-surface ocean warming is primarily a response to enhanced onshore advective heat transport across the shelf break. The deep shelf warming is initiated by onshore intrusions of relatively warm Circumpolar Deep Water (CDW), in density classes that access the shelf, as well as the reduction of the vertical mixing of heat. CO2-induced shelf freshening influences both warming mechanisms. The shelf freshening slows vertical mixing by limiting gravitational instabilities and the upward diffusion of heat associated with CDW, resulting in the build-up of heat at depth. Meanwhile, freshening near the shelf break enhances the lateral density gradient of the Antarctic Slope Front (ASF) and disconnect isopycnals between the shelf and CDW, making cross-ASF heat exchange more difficult. However, at several locations along the ASF, the cross-ASF heat transport is less inhibited and heat can move onshore. Once onshore, lateral and vertical heat advection work to disperse the heat anomalies across the shelf region. Understanding the inhomogeneous Antarctic shelf warming will lead to better projections of future ice sheet mass loss.

7. Hewitt, H T., M J Bell, E P Chassignet, A Czaja, D Ferreira, and Stephen M Griffies, et al., December 2017: Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales? Ocean Modelling, 120, DOI:10.1016/j.ocemod.2017.11.002 .
   [ Abstract ]

   As the importance of the ocean in the weather and climate system is increasingly recognised, operational systems are now moving towards coupled prediction not only for seasonal to climate timescales but also for short-range forecasts. A three-way tension exists between the allocation of computing resources to refine model resolution, the expansion of model complexity/capability, and the increase of ensemble size. Here we review evidence for the benefits of increased ocean resolution in global coupled models, where the ocean component explicitly represents transient mesoscale eddies and narrow boundary currents. We consider lessons learned from forced ocean/sea-ice simulations; from studies concerning the SST resolution required to impact atmospheric simulations; and from coupled predictions. Impacts of the mesoscale ocean in western boundary current regions on the large-scale atmospheric state have been identified. Understanding of air-sea feedback in western boundary currents is modifying our view of the dynamics in these key regions. It remains unclear whether variability associated with open ocean mesoscale eddies is equally important to the large-scale atmospheric state. We include a discussion of what processes can presently be parameterised in coupled models with coarse resolution non-eddying ocean models, and where parameterizations may fall short. We discuss the benefits of resolution and identify gaps in the current literature that leave important questions unanswered.

8. MacKinnon, J A., M H Alford, J K Ansong, B K Arbic, A Barna, B P Briegleb, F O Bryan, M C Buijsman, E P Chassignet, G Danabasoglu, S Diggs, Stephen M Griffies, Robert Hallberg, S R Jayne, M Jochum, J Klymak, E Kunze, W G Large, Sonya Legg, B Mater, and Angelique Melet, et al., November 2017: Climate Process Team on Internal-Wave Driven Ocean Mixing. Bulletin of the American Meteorological Society, 98(11), DOI:10.1175/BAMS-D-16-0030.1 .
   [ Abstract ]

   Recent advances in our understanding of internal-wave driven turbulent mixing in the ocean interior are summarized. New parameterizations for global climate ocean models, and their climate impacts, are introduced. Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

9. Orr, James C., R G Najjar, O Aumont, L Bopp, J L Bullister, G Danabasoglu, S C Doney, John P Dunne, J-C Dutay, H D Graven, Stephen M Griffies, and Jasmin G John, et al., June 2017: Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP). Geoscientific Model Development, 10(6), DOI:10.5194/gmd-10-2169-2017 .
   [ Abstract ]

   The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth System Models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) versus when integrated within fully coupled Earth System Models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled online to ocean circulation models, initialized with observational data or output from a model spin up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6 and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin up, preferably for 2000 years or more and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facililtate their implementation.

10. Spence, P, R Holmes, A McC Hogg, Stephen M Griffies, K D Stewart, and M H England, August 2017: Localized rapid warming of West Antarctic subsurface waters by remote winds. Nature Climate Change, 7(8), DOI:10.1038/nclimate3335 .
    [ Abstract ]

    The highest rates of Antarctic glacial ice mass loss are occurring to the west of the Antarctica Peninsula in regions where warming of subsurface continental shelf waters is also largest. However, the physical mechanisms responsible for this warming remain unknown. Here we show how localized changes in coastal winds off East Antarctica can produce significant subsurface temperature anomalies (>2 °C) around much of the continent. We demonstrate how coastal-trapped barotropic Kelvin waves communicate the wind disturbance around the Antarctic coastline. The warming is focused on the western flank of the Antarctic Peninsula because the circulation induced by the coastal-trapped waves is intensified by the steep continental slope there, and because of the presence of pre-existing warm subsurface water offshore. The adjustment to the coastal-trapped waves shoals the subsurface isotherms and brings warm deep water upwards onto the continental shelf and closer to the coast. This result demonstrates the vulnerability of the West Antarctic region to a changing climate.

11. Stewart, K D., A McC Hogg, and Stephen M Griffies, et al., May 2017: Vertical resolution of baroclinic modes in global ocean models. Ocean Modelling, 113, DOI:10.1016/j.ocemod.2017.03.012 .
    [ Abstract ]

    Improvements in the horizontal resolution of global ocean models, motivated by the horizontal resolution requirements for specific flow features, has advanced modelling capabilities into the dynamical regime dominated by mesoscale variability. In contrast, the choice of the vertical grid remains a subjective choice, and it is not clear that efforts to improve vertical resolution adequately support their horizontal counterparts. Indeed, considering that the bulk of the vertical ocean dynamics (including convection) are parameterized, it is not immediately obvious what the vertical grid is supposed to resolve. Here, we propose that the primary purpose of the vertical grid in a hydrostatic ocean model is to resolve the vertical structure of horizontal flows, rather than to resolve vertical motion. With this principle we construct vertical grids based on their abilities to represent baroclinic modal structures commensurate with the theoretical capabilities of a given horizontal grid. This approach is designed to ensure that the vertical grids of global ocean models complement (and, importantly, to not undermine) the resolution capabilities of the horizontal grid. We find that for z-coordinate global ocean models, at least 50 well-positioned vertical levels are required to resolve the first baroclinic mode, with an additional 25 levels per subsequent mode. High-resolution ocean-sea ice simulations are used to illustrate some of the dynamical enhancements gained by improving the vertical resolution of a 1/10° global ocean model. These enhancements include substantial increases in the sea surface height variance (∼ 30% increase south of 40°S), the barotropic and baroclinic eddy kinetic energies (up to 200% increase on and surrounding the Antarctic continental shelf and slopes), and the overturning streamfunction in potential density space (near-tripling of the Antarctic Bottom Water cell at 65°S).

12. Swapna, P, J Jyoti, R Krishnan, N Sandeep, and Stephen M Griffies, October 2017: Multi-decadal weakening of Indian summer monsoon circulation induces an increasing northern Indian Ocean sea level. Geophysical Research Letters, 44(20), DOI:10.1002/2017GL074706 .
    [ Abstract ]

    North Indian Ocean sea level has shown significant increase during last 3-4 decades. Analyses of long-term climate datasets and ocean model sensitivity experiments identify a mechanism for multi-decadal sea level variability relative to global mean. Our results indicate that north Indian Ocean sea level rise is accompanied by a weakening summer monsoon circulation. Given that Indian Ocean meridional heat transport is primarily regulated by the annual cycle of monsoon winds, weakening of summer monsoon circulation has resulted in reduced upwelling off Arabia and Somalia and decreased southward heat-transport, and corresponding increase of heat storage in the north Indian Ocean. These changes in-turn lead to increased retention of heat and increased thermosteric sea level rise in the north Indian Ocean, especially in the Arabian Sea. These findings imply that rising north Indian Ocean sea level due to weakening of monsoon circulation demand adaptive strategies to enable a resilient South Asian population.

13. Tamsitt, V, H F Drake, Adele K Morrison, L D Talley, C O Dufour, A R Gray, Stephen M Griffies, M R Mazloff, and Jorge L Sarmiento, et al., August 2017: Spiraling pathways of global deep waters to the surface of the Southern Ocean. Nature Communications, 8, 172, DOI:10.1038/s41467-017-00197-0 .
    [ Abstract ]

    Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.

14. Zhai, Ping, K B Rodgers, Stephen M Griffies, Richard D Slater, D Iudicone, Jorge L Sarmiento, and Laure Resplandy, September 2017: Mechanistic drivers of re-emergence of anthropogenic carbon in the Equatorial Pacific. Geophysical Research Letters, 44(18), DOI:10.1002/2017GL073758 .
    [ Abstract ]

    Relatively rapid re-emergence of anthropogenic carbon (Cant) in the Equatorial Pacific is of potential importance for its impact on the carbonate buffering capacity of surface seawater, and thereby impeding the ocean's ability to further absorb Cant from the atmosphere. We explore the mechanisms sustaining Cant re-emergence (upwelling) from the thermocline to surface layers by applying water mass transformation diagnostics to a global ocean/sea-ice/biogeochemistry model. We find that the upwelling rate of Cant (0.4 PgC yr-1) from the thermocline to the surface layer is almost twice as large as air-sea Cant fluxes (0.203 PgC yr-1). The upwelling of Cant from the thermocline to the surface layer can be understood as a two-step process: the first being due to diapycnal diffusive transformation fluxes and the second due to surface buoyancy fluxes. We also find that this re-emergence of Cant decreases dramatically during the 1982/1983 and 1997/1998 El Niño events.

15. Danabasoglu, G, S G Yeager, W M Kim, E Behrens, M Bentsen, D Bi, A Biastoch, R Bleck, C Boning, A Bozec, V M Canuto, C Cassou, E P Chassignet, A C Coward, S Danilov, N Diansky, H Drange, Riccardo Farneti, E Fernandez, P G Fogli, G Forget, Y Fujii, Stephen M Griffies, A Gusev, P Heimbach, A Howard, M Ilicak, T Jung, A R Karspeck, M Kelley, W G Large, A Leboissetier, J Lu, G Madec, S J Marsland, S Masina, A Navarra, A J George Nurser, A Pirani, A Romanou, D Salas y Mélia, and Bonita L Samuels, et al., January 2016: North Atlantic Simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part II: Inter-Annual to Decadal Variability. Ocean Modelling, 97, DOI:10.1016/j.ocemod.2015.11.007 .
    [ Abstract ]

    Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958−−2007 period from twenty global ocean – sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The study is Part II of our companion paper (Danabasoglu et al., 2014) which documented the mean states in the North Atlantic from the same models. A major focus of the present study is the representation of Atlantic meridional overturning circulation (AMOC) variability in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until the mid- to late-1970s, AMOC is relatively steady, remaining lower than its long-term (1958−−2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in the mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about the mid-1970s and the mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric datasets which include the influence of ocean dynamics from nature superimposed onto anthropogenic effects. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres.

16. Downes, S M., A McC Hogg, Stephen M Griffies, and Bonita L Samuels, August 2016: The transient response of Southern Ocean circulation to geothermal heating in a global climate model. Journal of Climate, 29(16), DOI:10.1175/JCLI-D-15-0458.1 .
    [ Abstract ]

    Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely-distributed Antarctic Bottom Waters. Here we test two distinct geothermal heat flux datasets under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. Our regional analysis reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. Whilst geothermal heating enhances Southern Ocean abyssal overturning circulation by 20-50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3 to 5 Sv. The transient responses in regional bottom temperature increases exceed 0.1°C. The combination of large scale features that we show act to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.

17. Gregory, J M., N Bouttes-Mauhourat, Stephen M Griffies, H Haak, William J Hurlin, J H Jungclaus, M Kelley, W G Lee, J Marshall, A Romanou, O A Saenko, D Stammer, and Michael Winton, November 2016: The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) contribution to CMIP6: Investigation of sea-level and ocean climate change in response to CO2 forcing. Geoscientific Model Development Discussion, 9(11), DOI:10.5194/gmd-9-3993-2016 .
    [ Abstract ]

    The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) aims to investigate the spread in simulations of sea-level and ocean climate change in response to CO2 forcing by atmosphere-ocean general circulation models (AOGCMs). It is particularly motivated by the uncertainties in projections of ocean heat uptake, global-mean sea-level rise due to thermal expansion and the geographical patterns of sea-level change due to ocean density and circulation change. FAFMIP has three tier-1 experiments, in which prescribed surface flux perturbations of momentum, heat and freshwater respectively are applied to the ocean in separate AOGCM simulations. All other conditions are as in the pre-industrial control. The prescribed fields are typical of pattern and magnitude of changes in these fluxes projected by AOGCMs for doubled CO2 concentration. Five groups have tested the experimental design with existing AOGCMs. Their results show diversity in the pattern and magnitude of changes, with some common qualitative features. Heat and water flux perturbation cause the dipole in sea-level change in the North Atlantic, while momentum and heat flux perturbation cause the gradient across the Antarctic Circumpolar Current. The Atlantic Meridional Overturning Circulation (AMOC) declines in response to the heat flux perturbation, and there is a strong positive feedback on this effect due to the consequent cooling of sea surface temperature in the North Atlantic, which enhances the local heat input to the ocean. The momentum and water flux perturbations do not substantially affect the AMOC. Heat is taken up largely as a passive tracer in the Southern Ocean, which is the region of greatest heat input, but elsewhere heat is actively redistributed towards lower latitude. Future analysis of these and other phenomena with the wider range of CMIP6 FAFMIP AOGCMs will benefit from new diagnostics of temperature and salinity tendencies, which will enable investigation of the model spread in behaviour in terms of physical processes as formulated in the models.

18. Griffies, Stephen M., G Danabasoglu, P J Durack, Alistair Adcroft, V Balaji, C Boning, E P Chassignet, E N Curchitser, J Deshayes, H Drange, B Fox-Kemper, P J Gleckler, J M Gregory, H Haak, Robert Hallberg, H T Hewitt, D M Holland, T Ilyina, J H Jungclaus, Y Komuro, John P Krasting, W G Large, S J Marsland, S Masina, T J McDougall, A J George Nurser, James C Orr, A Pirani, F Qiao, Ronald J Stouffer, K E Taylor, A M Treguier, H Tsujino, P Uotila, M Valdivieso, Michael Winton, and S G Yeager, September 2016: OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project. Geoscientific Model Development, 9(9), DOI:10.5194/gmd-9-3231-2016 .
    [ Abstract ]

    The Ocean Model Intercomparison Project (OMIP) aims to provide a framework for evaluating, understanding, and improving the ocean and sea-ice components of global climate and earth system models contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses these aims in two complementary manners: (A) by providing an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing, (B) by providing a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) offering details for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows that of the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II have become the standard method to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP (Scenario MIP), as well as the ocean-sea ice OMIP simulations. The bulk of this paper offers scientific rationale for saving these diagnostics.

19. Ilicak, M, H Drange, Q Wang, R Gerdes, Y Aksenov, D Bailey, M Bentsen, A Biastoch, A Bozec, C Boning, C Cassou, E P Chassignet, A C Coward, B Curry, G Danabasoglu, S Danilov, E Fernandez, P G Fogli, Y Fujii, Stephen M Griffies, D Iovino, A Jahn, T Jung, W G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., April 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part III: Hydrography and fluxes. Ocean Modelling, 100, DOI:10.1016/j.ocemod.2016.02.004 .
    [ Abstract ]

    In this paper we compare the simulated Arctic Ocean in fifteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves.

20. Morrison, Adele K., Stephen M Griffies, Michael Winton, Whit G Anderson, and Jorge L Sarmiento, March 2016: Mechanisms of Southern Ocean heat uptake and transport in a global eddying climate model. Journal of Climate, 29(6), DOI:10.1175/JCLI-D-15-0579.1 .
    [ Abstract ]

    The Southern Ocean plays a dominant role in anthropogenic oceanic heat uptake. Strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region and allows the heat uptake to be sustained. Using an eddy-rich global climate model, the processes controlling the northward transport and convergence of the heat anomaly in the mid-latitude Southern Ocean are investigated in an idealized 1% yr−1 increasing CO2 simulation. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the uptake region in the south is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller 20% contribution from enhanced upwelling. The heat anomaly converges in the mid-latitude deep mixed layers, because there is not a corresponding increase in the mean heat transport out of the deep mixed layers northward into the mode waters. To the north of the deep mixed layers, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. The eddy transport mechanism results from a reduction in both the diffusive and advective southward eddy heat transports, driven by decreasing isopycnal slopes and decreasing along-isopycnal temperature gradients on the northern edge of the peak warming.

21. Saba, Vincent S., Stephen M Griffies, Whit G Anderson, Michael Winton, M A Alexander, Thomas L Delworth, J A Hare, Matthew J Harrison, Anthony Rosati, Gabriel A Vecchi, and Rong Zhang, January 2016: Enhanced warming of the northwest Atlantic Ocean under climate change. Journal of Geophysical Research, 121(1), DOI:10.1002/2015JC011346 .
    [ Abstract ]

    The Intergovernmental Panel on Climate Change (IPCC) fifth assessment of projected global and regional ocean temperature change is based on global climate models that have coarse (∼100-km) ocean and atmosphere resolutions. In the Northwest Atlantic, the ensemble of global climate models has a warm bias in sea surface temperature due to a misrepresentation of the Gulf Stream position; thus, existing climate change projections are based on unrealistic regional ocean circulation. Here we compare simulations and an atmospheric CO2 doubling response from four global climate models of varying ocean and atmosphere resolution. We find that the highest resolution climate model (∼10-km ocean, ∼50-km atmosphere) resolves Northwest Atlantic circulation and water mass distribution most accurately. The CO2 doubling response from this model shows that upper-ocean (0-300 m) temperature in the Northwest Atlantic Shelf warms at a rate nearly twice as fast as the coarser models and nearly three times faster than the global average. This enhanced warming is accompanied by an increase in salinity due to a change in water mass distribution that is related to a retreat of the Labrador Current and a northerly shift of the Gulf Stream. Both observations and the climate model demonstrate a robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm-Temperate Slope Water entering the Northwest Atlantic Shelf. Therefore, prior climate change projections for the Northwest Atlantic may be far too conservative. These results point to the need to improve simulations of basin and regional-scale ocean circulation.

22. Tseng, Y-H, H Lin, H-C Chen, K Thompson, M Bentsen, C Boning, A Bozec, C Cassou, E P Chassignet, C H Chow, G Danabasoglu, S Danilov, Riccardo Farneti, P G Fogli, Y Fujii, Stephen M Griffies, M Ilicak, T Jung, S Masina, A Navarra, L Patara, and Bonita L Samuels, et al., August 2016: North and Equatorial Pacific Ocean Circulation in the CORE-II Hindcast Simulations. Ocean Modelling, 104, DOI:10.1016/j.ocemod.2016.06.003 .
    [ Abstract ]

    We evaluate the mean circulation patterns, water mass distributions, and tropical dynamics of the North and Equatorial Pacific Ocean based on a suite of global ocean-sea ice simulations driven by the CORE-II atmospheric forcing from 1963-2007. The first three moments (mean, standard deviation and skewness) of sea surface height and surface temperature variability are assessed against observations. Large discrepancies are found in the variance and skewness of sea surface height and in the skewness of sea surface temperature. Comparing with the observation, most models underestimate the Kuroshio transport in the Asian Marginal seas due to the missing influence of the unresolved western boundary current and meso-scale eddies. In terms of the Mixed Layer Depths (MLDs) in the North Pacific, the two observed maxima associated with Subtropical Mode Water and Central Mode Water formation coalesce into a large pool of deep MLDs in all participating models, but another local maximum associated with the formation of Eastern Subtropical Mode Water can be found in all models with different magnitudes. The main model bias of deep MLDs results from excessive Subtropical Mode Water formation due to inaccurate representation of the Kuroshio separation and of the associated excessively warm and salty Kuroshio water. Further water mass analysis shows that the North Pacific Intermediate Water can penetrate southward in most models, but its distribution greatly varies among models depending not only on grid resolution and vertical coordinate but also on the model dynamics. All simulations show overall similar large scale tropical current system, but with differences in the structures of the Equatorial Undercurrent. We also confirm the key role of the meridional gradient of the wind stress curl in driving the equatorial transport, leading to a generally weak North Equatorial Counter Current in all models due to inaccurate CORE-II equatorial wind fields. Most models show a larger interior transport of Pacific subtropical cells than the observation due to the overestimated transport in the Northern Hemisphere likely resulting from the deep pycnocline.

23. Wang, Q, M Ilicak, R Gerdes, H Drange, Y Aksenov, D Bailey, M Bentsen, A Biastoch, A Bozec, C Boning, C Cassou, E P Chassignet, A C Coward, B Curry, G Danabasoglu, S Danilov, E Fernandez, P G Fogli, Y Fujii, Stephen M Griffies, D Iovino, A Jahn, T Jung, W G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, B Rabe, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., March 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part I: Sea ice and solid freshwater. Ocean Modelling, 99, DOI:10.1016/j.ocemod.2015.12.008 .
    [ Abstract ]

    The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE II) is analyzed. The focus is on the Arctic sea ice extent, the solid freshwater (FW) sources and solid freshwater content (FWC). Available observations are used for model evaluation. The variability of sea ice extent and solid FW budget is more consistently reproduced than their mean state in the models. The descending trend of September sea ice extent is well simulated in terms of the model ensemble mean. Models overestimating sea ice thickness tend to underestimate the descending trend of September sea ice extent. The models underestimate the observed sea ice thinning trend by a factor of two. When averaged on decadal time scales, the variation of Arctic solid FWC is contributed by those of both sea ice production and sea ice transport, which are out of phase in time. The solid FWC decreased in the recent decades, caused mainly by the reduction in sea ice thickness. The models did not simulate the acceleration of sea ice thickness decline, leading to an underestimation of solid FWC trend after 2000. The common model behaviour, including the tendency to underestimate the trend of sea ice thickness and March sea ice extent, remains to be improved.

24. Wang, Q, M Ilicak, R Gerdes, H Drange, Y Aksenov, D Bailey, M Bentsen, A Biastoch, A Bozec, C Boning, C Cassou, E P Chassignet, A C Coward, B Curry, G Danabasoglu, S Danilov, E Fernandez, P G Fogli, Y Fujii, Stephen M Griffies, D Iovino, A Jahn, T Jung, W G Large, Craig Lee, C Lique, J Lu, S Masina, A J George Nurser, B Rabe, C Roth, D Salas y Mélia, and Bonita L Samuels, et al., March 2016: An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part II: Liquid freshwater. Ocean Modelling, 99, DOI:10.1016/j.ocemod.2015.12.009 .
    [ Abstract ]

    The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II) is analyzed in this study. The focus is on the Arctic liquid freshwater (FW) sources and freshwater content (FWC). The models agree on the interannual variability of liquid FW transport at the gateways where the ocean volume transport determines the FW transport variability. The variation of liquid FWC is induced by both the surface FW flux (associated with sea ice production) and lateral liquid FW transport, which are in phase when averaged on decadal time scales. The liquid FWC shows an increase starting from the mid-1990s, caused by the reduction of both sea ice formation and liquid FW export, with the former being more significant in most of the models. The mean state of the FW budget is less consistently simulated than the temporal variability. The model ensemble means of liquid FW transport through the Arctic gateways compare well with observations. On average, the models have too high mean FWC, weaker upward trends of FWC in the recent decade than the observation, and low consistency in the temporal variation of FWC spatial distribution, which needs to be further explored for the purpose of model development.

25. Biastoch, A, J V Durgadoo, Adele K Morrison, E van Sebille, W Weijer, and Stephen M Griffies, December 2015: Atlantic multi-decadal oscillation covaries with Agulhas leakage. Nature Communications, 6, 10082, DOI:10.1038/ncomms10082 .
    [ Abstract ]

    The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlantic.

26. Downes, S M., Riccardo Farneti, P Uotila, Stephen M Griffies, S J Marsland, D Bailey, E Behrens, M Bentsen, D Bi, A Biastoch, C Boning, A Bozec, V M Canuto, E P Chassignet, G Danabasoglu, S Danilov, N Diansky, H Drange, P G Fogli, A Gusev, A Howard, M Ilicak, T Jung, M Kelley, W G Large, A Leboissetier, M Long, J Lu, S Masina, A Mishra, A Navarra, A J George Nurser, L Patara, and Bonita L Samuels, et al., October 2015: An assessment of Southern Ocean water masses and sea ice during 1988-2007 in a suite of inter-annual CORE-II simulations. Ocean Modelling, 94, DOI:10.1016/j.ocemod.2015.07.022 .
    [ Abstract ]

    We characterize the representation of the Southern Ocean water mass structure and sea ice within a suite of 15 global ocean-ice models run with the Coordinated Ocean-ice Reference Experiment Phase II (CORE-II) protocol. The main focus is the representation of the present (1988–2007) mode and intermediate waters, thus framing an analysis of winter and summer mixed layer depths; temperature, salinity, and potential vorticity structure; and temporal variability of sea ice distributions. We also consider the inter-annual variability over the same 20 year period. Comparisons are made between models as well as to observation-based analyses where available. The CORE-II models exhibit several biases relative to Southern Ocean observations, including an underestimation of the model mean mixed layer depths of mode and intermediate water masses in March (associated with greater ocean surface heat gain), and an overestimation in September (associated with greater high latitude ocean heat loss and a more northward winter sea-ice extent). In addition, the models have cold and fresh/ warm and salty water column biases centered near 50°S. Over the 1988–2007 period, the CORE-II models consistently simulate spatially variable trends in sea-ice concentration, surface freshwater fluxes, mixed layer depths, and 200–700 m ocean heat content. In particular, sea-ice coverage around most of the Antarctic continental shelf is reduced, leading to a cooling and freshening of the near surface waters. The shoaling of the mixed layer is associated with increased surface buoyancy gain, except in the Pacific where sea ice is also influential. The models are in disagreement, despite the common CORE-II atmospheric state, in their spatial pattern of the 20-year trends in the mixed layer depth and sea-ice.

27. Dufour, C O., Stephen M Griffies, Gregory F de Souza, I Frenger, Adele K Morrison, J B Palter, Jorge L Sarmiento, E D Galbraith, John P Dunne, Whit G Anderson, and Richard D Slater, December 2015: Role of mesoscale eddies in cross-frontal transport of heat and biogeochemical tracers in the Southern Ocean. Journal of Physical Oceanography, 45(12), DOI:10.1175/JPO-D-14-0240.1 .
    [ Abstract ]

    This study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which we refer to as the “PF core”. A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model (CM2.6-miniBLING, 0.1° ocean model) is used to investigate the transport of heat, carbon, oxygen and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. We find that the total transport across the PF core results from an ubiquitous Ekman transport that drives the upwelled tracers to the north, and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the southward eddy transport dominates the total transport but remains much smaller in magnitude than the near-surface northward transport. Most of the southward branch of the total transport is achieved below the PF core, mainly through geostrophic currents. We find that the eddy diffusive transport reinforces the southward eddy advective transport for carbon and heat, and opposes it for oxygen and phosphate. Eddy advective transport is likely to be the leading-order component of eddy-induced transport for all four tracers. However, eddy diffusive transport may provide a significant contribution to the southward eddy heat transport due to strong along-isopycnal temperature gradients.

28. Farneti, Riccardo, S M Downes, Stephen M Griffies, S J Marsland, E Behrens, M Bentsen, D Bi, A Biastoch, C Boning, A Bozec, V M Canuto, E P Chassignet, G Danabasoglu, S Danilov, N Diansky, H Drange, P G Fogli, A Gusev, Robert Hallberg, A Howard, M Ilicak, T Jung, M Kelley, W G Large, A Leboissetier, M Long, J Lu, S Masina, A Mishra, A Navarra, A J George Nurser, L Patara, and Bonita L Samuels, et al., September 2015: An assessment of Antarctic Circumpolar Current and Southern Ocean Meridional Overturning Circulation during 1958–2007 in a suite of interannual CORE-II simulations. Ocean Modelling, 93, DOI:10.1016/j.ocemod.2015.07.009 .
    [ Abstract ]

    In the framework of the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II), we present an analysis of the representation of the Antarctic Circumpolar Current (ACC) and Southern Ocean Meridional Overturning Circulation (MOC) in a suite of seventeen global ocean-sea ice models. We focus on the mean, variability and trends of both the ACC and MOC over the 1958–2007 period, and discuss their relationship with the surface forcing. We aim to quantify the degree of eddy saturation and eddy compensation in the models participating in CORE-II, and compare our results with available observations, previous fine-resolution numerical studies and theoretical constraints. Most models show weak ACC transport sensitivity to changes in forcing during the past five decades, and they can be considered to be in an eddy saturated regime. Larger contrasts arise when considering MOC trends, with a majority of models exhibiting significant strengthening of the MOC during the late 20th and early 21st century. Only a few models show a relatively small sensitivity to forcing changes, responding with an intensified eddy-induced circulation that provides some degree of eddy compensation, while still showing considerable decadal trends. Both ACC and MOC interannual variability are largely controlled by the Southern Annular Mode (SAM). Based on these results, models are clustered into two groups. Models with constant or two-dimensional (horizontal) specification of the eddy-induced advection coefficient κ show larger ocean interior decadal trends, larger ACC transport decadal trends and no eddy compensation in the MOC. Eddy-permitting models or models with a three-dimensional time varying κ show smaller changes in isopycnal slopes and associated ACC trends, and partial eddy compensation. As previously argued, a constant in time or space κ is responsible for a poor representation of mesoscale eddy effects and cannot properly simulate the sensitivity of the ACC and MOC to changing surface forcing. Evidence is given for a larger sensitivity of the MOC as compared to the ACC transport, even when approaching eddy saturation. Future process studies designed for disentangling the role of momentum and buoyancy forcing in driving the ACC and MOC are proposed.

29. Goddard, P, Jianjun Yin, Stephen M Griffies, and Shaoqing Zhang, February 2015: An extreme event of sea-level rise along the Northeast coast of North America in 2009–2010. Nature Communications, 6, 6346, DOI:10.1038/ncomms7346 .
    [ Abstract ]

    The coastal sea levels along the Northeast Coast of North America show significant year-to-year fluctuations in a general upward trend. The analysis of long-term tide gauge records identified an extreme sea-level rise (SLR) event during 2009–10. Within this 2-year period, the coastal sea level north of New York City jumped by 128 mm. This magnitude of interannual SLR is unprecedented (a 1-in-850 year event) during the entire history of the tide gauge records. Here we show that this extreme SLR event is a combined effect of two factors: an observed 30% downturn of the Atlantic meridional overturning circulation during 2009–10, and a significant negative North Atlantic Oscillation index. The extreme nature of the 2009–10 SLR event suggests that such a significant downturn of the Atlantic overturning circulation is very unusual. During the twenty-first century, climate models project an increase in magnitude and frequency of extreme interannual SLR events along this densely populated coast.

30. Griffies, Stephen M., Michael Winton, Whit G Anderson, Rusty Benson, Thomas L Delworth, C O Dufour, John P Dunne, P Goddard, Adele K Morrison, Andrew T Wittenberg, Jianjun Yin, and Rong Zhang, February 2015: Impacts on ocean heat from transient mesoscale eddies in a hierarchy of climate models. Journal of Climate, 28(3), DOI:10.1175/JCLI-D-14-00353.1 .
    [ Abstract ]

    We characterize impacts on heat in the ocean climate system from transient ocean mesoscale eddies. Our tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three GFDL coupled models comprising the CM2-O model suite. CM2-O models differ in their ocean resolution: CM2.6 uses a 0.1° ocean grid, CM2.5 uses an intermediate grid with 0.25° spacing, and CM2-1deg uses a nominally 1.0° grid. Analysis of the ocean heat budget reveals that mesoscale eddies act to transport heat upward in a manner that partially compensates (or offsets) for the downward heat transport from the time mean currents. Stronger vertical eddy heat transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also imparts an upward heat transport, yet it differs systematically from that found in CM2.6. This analysis points to the fundamental role that ocean mesoscale features play in transient ocean heat uptake. In general, the more accurate simulation found in CM2.6 provides an argument for either including a rich representation of the ocean mesoscale in model simulations of the mean and transient climate, or for employing parameterizations that faithfully reflect the role of eddies in both lateral and vertical heat transport.

31. Sitz, L E., Riccardo Farneti, and Stephen M Griffies, July 2015: Simulated South Atlantic transports and their variability during 1958-2007. Ocean Modelling, 91, DOI:10.1016/j.ocemod.2015.05.001 .
    [ Abstract ]

    South Atlantic transports, as simulated by a global ocean-sea ice model forced with the Coordinated Ocean-ice Reference Experiments version 2 (CORE-II) interannually varying air-sea reanalysis data sets, are analyzed for the period 1958–2007. The ocean-sea ice model is configured at three different resolutions: from eddy-permitting to coarsened grid spacing. A particular focus is given to the effect of eddy fluxes and inter-ocean exchanges on the South Atlantic Meridional Overturning Circulation (SAMOC), as well as on the main factors contributing to the interannual variability during the integration period. Differences between refined and coarsened grid spacing models are more evident in coastal areas and in regions of high eddy activities. Major discrepancies are associated to both the parametrization of eddy fluxes and the coarse representation of the bathymetry. The refined grid spacing model produces higher values of both SAMOC index, defined as the maximum of the zonally-integrated northward cumulative volume transport (CVT) from surface to bottom across ∼ 34° S, and meridional heat transport (MHT). All models show high correlations between SAMOC index and MHT, as well as a strengthening of the transports in the 1980-2007 period. The strengthening of the SAMOC index is mainly dominated by surface and mode waters in all models. In surface and intermediate layers, the regions contributing to this trend are located east of 40° W. These changes are compensated by the strengthening of the poleward transport in deeper layers, mostly in the western part of the basin. The MHT trend is connected with the combined effect of a heat transport increase through the Drake Passage and a reduction of the heat loss through the eastern section between Africa and Antarctica, mainly associated with a strengthening in heat entering into the basin through the Agulhas system.

32. Snow, K, A McC Hogg, S M Downes, B M Sloyan, M L Bates, and Stephen M Griffies, May 2015: Sensitivity of abyssal water masses to overflow parameterisations. Ocean Modelling, 89, DOI:10.1016/j.ocemod.2015.03.004 .
    [ Abstract ]

    Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) control the abyssal limb of the global overturning circulation and play a major role in oceanic heat uptake and carbon storage. However, current general circulation models are unable to resolve the observed AABW and NADW formation and transport processes. One key process, that of overflows, motivates the application of overflow parameterisations. We present a sensitivity study of both AABW and NADW properties to three current parameterisations using a z∗z∗-coordinate ocean-sea ice model within a realistic-topography sector of the Atlantic Ocean.

    Overflow parameterisations that affect only tracer equations are compared to a fully dynamical Lagrangian point particle method. An overflow parameterisation involving partial convective mixing of tracers is most efficient at transporting dense NADW water downslope. This parameterisation leads to a maximum mean increase in density in the north of 0.027 kg m−3 and a decrease in age of 525 years (53%). The relative change in density and age in the south is less than 30% of that in the north for all overflow parameterisations. The reduced response in the south may result from the differing dense water formation and overflow characteristics of AABW compared to NADW. Alternative approaches may be necessary to improve AABW representation in z∗z∗-coordinate ocean climate models.

33. Bueti, M R., I Ginis, L Rothstein, and Stephen M Griffies, September 2014: Tropical Cyclone-Induced Thermocline Warming and its Regional and Global Impacts. Journal of Climate, 27(18), DOI:10.1175/JCLI-D-14-00152.1 .
    [ Abstract ]

    Strong surface winds of a hurricane locally cool the surface and warm the subsurface waters via turbulent mixing processes. While the surface cool anomalies generally decay in roughly a month, the warm subsurface anomalies can persist over a seasonal cycle. We examine questions related to the magnitude and cumulative footprint of subsurface warm anomalies forced by tropical cyclones during the combined global tropical cyclone seasons, making use of a global ocean model forced by tropical cyclones. Simulations of the 2004-2005 tropical cyclone season are conducted with and without tropical cyclone wind forcing, blended with the daily Coordinated Ocean-ice Reference Experiments (COREs) atmospheric state. Physical characteristics of cyclone-forced surface and subsurface anomalies are elucidated. In particular, we examine the spatial extent and magnitude of tropical cyclone forced subsurface warm anomalies over the course of an entire season. This analysis allows us to estimate the contribution of cyclone-induced anomalies to the ocean heat content and sea surface temperature, and to understand anomalous meridional heat transport. Globally, there is a maximum accumulated heat uptake 4.1·1021J, with the greatest regional contributions in the North Atlantic (1.7·1021J), West Pacific (1.5·1021J), and East Pacific (1.7·1021J). We find an export of heat from the subtropics to the tropics via rapid advective pathways, most notably in the West Pacific. These warm anomalies tend to remain in the equatorial band, with potential implications for the tropical climate system.

34. Danabasoglu, G, Stephen M Griffies, and Bonita L Samuels, et al., January 2014: North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part I: Mean states. Ocean Modelling, 73, DOI:10.1016/j.ocemod.2013.10.005 .
    [ Abstract ]

    Simulation characteristics from eighteen global ocean–sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort.

35. Fan, Yalin, Shian-Jiann Lin, Stephen M Griffies, and M A Hemer, May 2014: Simulated Global Swell and Wind Sea Climate and Their responses to Anthropogenic Climate Change at the End of the 21st Century. Journal of Climate, 27(10), DOI:10.1175/JCLI-D-13-00198.1 .
36. Fan, Yalin, and Stephen M Griffies, June 2014: Impacts of parameterized Langmuir turbulence and non-breaking wave mixing in global climate simulations. Journal of Climate, 27(12), DOI:10.1175/JCLI-D-13-00583.1 .
    [ Abstract ]

    We assess the impacts of parameterized upper ocean wave mixing on global climate simulations through modification to the K-profile ocean boundary layer parameterization (KPP; Large et al 1994) in a coupled atmosphere-ocean-wave global climate model. We consider three parameterizations and focus on impacts to high latitude ocean mixed layer depths and related ocean diagnostics. The McWilliams and Sullivan (2000) parameterization (MS2000) adds a Langmuir turbulence enhancement to the non-local component of KPP. We find that the Langmuir turbulence induced mixing provided by this parameterization is too strong in winter, producing overly deep mixed layers, and of minimal impact in summer. The Smyth et al (2002) parameterization modifies MS2000 by adding a stratification effect to restrain the turbulence enhancement under weak stratification conditions (e.g., winter) and to magnify the enhancement under strong stratification conditions. The Smyth et al (2002) scheme improves the simulated winter Mixed Layer Depth in our simulations, with mixed layer deepening in the Labrador Sea and shoaling in the Weddell and Ross Seas. Enhanced vertical mixing through parameterized Langmuir turbulence, coupled with enhanced lateral transport associated with parameterized mesoscale/submesoscale eddies, are found to be key elements for improving mixed layer simulations. Secondary impacts include strengthening the Atlantic Meridional Overturning Circulation and reducing the Antarctic Circumpolar Current. The Qiao et al (2004) non-breaking wave parameterization is the third scheme we assess. It adds a wave orbital velocity to the Reynolds stress calculation, and provides the strongest summer mixed layer deepening in the Southern Ocean among the three experiments, but with weak impacts during winter.

37. Farneti, Riccardo, S Dwivedi, F Kucharski, F Molteni, and Stephen M Griffies, September 2014: On Pacific Subtropical Cell Variability over the Second Half of the Twentieth Century. Journal of Climate, 27(18), DOI:10.1175/JCLI-D-13-00707.1 .
    [ Abstract ]

    The evolution of the Pacific subtropical cells (STC) is presented for the period 1948-2007. Using ocean models of different resolutions forced with interannually varying atmospheric forcing data sets, the mechanisms responsible for the observed STC weakening and late recovery during the period of study are analyzed. As a result of the STC weakening (strengthening), warming (cooling) trends are found in the equatorial Pacific sea surface temperatures (SSTs). Model results agree well with observed estimates of STC transport, convergence, and equatorial SST anomalies. It is shown that subtropical atmospheric variability is the primary driver of the STC and equatorial SST low-frequency evolution, and responsible for both the slowdown during the second half of the twentieth century and the rebound at the end of the century. Subtropically-forced STC variability is identified as a major player in the generation of equatorial Pacific decadal SST anomalies, pacing tropical Pacific natural climate variability on inter-decadal time scales, as observed in historical records. The natural mode of variability has implications for the evolution of equatorial SST in the coming decades under the concomitant effects of climate change.

38. Griffies, Stephen M., Bonita L Samuels, and Michael Winton, et al., June 2014: An assessment of global and regional sea level for years 1993-2007 in a suite of interannual CORE-II simulations. Ocean Modelling, 78, DOI:10.1016/j.ocemod.2014.03.004 .
    [ Abstract ]

    We provide an assessment of sea level simulated in a suite of global ocean-sea ice models using the interannual CORE atmospheric state to determine surface ocean boundary buoyancy and momentum fluxes. These CORE-II simulations are compared amongst themselves as well as to observation-based estimates. We focus on the final 15 years of the simulations (1993-2007), as this is a period where the CORE-II atmospheric state is well sampled, and it allows us to compare sea level related fields to both satellite and in situ analyses. The ensemble mean of the CORE-II simulations broadly agree with various global and regional observation-based analyses during this period, though with the global mean thermosteric sea level rise biased low relative to observation-based analyses. The simulations reveal a positive trend in dynamic sea level in the west Pacific and negative trend in the east, with this trend arising from wind shifts and regional changes in upper 700 m ocean heat content. The models also exhibit a thermosteric sea level rise in the subpolar North Atlantic associated with a transition around 1995/1996 of the North Atlantic Oscillation to its negative phase, and the advection of warm subtropical waters into the subpolar gyre. Sea level trends are predominantly associated with steric trends, with thermosteric effects generally far larger than halosteric effects, except in the Arctic and North Atlantic. There is a general anti-correlation between thermosteric and halosteric effects for much of the World Ocean, associated with density compensated changes.

39. McDougall, T J., S Groeskamp, and Stephen M Griffies, August 2014: On geometrical aspects of interior ocean mixing. Journal of Physical Oceanography, 44(8), DOI:10.1175/JPO-D-13-0270.1 .
    [ Abstract ]

    The small-slope approximation to the full three-dimensional diffusion tensor of epineutral diffusion gives exactly the same tracer flux as the commonly-used projected non-orthogonal diffusive flux of layered ocean models and of theoretical studies. The epineutral diffusion achieved by this small-slope approximation is not exactly in the direction of the correct epineutral tracer gradient. That is, the use of the small-slope approximation leads to a very small flux of tracer in a direction in which there is no epineutral gradient of tracer. For (the tracer) temperature or salinity, the difference between the correct epineutral gradient and the small-slope approximation to it is proportional to neutral helicity. We also make the point that small-scale turbulent mixing processes act to diffuse tracer isotropically (i.e. the same in each spatial direction) and hence it is strictly a misnomer to call this process “dianeutral diffusion” or “vertical diffusion”. This realization also has implications for the diffusion tensor.

40. Palter, J B., Stephen M Griffies, E D Galbraith, Anand Gnanadesikan, Bonita L Samuels, and A Klocker, January 2014: The deep ocean buoyancy budget and its temporal variability. Journal of Climate, 27(2), DOI:10.1175/JCLI-D-13-00016.1 .
    [ Abstract ]

    Despite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this redistribution remain poorly understood. This work uses an Earth System Model to evaluate the global steady state ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical advective-diffusive balance for the deep ocean holds at low- to mid-latitudes. At higher latitudes, the balance depends on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes then influence the large-scale advection of buoyancy to create transient deep buoyancy anomalies at lower latitudes. Following a doubling of atmospheric carbon dioxide concentrations, the high latitude buoyancy sinks are suppressed by a slowdown in convection and reduced dense water formation. This change is accompanied by a slowing of both upper and lower cells of the global meridional overturning circulation, reducing the supply of dense water to low latitudes beneath the pycnocline and the commensurate flow of light waters to high latitudes above the pycnocline. By this mechanism, changes in high latitude buoyancy are communicated to the global deep ocean on relatively fast advective timescales.

41. Rahaman, H, M Ravichandran, D Sengupta, Matthew J Harrison, and Stephen M Griffies, March 2014: Development of a regional model for the north Indian Ocean. Ocean Modelling, 75, DOI:10.1016/j.ocemod.2013.12.005 .
    [ Abstract ]

    We have developed a one-way nested Indian Ocean regional model. The model combines the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory’s (GFDL) Modular Ocean Model (MOM4p1) at global climate model resolution (nominally one degree), and a regional Indian Ocean MOM4p1 configuration with 25 km horizontal resolution and 1 meter vertical resolution near the surface. Inter-annual global simulations with Coordinated Ocean-Ice Reference Experiments (CORE-II) surface forcing over years 1992-2005 provide surface boundary conditions. We show that relative to the global simulation, (i) biases in upper ocean temperature, salinity and mixed layer depth are reduced, (ii) sea surface height and upper ocean circulation are closer to observations, and (iii) improvements in model simulation can be attributed to refined resolution, more realistic topography and inclusion of seasonal river runoff. Notably, the surface salinity bias is reduced to less than 0.1 psu over the Bay of Bengal using relatively weak restoring to observations, and the model simulates the strong, shallow halocline often observed in the north Bay of Bengal. There is marked improvement in subsurface salinity and temperature, as well as mixed layer depth in the Bay of Bengal. Major seasonal signatures in observed sea surface height anomaly in the tropical Indian Ocean, including the coastal waveguide around the Indian peninsula, are simulated with great fidelity. The use of realistic topography and seasonal river runoff brings the three dimensional structure of the East India Coastal Current and West India Coastal Current much closer to observations. As a result, the incursion of low salinity Bay of Bengal water into the south-eastern Arabian Sea is more realistic.

42. Spence, P, and Stephen M Griffies, et al., July 2014: Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds. Geophysical Research Letters, 41(13), DOI:10.1002/2014GL060613 .
    [ Abstract ]

    The southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s. This wind trend is projected to persist under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Here we show that a poleward wind shift at the latitudes of the Antarctic Peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 °C at 200-700 m depth. The model simulated warming results from a rapid advective heat flux induced by weakened near-shore Ekman pumping, and is associated with weakened coastal currents. This analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around Antarctica, with potentially significant ramifications for global sea level rise.

43. Winton, Michael, Whit G Anderson, Thomas L Delworth, Stephen M Griffies, William J Hurlin, and Anthony Rosati, December 2014: Has Coarse Ocean Resolution Biased Simulations of Transient Climate Sensitivity? Geophysical Research Letters, 41(23), DOI:10.1002/2014GL061523 .
    [ Abstract ]

    We investigate the influence of ocean component resolution on simulation of climate sensitivity using variants of the GFDL CM2.5 climate model incorporating eddy-resolving (1/10o) and eddy-parameterizing (1o) ocean resolutions. Two parameterization configurations of the coarse-resolution model are used yielding a three-model suite with significant variation in the transient climate response (TCR). The variation of TCR in this suite and in an enhanced group of 10 GFDL models is found to be strongly associated with the control climate Atlantic meridional overturning circulation (AMOC) magnitude and its decline under forcing. We find it is the AMOC behavior rather than resolution per se that accounts for most of the TCR differences. A smaller difference in TCR stems from the eddy-resolving model having more Southern Ocean surface warming than the coarse models.

44. Bi, D, and Stephen M Griffies, et al., March 2013: The ACCESS coupled model: description, control climate and evaluation. Australian Meteorological and Oceanographic Journal, 63(1), 41-64.
    [ Abstract ]

    The Australian Community Climate and Earth System Simulator coupled model (ACCESS-CM) has been developed at the Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO1 and the Bureau of Meteorology. It is built by coupling the UK Met Office atmospheric unified model (UM), and other sub-models as required, to the ACCESS ocean model, which consists of the NOAA/GFDL2 ocean model MOM4p1 and the LANL3 sea-ice model CICE4.1, under the CERFACS4 OASIS3.2–5 coupling framework. The primary goal of the ACCESS-CM development is to provide the Australian climate community with a new generation fully coupled climate model for climate research, and to participate in phase five of the Coupled Model Inter-comparison Project (CMIP5). This paper describes the ACCESS-CM framework and components, and presents the control climates from two versions of the ACCESS-CM, ACCESS1.0 and ACCESS1.3, together with some fields from the 20th century historical experiments, as part of model evaluation. While sharing the same ocean sea-ice model (except different setups for a few parameters), ACCESS1.0 and ACCESS1.3 differ from each other in their atmospheric and land surface components: the former is configured with the UK Met Office HadGEM2 (r1.1) atmospheric physics and the Met Office Surface Exchange Scheme land surface model version 2, and the latter with atmospheric physics similar to the UK Met Office Global Atmosphere 1.0 including modifications performed at CAWCR and the CSIRO Community Atmosphere Biosphere Land Exchange land surface model version 1.8. The global average annual mean surface air temperature across the 500-year preindustrial control integrations show a warming drift of 0.35 °C in ACCESS1.0 and 0.04 °C in ACCESS1.3. The overall skills of ACCESS-CM in simulating a set of key climatic fields both globally and over Australia significantly surpass those from the preceding CSIRO Mk3.5 model delivered to the previous coupled model inter-comparison. However, ACCESS-CM, like other CMIP5 models, has deficiencies in various aspects, and these are also discussed.

45. Bi, D, and Stephen M Griffies, et al., March 2013: ACCESS-OM: the ocean and sea-ice core of the ACCESS coupled model. Australian Meteorological and Oceanographic Journal, 63(1), 213-232.
    [ Abstract ]

    The Australian Community Climate and Earth System Simulator Ocean Model (ACCESS-OM), a global coupled ocean and sea-ice model, has been developed at the Centre for Australian Weather and Climate Research 1. It is aimed to serve the Australian climate sciences community, including the Bureau of Meteorology, CSIRO2 and Australian universities, for ocean climate research. ACCESS-OM comprises the NOAA/GFDL3 Modular Ocean Model version 4p1; the LANL4 Sea-ice Model version 4.1, a data atmospheric model; and the CERFACS5 OASIS3.25 coupler, which constrains data exchange between the sub-models. ACCESS-OM has been functioning as the ocean and sea-ice coupling core of the ACCESS coupled model, one of the Australian models participating in the Coupled Model Inter-comparison Project phase 5. This paper describes the ACCESS-OM sub-models, coupler, coupling strategy and framework. A selection of key metrics from an ACCESS-OM benchmark simulation, which has run for 500 years using the Coordinated Ocean-ice Reference Experiments normal year forcing, is presented and compared with observations to evaluate the model performance. It shows ACCESS-OM simulates the global ocean and sea-ice climate generally comparably to the results from other ocean sea-ice models of the same class (Griffies et al. 2009). For example, the global ocean volume-averaged temperature undergoes minor evolution. The maximum transport of North Atlantic overturning circulation is 18.5 Sv and the Antarctic Circumpolar Current transport through Drake Passage is 150 Sv, both in fair agreement with the observations; and the sea-ice coverage has reasonable distribution and annual cycle. Measured against other ocean sea-ice models and observations, ACCESS-OM is an appropriate tool for Australia’s future ocean climate modelling efforts.

46. Bony, S, Isaac M Held, and Stephen M Griffies, et al., September 2013: Carbon Dioxide and Climate: Perspectives on a Scientific Assessment In Climate Science for Serving Society Research, Modeling and Prediction Priorities, DOI:10.1007/978-94-007-6692-1_14 .
    [ Abstract ]

    Many of the findings of the Charney Report on CO2-induced climate change published in 1979 are still valid, even after 30 additional years of climate research and observations. This paper considers the reasons why the report was so prescient, and assesses the progress achieved since its publication. We suggest that emphasis on the importance of physical understanding gained through the use of theory and simple models, both in isolation and as an aid in the interpretation of the results of General Circulation Models, provided much of the authors’ insight at the time. Increased emphasis on these aspects of research is likely to continue to be productive in the future, and even to constitute one of the most efficient routes towards improved climate change assessments.

47. Dunne, John P., Jasmin G John, Elena Shevliakova, Ronald J Stouffer, John P Krasting, Sergey Malyshev, P C D Milly, Lori T Sentman, Alistair Adcroft, William F Cooke, Krista A Dunne, Stephen M Griffies, Robert Hallberg, Matthew J Harrison, Hiram Levy II, Andrew T Wittenberg, Peter Phillipps, and Niki Zadeh, April 2013: GFDL's ESM2 global coupled climate-carbon Earth System Models Part II: Carbon system formulation and baseline simulation characteristics. Journal of Climate, 26(7), DOI:10.1175/JCLI-D-12-00150.1 .
    [ Abstract ]

    We describe carbon system formulation and simulation characteristics of two new global coupled carbon-climate Earth System Models, ESM2M and ESM2G. These models demonstrate good climate fidelity as described in Part I while incorporating explicit and consistent carbon dynamics. The two models differ almost exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4.1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. On land, both ESMs include a revised land model to simulate competitive vegetation distributions and functioning, including carbon cycling among vegetation, soil and atmosphere. In the ocean, both models include new biogeochemical algorithms including phytoplankton functional group dynamics with flexible stoichiometry. Preindustrial simulations are spun up to give stable, realistic carbon cycle means and variability. Significant differences in simulation characteristics of these two models are described. Due to differences in oceanic ventilation rates (Part I) ESM2M has a stronger biological carbon pump but weaker northward implied atmospheric CO2 transport than ESM2G. The major advantages of ESM2G over ESM2M are: improved representation of surface chlorophyll in the Atlantic and Indian Oceans and thermocline nutrients and oxygen in the North Pacific. Improved tree mortality parameters in ESM2G produced more realistic carbon accumulation in vegetation pools. The major advantages of ESM2M over ESM2G are reduced nutrient and oxygen biases in the Southern and Tropical Oceans.

48. Frankcombe, L M., P Spence, A McC Hogg, M H England, and Stephen M Griffies, November 2013: Sea level changes forced by Southern Ocean winds. Geophysical Research Letters, 40(21), DOI:10.1002/2013GL058104 .
    [ Abstract ]

    On regional scales, changes in sea level are significantly affected by local dynamical changes. Westerly winds over the Southern Ocean have been strengthening and shifting southward in recent decades, and this change is projected to continue in the future. This study applies wind forcing anomalies to an eddy-permitting ocean model to study the dynamical response to a Southern Hemisphere westerly wind increase and/or southward shift. It is shown that the applied wind anomalies result in a change in sea surface slope across the Antarctic Circumpolar Current such that a fall in sea level occurs around the Antarctic continental margin. The Antarctic Circumpolar Current transport and regional sea level are particularly sensitive to latitudinal shifts in the wind, with a much more muted response found when only wind strengthening is applied. In addition to the local sea level changes, Southern Ocean winds also have a global effect through changing ocean heat content and the global overturning circulation.

49. Griffies, Stephen M., and A M Treguier, 2013: Ocean Circulation Models and Modeling In Ocean Circulation and Climate: A 21st Century Perspective, Second Edition, International Geophysics Vol 103, Elsevier, 521-551.
    [ Abstract ]

    This chapter focuses on numerical models used to understand and predict large-scale circulation, such as the circulation comprising basin and global scales. It is organized according to two themes. The first addresses physical and numerical topics forming a foundation for ocean models. We focus here on the science of ocean models, in which we ask questions about fundamental processes and develop the mathematical equations for ocean thermo-hydrodynamics. We also touch upon various methods used to represent the continuum ocean fluid with a discrete computer model, raising such topics as the finite volume formulation of the ocean equations; the choice for vertical coordinate; the complementary issues related to horizontal gridding; and the pervasive questions of subgrid scale parameterizations. The second theme of this chapter concerns the applications of ocean models, in particular how to design an experiment and how to analyze results. This material forms the basis for ocean modelling, with the aim being to mechanistically describe, interpret, understand, and predict emergent features of the simulated, and ultimately the observed, ocean.

50. Lee, Hyun-Chul, Thomas L Delworth, Anthony Rosati, Rong Zhang, Whit G Anderson, Fanrong Zeng, Charles A Stock, Anand Gnanadesikan, Keith W Dixon, and Stephen M Griffies, January 2013: Impact of climate warming on upper layer of the Bering Sea. Climate Dynamics, 40(1-2), DOI:10.1007/s00382-012-1301-8 .
    [ Abstract ]

    The impact of climate warming on the upper layer of the Bering Sea is investigated by using a high-resolution coupled global climate model. The model is forced by increasing atmospheric CO2 at a rate of 1% per year until CO2 reaches double its initial value (after 70 years), after which it is held constant. In response to this forcing, the upper layer of the Bering Sea warms by about 2�C in the southeastern shelf and by a little more than 1�C in the western basin. The wintertime ventilation to the permanent thermocline weakens in the western Bering Sea. After CO2 doubling, the southeastern shelf of the Bering Sea becomes almost ice-free in March, and the stratification of the upper layer strengthens in May and June. Changes of physical condition due to the climate warming would impact the pre-condition of spring bio-productivity in the southeastern shelf.

51. Marsland, S J., and Stephen M Griffies, et al., March 2013: Evaluation of ACCESS climate model ocean metrics in CMIP5 simulations. Australian Meteorological and Oceanographic Journal, 63(1), 101-119.
    [ Abstract ]

    Global and regional diagnostics are used to evaluate the ocean performance of the Australian Community Climate and Earth System Simulator coupled model (ACCESS-CM) contributions to the Climate Model Intercomparison Project phase 5 (CMIP5). Two versions of ACCESS-CM have been submitted to CMIP; namely CSIRO-BOM ACCESS1.0 and CSIRO-BOM ACCESS1.3. Results from six of the core CMIP5 experiments (piControl, historical, rcp45, rcp85, 1pctCO2, and abrupt 4xCO2) are evaluated for each of the two ACCESS-CM model versions. Overall, both model versions exhibit a reasonable and stable representation of key diagnostics of ocean climate performance in the pre-industrial control simulations, including a meridional overturning circulation with North Atlantic Deep Water maxima in the range 22–24 Sv, and a poleward heat transport maximum of around 1.5 PW. For the projected climate change scenarios considered the ACCESS-CM results are in reasonable agreement with responses found in other CMIP models, with the familiar ocean warming, and reduction in strength of meridional overturning and poleward heat transport. Drifts in the control simulations of both global ocean salinity and global sea-level are opposite in sign for ACCESS1.0 and ACCESS1.3, suggesting problems exist in the closure of the hydrological cycle. The simulation of ocean climate change over the historical period shows a weak response compared to observations, which manifests as a late response of ocean warming and sea level rise starting around 1990 in the model, compared to the mid 1960s in observations. Further historical simulations are underway to ascertain if this late response in ACCESS is a robust model feature, or just low frequency variability. If the weak response over the historical period proves robust, the likely cause is a too strong cooling from atmospheric aerosols. Broadening the set of experiments to further investigate the relative warming response of the ACCESS-CM to greenhouse gases compared to the cooling response to aerosols is underway, and preliminary results do suggest that the cooling due to aerosols is strong in the historical simulations.

52. Rugenstein, M, Michael Winton, Ronald J Stouffer, Stephen M Griffies, and Robert Hallberg, January 2013: Northern high latitude heat budget decomposition and transient warming. Journal of Climate, 26(2), DOI:10.1175/JCLI-D-11-00695.1 .
    [ Abstract ]

    Climate models simulate a wide range of climate changes at high northern latitudes in response to increased CO2. They also have substantial disagreement on projected changes of the Atlantic meridional overturning circulation (AMOC). Here we use two pairs of closely related climate models - each containing members with large and small AMOC declines - to explore the influence of AMOC decline on the high latitude response to increased CO2. The models with larger AMOC decline have less high latitude warming and sea ice decline than their small AMOC decline counterpart. By examining differences in the perturbation heat budget of the 40�90�N region, it is shown that AMOC decline diminishes the warming by weakening poleward ocean heat transport and increasing the ocean heat uptake. The cooling impact of this AMOC forced surface heat flux perturbation difference is enhanced by shortwave feedback and diminished by longwave feedback and atmospheric heat transport differences. The magnitude of the AMOC decline within model pairs is positively related to the magnitudes of control climate AMOC and Labrador Sea convection. Because the 40degree 90degree N region accounts for up to 40% of the simulated global ocean heat uptake over one hundred years, the process described here influences the global heat uptake efficiency.

53. Siedler, G A., Stephen M Griffies, J Gould, and J A Church, December 2013: Ocean Circulation and Climate: A 21st Century Perspective , Second Edition, International Geophysics V. 103: Academic Press, 904pp.
54. Straneo, F, P Heimbach, Olga V Sergienko, G Hamilton, G Catania, Stephen M Griffies, and Robert Hallberg, et al., August 2013: Challenges to Understand the Dynamic Response of Greenland's Marine Terminating Glaciers to Oceanic and Atmospheric Forcing. Bulletin of the American Meteorological Society, 94(8), DOI:10.1175/BAMS-D-12-00100.1 .
    [ Abstract ]

    The recent retreat and speedup of outlet glaciers, as well as enhanced surface melting around the ice sheet margin, have increased Greenland's contribution to sea level rise to 0.6±0.1 mm/yr and its discharge of freshwater into the North Atlantic. The widespread, near-synchronous glacier retreat, and its coincidence with a period of oceanic and atmospheric warming, suggest a common climate driver. Evidence points to the marine margins of these glaciers as the region from which changes propagated inland. Yet the forcings and mechanisms behind these dynamic responses are poorly understood and either missing or crudely parameterized in climate and ice sheet models. Resulting projected sea level rise contributions from Greenland by 2100 remain highly uncertain. This paper summarizes current state of knowledge and highlights key physical aspects of Greenland's coupled ice-sheet/ocean/atmosphere system. Three research thrusts are identified to yield fundamental insights into ice sheet, ocean, sea ice and atmosphere interactions, their role in Earth's climate system, and probable trajectories of future changes: (1) focused process studies addressing critical glacier, ocean, atmosphere and coupled dynamics; (2) sustained observations at key sites; and (3) inclusion of relevant dynamics in Earth System Models. Understanding the dynamic response of Greenland's glaciers to climate forcing constitutes both a scientific and technological frontier given the challenges of obtaining the appropriate measurements from the glaciers' marine termini and the complexity of the dynamics involved, including the coupling of the ocean, atmosphere, glacier and sea ice systems. Interdisciplinary and international cooperation are crucial to making progress on this novel and complex problem. Capsule: An interdisciplinary and multi-faceted approach is needed to understand the forcings and mechanisms behind the recent retreat and acceleration of Greenland's glaciers and its implications for future sea level rise

55. Winton, Michael, Alistair Adcroft, Stephen M Griffies, Robert Hallberg, Larry W Horowitz, and Ronald J Stouffer, January 2013: Influence of ocean and atmosphere components on simulated climate sensitivities. Journal of Climate, 26(1), DOI:10.1175/JCLI-D-12-00121.1 .
    [ Abstract ]

    We examine the influence of alternative ocean and atmosphere subcomponents on climate model simulation of transient sensitivities by comparing three GFDL climate models used for the CMIP5. The base model ESM2M is closely related to GFDL's CMIP3 climate model CM2.1, and makes use of a depth coordinate ocean component. The second model, ESM2G, is identical to ESM2M but makes use of an isopycnal coordinate ocean model. We compare the impact of this "ocean swap" with an "atmosphere swap" that produces the CM3 climate model by replacing the AM2 atmosphere with AM3 while retaining a depth coordinate ocean model. The atmosphere swap is found to have much larger influence on sensitivities of global surface temperature and Northern Hemisphere sea ice cover. The atmosphere swap also introduces a multi-decadal response timescale through its indirect influence on heat uptake. Despite significant differences in their interior ocean mean states, the ESM2M and ESM2G simulations of these metrics of climate change are very similar, except for an enhanced high latitude salinity response accompanied by temporarily advancing sea ice in ESM2G. In the ESM2G historical simulation this behavior results in the establishment of a strong halocline in the subpolar North Atlantic during the early 20th century and an associated cooling which are counter to observations in that region. The Atlantic meridional overturning declines comparably in all three models.

56. Winton, Michael, Stephen M Griffies, Bonita L Samuels, Jorge L Sarmiento, and T L Frölicher, April 2013: Connecting Changing Ocean Circulation with Changing Climate. Journal of Climate, 26(7), DOI:10.1175/JCLI-D-12-00296.1 .
    [ Abstract ]

    The influence of changing ocean currents on climate change is evaluated by comparing an earth system model’s response to increased CO2 with and without an ocean circulation response. Inhibiting the ocean circulation response, by specifying a seasonally-varying preindustrial climatology of currents, has a much larger influence on the heat storage pattern than on the carbon storage pattern. The heat storage pattern without circulation changes resembles carbon storage (either with or without circulation changes) more than it resembles the heat storage when currents are allowed to respond. This is shown to be due to the larger magnitude of the redistribution transport – the change in transport due to circulation anomalies acting on control climate gradients – for heat than for carbon. The net ocean heat and carbon uptake are slightly reduced when currents are allowed to respond. Hence, ocean circulation changes potentially act to warm the surface climate. However, the impact of the reduced carbon uptake on radiative forcing is estimated to be small while the redistribution heat transport shifts ocean heat uptake from low to high latitudes increasing its cooling power. Consequently, global surface warming is significantly reduced by circulation changes. Circulation changes also shift the pattern of warming from broad northern hemisphere amplification to a more structured pattern with reduced warming at subpolar latitudes in both hemispheres and enhanced warming near the equator.

57. Bates, M L., Stephen M Griffies, and M H England, December 2012: A dynamic, embedded Lagrangian model for ocean climate models, Part I: Theory and implementation. Ocean Modelling, 59-60, DOI:10.1016/j.ocemod.2012.05.004 .
    [ Abstract ]

    A framework for embedding a Lagrangian model within ocean climate models that employ horizontal Eulerian grids is presented. The embedded Lagrangian model can be used to explicitly represent processes that are at the subgrid scale to the Eulerian model. The framework is applied to open ocean deep convection and gravity driven downslope flows, both of which are subgridscale in the present generation of level coordinate ocean climate models. In order to apply the embedded Lagrangian framework to these processes, it is necessary to partition the mass and momentum of the model into an Eulerian system and a Lagrangian system. This partitioning allows the Lagrangian model to transport seawater using a more appropriate set of dynamics. A number of schemes suitable for implementation in the embedded Lagrangian model are derived. Two dynamically passive schemes are derived that emulate existing parameterisations and two dynamically active schemes are also derived that evolve Lagrangian parcels of water (termed ”blobs”) according to a set of physical equations. Some details of the implementation into the Geophysical Fluid Dynamics Modular Ocean Model are also given. Finally, results are presented that show that the dynamically passive schemes are able to emulate their Eulerian counterparts to within roundoff error in idealised test cases.

58. Bates, M L., Stephen M Griffies, and M H England, December 2012: A dynamic, embedded Lagrangian model for ocean climate models, Part II: Idealised overflow tests. Ocean Modelling, 59-60, DOI:10.1016/j.ocemod.2012.08.003 .
    [ Abstract ]

    Dense gravity current overflows occur in several regions throughout the world and are an important process in the meridional overturning circulation. Overflows are poorly represented in coarse resolution level coordinate ocean climate models. Here, the embedded Lagrangian model formulated in the companion paper of Bates et al., 2012 is used in two idealised test cases to examine the effect on the representation of dense gravity driven plumes, as well as the effect on the circulation of the bulk ocean in the Eulerian model. The results are compared with simulations with no parameterisation for overflows, as well as simulations that use traditional hydrostatic overflow schemes. The use of Lagrangian “blobs” is shown to improve three key characteristics that are poorly represented in coarse resolution level coordinate models: (1) the depth of the plume, (2) the along slope velocity of the plume, and (3) the response of the bulk ocean to the bottom boundary layer. These improvements are associated with the more appropriate set of dynamics satisfied by the blobs, leading to a more physically sound representation. Experiments are also conducted to examine sensitivity to blob parameters. The blob parameters are examined over a large parameter space.

59. Delworth, Thomas L., Anthony Rosati, Whit G Anderson, Alistair Adcroft, V Balaji, Rusty Benson, Keith W Dixon, Stephen M Griffies, Hyun-Chul Lee, Ronald C Pacanowski, Gabriel A Vecchi, Andrew T Wittenberg, Fanrong Zeng, and Rong Zhang, April 2012: Simulated climate and climate change in the GFDL CM2.5 high-resolution coupled climate model. Journal of Climate, 25(8), DOI:10.1175/JCLI-D-11-00316.1 .
    [ Abstract ]

    We present results for simulated climate and climate change from a newly developed high-resolution global climate model (GFDL CM2.5). The GFDL CM2.5 model has an atmospheric resolution of approximately 50 Km in the horizontal, with 32 vertical levels. The horizontal resolution in the ocean ranges from 28 Km in the tropics to 8 Km at high latitudes, with 50 vertical levels. This resolution allows the explicit simulation of some mesoscale eddies in the ocean, particularly at lower latitudes. We present analyses based on the output of a 280 year control simulation; we also present results based on a 140 year simulation in which atmospheric CO2 increases at 1% per year until doubling after 70 years. Results are compared to the GFDL CM2.1 climate model, which has somewhat similar physics but coarser resolution. The simulated climate in CM2.5 shows marked improvement over many regions, especially the tropics, including a reduction in the double ITCZ and an improved simulation of ENSO. Regional precipitation features are much improved. The Indian monsoon and Amazonian rainfall are also substantially more realistic in CM2.5. The response of CM2.5 to a doubling of atmospheric CO2 has many features in common with CM2.1, with some notable differences. For example, rainfall changes over the Mediterranean appear to be tightly linked to topography in CM2.5, in contrast to CM2.1 where the response is more spatially homogeneous. In addition, in CM2.5 the near-surface ocean warms substantially in the high latitudes of the Southern Ocean, in contrast to simulations using CM2.1.

60. Dunne, John P., Jasmin G John, Alistair Adcroft, Stephen M Griffies, Robert Hallberg, Elena Shevliakova, Ronald J Stouffer, William F Cooke, Krista A Dunne, Matthew J Harrison, John P Krasting, Sergey Malyshev, P C D Milly, Peter Phillipps, Lori T Sentman, Bonita L Samuels, Michael J Spelman, Michael Winton, Andrew T Wittenberg, and Niki Zadeh, October 2012: GFDL's ESM2 global coupled climate-carbon Earth System Models Part I: Physical formulation and baseline simulation characteristics. Journal of Climate, 25(19), DOI:10.1175/JCLI-D-11-00560.1 .
    [ Abstract ]

    We describe the physical climate formulation and simulation characteristics of two new global coupled carbon-climate Earth System Models, ESM2M and ESM2G. These models demonstrate similar climate fidelity as the Geophysical Fluid Dynamics Laboratory’s previous CM2.1 climate model while incorporating explicit and consistent carbon dynamics. The two models differ exclusively in the physical ocean component; ESM2M uses Modular Ocean Model version 4.1 with vertical pressure layers while ESM2G uses Generalized Ocean Layer Dynamics with a bulk mixed layer and interior isopycnal layers. Differences in the ocean mean state include the thermocline depth being relatively deep in ESM2M and relatively shallow in ESM2G compared to observations. The crucial role of ocean dynamics on climate variability is highlighted in the El Niño-Southern Oscillation being overly strong in ESM2M and overly weak ESM2G relative to observations. Thus, while ESM2G might better represent climate changes relating to: total heat content variability given its lack of long term drift, gyre circulation and ventilation in the North Pacific, tropical Atlantic and Indian Oceans, and depth structure in the overturning and abyssal flows, ESM2M might better represent climate changes relating to: surface circulation given its superior surface temperature, salinity and height patterns, tropical Pacific circulation and variability, and Southern Ocean dynamics. Our overall assessment is that neither model is fundamentally superior to the other, and that both models achieve sufficient fidelity to allow meaningful climate and earth system modeling applications. This affords us the ability to assess the role of ocean configuration on earth system interactions in the context of two state-of-the-art coupled carbon-climate models.

61. Griffies, Stephen M., and R J Greatbatch, July 2012: Physical processes that impact the evolution of global mean sea level in ocean climate models. Ocean Modelling, 51, DOI:10.1016/j.ocemod.2012.04.003 .
    [ Abstract ]

    This paper develops an analysis framework to identify how physical processes, as represented in ocean climate models, impact the evolution of global mean sea level. The formulation utilizes the coarse grained equations appropriate for an ocean model, and starts from the vertically integrated mass conservation equation in its Lagrangian form. Global integration of this kinematic equation results in an evolution equation for global mean sea level that depends on two physical processes: boundary fluxes of mass and the non-Boussinesq steric effect. The non-Boussinesq steric effect itself contains contributions from boundary fluxes of buoyancy; interior buoyancy changes associated with parameterized subgrid scale processes; and motion across pressure surfaces. The non-Boussinesq steric effect can be diagnosed in either volume conserving Boussinesq or mass conserving non-Boussinesq ocean circulation models, with differences found to be negligible. We find that surface heating is the dominant term affecting sea level arising from buoyancy fluxes, contributing to a net positive tendency to global mean sea level, largely due to low latitude heating and because the thermal expansion coefficient is much larger in the tropics than high latitudes. Subgrid scale effects from parameterized quasi-Stokes transport, vertical diffusion, cabbeling, and thermobaricity are also found to be significant, each resulting in a reduction of global mean sea level. Sea level rise through low latitude heating is largely compensated by a sea level drop from poleward eddy heat transport and ocean mixing. Spatial variations in the thermal expansion coefficient provide an essential modulation of how physical effects from mixing and eddy induced advective transport impact global mean sea level.

62. Ilicak, M, Alistair Adcroft, Stephen M Griffies, and Robert Hallberg, February 2012: Spurious dianeutral mixing and the role of momentum closure. Ocean Modelling, 45-46, DOI:10.1016/j.ocemod.2011.10.003 .
    [ Abstract ]

    This paper examines spurious dianeutral transport within a suite of ocean models (GOLD, MITgcm, MOM, and ROMS). We quantify such transport through a global diagnostic that computes the reference potential energy, whose evolution arises solely through transport between density classes. Previous studies have focused on the importance of accurate tracer advection schemes in reducing the spurious transport and closure. The present study highlights complementary issues associated with momentum transport. Spurious dianeutral transport is shown to be directly proportional to the lateral grid Reynolds number (ReΔ), with such transport significantly reduced when ReΔ<10. Simulations with the isopycnal model GOLD provide a benchmark for the smallest level of spurious dianeutral transport realizable in our model suite. For idealized simulations with a linear equation of state, GOLD exhibits identically zero spurious dianeutral mixing, and thus maintains a constant reference potential energy when all physical mixing processes are omitted. Amongst the non-isopycnal models tested in idealized simulations, ROMS generally produces smaller spurious dianeutral mixing than MITgcm or MOM, since ROMS makes use of a higher order upwind-biased scheme for momentum transport that enforces a small ReΔ. In contrast, MITgcm and MOM both employ unbiased (centered) discretizations of momentum transport, and therefore rely on lateral friction operators to control the grid Reynolds number. We find that a lateral shear-dependent Smagorinsky viscosity provides an effective means to locally reduce ReΔ, and thus to reduce spurious dianeutral transport in MITgcm and MOM. In addition to four idealized simulations, we quantify spurious dianeutral transport in realistic global ocean climate simulations using GOLD and MOM with a realistic equation of state for seawater, both with and without mesoscale eddies in the resolved flow field. The GOLD simulations have detectable levels of spurious cabbeling from along isopycnal advective truncation errors. Significantly larger spurious dianeutral transport arises in a non-eddying MOM simulation. In an eddying MOM simulation, spurious dianeutral transport is larger still but is reduced by increasing momentum friction.

63. Lorbacher, K, S J Marsland, J A Church, Stephen M Griffies, and D Stammer, June 2012: Rapid barotropic sea-level rise from ice-sheet melting. Journal of Geophysical Research, 117, C06003, DOI:10.1029/2011JC007733 .
    [ Abstract ]

    Sea-level rise associated with idealized Greenland and Antarctic ice-sheet melting events is examined using a global coupled ocean sea-ice model that has a free surface formulation and thus can simulate fast barotropic motions. The perturbation experiments follow the Coordinated Ocean-ice Reference Experiment (CORE) version III. All regions of the global ocean experience a sea-level rise within 7-8 days of the initialization of a polar meltwater input of 0.1 Sv (1 Sv = 106 m3 s−1). The fast adjustment contrasts sharply with the slower adjustment associated with the smaller steric sea-level evolution that is also connected with melt events. The global mean sea-level rises by 9 mm yr−1 when this forcing is applied either from Greenland or Antarctica. Nevertheless, horizontal inter-basin gradients in sea level remain. For climate adaption in low-lying coastal and island regions, it is critical that the barotropic sea-level signal associated with melt events is taken into consideration, as it leads to a fast sea-level rise from melting ice-sheets for the bulk of the global ocean. A linear relation between sea-level rise and global meltwater input is further supported by experiments in which idealized melting occurs only in a region east or west of the Antarctic Peninsula, and when melting rates are varied between 0.01 Sv and 1.0 Sv. The results indicate that in ocean models that do not explicitly represent the barotropic signal, the barotropic component of sea-level rise can be added off-line to the simulated steric signal.

64. Donner, Leo J., Bruce Wyman, Richard S Hemler, Larry W Horowitz, Yi Ming, Ming Zhao, J-C Golaz, Paul Ginoux, Shian-Jiann Lin, M Daniel Schwarzkopf, John Austin, G Alaka, William F Cooke, Thomas L Delworth, Stuart Freidenreich, C Tony Gordon, Stephen M Griffies, Isaac M Held, William J Hurlin, Stephen A Klein, Thomas R Knutson, Amy R Langenhorst, Hyun-Chul Lee, Yanluan Lin, B I Magi, Sergey Malyshev, P C D Milly, Vaishali Naik, Mary Jo Nath, R Pincus, Jeff J Ploshay, V Ramaswamy, Charles J Seman, Elena Shevliakova, Joseph J Sirutis, William F Stern, Ronald J Stouffer, R John Wilson, Michael Winton, Andrew T Wittenberg, and Fanrong Zeng, July 2011: The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component AM3 of the GFDL Global Coupled Model CM3. Journal of Climate, 24(13), DOI:10.1175/2011JCLI3955.1 .
    [ Abstract ]

    The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology. Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2. The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.

65. Downes, S M., Anand Gnanadesikan, Stephen M Griffies, and Jorge L Sarmiento, September 2011: Water mass exchange in the Southern Ocean in coupled climate models. Journal of Physical Oceanography, 41(9), DOI:10.1175/2011JPO4586.1 .
    [ Abstract ]

    We estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the ECCO model based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean, but not in the Indian and Pacific where the model based estimates are much smaller. Associated with this, in the models about half the southward flowing deep water is converted into lighter waters and half to denser bottom waters, whereas the observationally-based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwater fluxes. Given the small vertical diffusivity specified in the models in this region, we conclude that other processes such as cabbeling and thermobaricity must be playing an important role in water mass transformation. Finally, in the models, the largest contribution of the surface buoyancy fluxes in the Southern Ocean is to convert Upper Circumpolar Deep Water (UCDW) and Antarctic Intermediate Water (AAIW) into lighter Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW).

66. Fox-Kemper, B, G Danabasoglu, R Ferrari, Stephen M Griffies, Robert Hallberg, M M Holland, M E Maltrud, S L Peacock, and Bonita L Samuels, July 2011: Parameterization of mixed layer eddies. III: Implementation and impact in global ocean climate simulations. Ocean Modelling, 39(1-2), DOI:10.1016/j.ocemod.2010.09.002 .
    [ Abstract ]

    A parameterization for the restratification by finite-amplitude, submesoscale, mixed layer eddies, formulated as an overturning streamfunction, has been recently proposed to approximate eddy fluxes of density and other tracers. Here, the technicalities of implementing the parameterization in the coarse-resolution ocean component of global climate models are made explicit, and the primary impacts on model solutions of implementing the parameterization are discussed. Three global ocean general circulation models including this parameterization are contrasted with control simulations lacking the parameterization. The MLE parameterization behaves as expected and fairly consistently in models differing in discretization, boundary layer mixing, resolution, and other parameterizations. The primary impact of the parameterization is a shoaling of the mixed layer, with the largest effect in polar winter regions. Secondary impacts include strengthening the Atlantic meridional overturning while reducing its variability, reducing CFC and tracer ventilation, modest changes to sea surface temperature and air-sea fluxes, and an apparent reduction of sea ice basal melting.

67. Galbraith, E D., E Y Kwon, Anand Gnanadesikan, K B Rodgers, Stephen M Griffies, D Bianchi, Jorge L Sarmiento, John P Dunne, J Simeon, Richard D Slater, Andrew T Wittenberg, and Isaac M Held, August 2011: Climate Variability and Radiocarbon in the CM2Mc Earth System Model. Journal of Climate, 24(16), DOI:10.1175/2011JCLI3919.1 .
    [ Abstract ]

    The distribution of radiocarbon (¹⁴C) in the ocean and atmosphere has fluctuated on timescales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute ¹⁴C within the Earth system on inter-annual to centennial timescales. The model, CM2Mc, is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory's CM2M model, uses no flux adjustments, and incorporates a simple prognostic ocean biogeochemistry model including ¹⁴C. The atmospheric ¹⁴C and radiative boundary conditions are held constant, so that the oceanic distribution of ¹⁴C is only a function of internal climate variability. The simulation displays previously-described relationships between tropical sea surface ¹⁴C and the model-equivalents of the El Niño Southern Oscillation and Indonesian Throughflow. Sea surface ¹⁴C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability, and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974–1976. Interannual variability in the air-sea balance of ¹⁴C is dominated by exchange within the belt of intense Southern Westerly winds, rather than at the convective locations where the surface ¹⁴C is most variable. Despite significant interannual variability, the simulated impact on air-sea exchange is an order of magnitude smaller than the recorded atmospheric ¹⁴C variability of the past millennium. This result partly reflects the importance of variability in the production rate of ¹⁴C in determining atmospheric ¹⁴C, but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.

68. Griffies, Stephen M., Michael Winton, Leo J Donner, Larry W Horowitz, S M Downes, Riccardo Farneti, Anand Gnanadesikan, William J Hurlin, Hyun-Chul Lee, Zhi Liang, J B Palter, Bonita L Samuels, Andrew T Wittenberg, Bruce Wyman, Jianjun Yin, and Niki Zadeh, July 2011: The GFDL CM3 Coupled Climate Model: Characteristics of the ocean and sea ice simulations. Journal of Climate, 24(13), DOI:10.1175/2011JCLI3964.1 .
    [ Abstract ]

    This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The climate model, known as CM3, is formulated with effectively the same ocean and sea ice components as the earlier GFDL climate model, CM2.1, yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. We anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than CM2.1, which adversely impacts the interior biases.

69. Griffies, Stephen M., and G Danabasoglu, May 2011: Physical ocean fields in CMIP5. Clivar Exchanges, 16(2), 32-34.
    [ PDF ]
70. Griffies, Stephen M., July 2011: Preface to the Ocean Modelling special issue on ocean eddies. Ocean Modelling, 39(1-2), 1.
    [ PDF ]
71. Herzfeld, M, M Schmidt, Stephen M Griffies, and Zhi Liang, February 2011: Realistic test cases for limited area ocean modelling. Ocean Modelling, 37(1-2), DOI:10.1016/j.ocemod.2010.12.008 .
    [ Abstract ]

    The applicability of Modular Ocean Model version 4 (MOM4p1) as a code base to study regional physical oceanographic phenomena is presented, highlighting features recently implemented for use in limited area domains. Central to the successful operation of limited area model applications are the inclusion of a comprehensive suite of open boundary conditions, turbulence closure and vertical discretization. The open boundary problem, in particular, is considered and we present the open boundary condition implementation and performance in limited area model configurations corresponding to three realistic test cases. These tests represent typical configurations the physical oceanographer may encounter, and consist of (1) a coastal shelf model application where a two-tiered model configuration is used for down-scaling from a coarse grid model to supply sufficiently resolved boundary values for active cross-shelf open boundaries of a regional model; (2) tidal response of a gulf with one open boundary across the mouth of the gulf; (3) response of a coastal region to the passage of a tropical cyclone, where the open boundaries behave in primarily a passive capacity. Although the code base used in the test cases is MOM4p1, emphasis is placed on general features of the tests that are necessary for the scientific and operational use of any limited area model, hence key findings may be applied to limited area models in general.

72. Stock, Charles A., Thomas L Delworth, John P Dunne, Stephen M Griffies, R Rykaczewski, Jorge L Sarmiento, Ronald J Stouffer, and Gabriel A Vecchi, et al., January 2011: On the use of IPCC-class models to assess the impact of climate on Living Marine Resources. Progress in Oceanography, 88(1-4), DOI:10.1016/j.pocean.2010.09.001 .
    [ Abstract ]

    The study of climate impacts on Living Marine Resources (LMRs) has increased rapidly in recent years with the availability of climate model simulations contributed to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Collaboration between climate and LMR scientists and shared understanding of critical challenges for such applications are essential for developing robust projections of climate impacts on LMRs. This paper assesses present approaches for generating projections of climate impacts on LMRs using IPCC-class climate models, recommends practices that should be followed for these applications, and identifies priority developments that could improve current projections. Understanding of the climate system and its representation within climate models has progressed to a point where many climate model outputs can now be used effectively to make LMR projections. However, uncertainty in climate model projections (particularly biases and inter-model spread at regional to local scales), coarse climate model resolution, and the uncertainty and potential complexity of the mechanisms underlying the response of LMRs to climate limit the robustness and precision of LMR projections. A variety of techniques including the analysis of multi-model ensembles, bias corrections, and statistical and dynamical downscaling can ameliorate some limitations, though the assumptions underlying these approaches and the sensitivity of results to their application must be assessed for each application. Developments in LMR science that could improve current projections of climate impacts on LMRs include improved understanding of the multi-scale mechanisms that link climate and LMRs and better representations of these mechanisms within more holistic LMR models. These developments require a strong baseline of field and laboratory observations including long time-series and measurements over the broad range of spatial and temporal scales over which LMRs and climate interact. Priority developments for IPCC-class climate models include improved model accuracy (particularly at regional and local scales), inter-annual to decadal-scale predictions, and the continued development of earth system models capable of simulating the evolution of both the physical climate system and biosphere. Efforts to address these issues should occur in parallel and be informed by the continued application of existing climate and LMR models.

73. Yin, Jianjun, J E Overland, Stephen M Griffies, A Hu, J L Russell, and Ronald J Stouffer, August 2011: Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica. Nature Geoscience, 4(8), DOI:10.1038/ngeo1189 .
    [ Abstract ]

    The observed acceleration of outlet glaciers and ice flows in Greenland and Antarctica is closely linked to ocean warming, especially in the subsurface layer. Accurate projections of ice-sheet dynamics and global sea-level rise therefore require information of future ocean warming in the vicinity of the large ice sheets. Here we use a set of 19 state-of-the-art climate models to quantify this ocean warming in the next two centuries. We find that in response to a mid-range increase in atmospheric greenhouse-gas concentrations, the subsurface oceans surrounding the two polar ice sheets at depths of 200–500 m warm substantially compared with the observed changes thus far6, 7, 8. Model projections suggest that over the course of the twenty-first century, the maximum ocean warming around Greenland will be almost double the global mean, with a magnitude of 1.7–2.0 °C. By contrast, ocean warming around Antarctica will be only about half as large as global mean warming, with a magnitude of 0.5–0.6 °C. A more detailed evaluation indicates that ocean warming is controlled by different mechanisms around Greenland and Antarctica. We conclude that projected subsurface ocean warming could drive significant increases in ice-mass loss, and heighten the risk of future large sea-level rise.

74. Farneti, Riccardo, Thomas L Delworth, Anthony Rosati, Stephen M Griffies, and Fanrong Zeng, July 2010: The role of mesoscale eddies in the rectification of the Southern Ocean response to climate change. Journal of Physical Oceanography, 40(7), DOI:10.1175/2010JPO4353.1 .
    [ Abstract ]

    Simulations from a fine-resolution global coupled model, the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.4 (CM2.4), are presented, and the results are compared with a coarse version of the same coupled model, CM2.1, under idealized climate change scenarios. A particular focus is given to the dynamical response of the Southern Ocean and the role played by the eddies—parameterized or permitted— in setting the residual circulation and meridional density structure. Compared to the case in which eddies are parameterized and consistent with recent observational and idealized modeling studies, the eddy-permitting integrations of CM2.4 show that eddy activity is greatly energized with increasing mechanical and buoyancy forcings, buffering the ocean to atmospheric changes, and the magnitude of the residual oceanic circulation response is thus greatly reduced. Although compensation is far from being perfect, changes in poleward eddy fluxes partially compensate for the enhanced equatorward Ekman transport, leading to weak modifications in local isopycnal slopes, transport by the Antarctic Circumpolar Current, and overturning circulation. Since the presence of active ocean eddy dynamics buffers the oceanic response to atmospheric changes, the associated atmospheric response to those reduced ocean changes is also weakened. Further, it is hypothesized that present numerical approaches for the parameterization of eddy-induced transports could be too restrictive and prevent coarse-resolution models from faithfully representing the eddy response to variability and change in the forcing fields.

75. Ferrari, R, Stephen M Griffies, A J George Nurser, and Geoffrey K Vallis, April 2010: A boundary-value problem for the parameterized mesoscale eddy transport. Ocean Modelling, 32(3-4), DOI:10.1016/j.ocemod.2010.01.004 .
    [ Abstract ]

    We present a physically and numerically motivated boundary-value problem for each vertical ocean column, whose solution yields a parameterized mesoscale eddy-induced transport streamfunction. The new streamfunction is a nonlocal function of the properties of the fluid column. It is constructed to have a low baroclinic mode vertical structure and to smoothly transition through regions of weak stratification such as boundary layers or mode waters. It requires no matching conditions or regularization in unstratified regions; it satisfies boundary conditions of zero transport at the ocean surface and bottom; and it provides a sink of available potential energy for each vertical seawater column, but not necessarily at each location within the column. Numerical implementation of the methodology requires the solution of a one-dimensional tridiagonal problem for each vertical column. To illustrate the approach, we present an analytical example based on the nonlinear Eady problem and two numerical simulations.

76. Griffies, Stephen M., Alistair Adcroft, Anand Gnanadesikan, Robert Hallberg, Matthew J Harrison, Sonya Legg, C M Little, M Nikurashin, A Pirani, Bonita L Samuels, J R Toggweiler, and Geoffrey K Vallis, et al., September 2010: Problems and prospects in large-scale ocean circulation models In Ocean Obs '09, 21-25 September, Venice, Italy, ESA Special Publication, DOI:10.5270/OceanObs09.cwp.38 .
    [ Abstract ]

    We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements of observational measures rendering information to help evaluate simulations and to guide development priorities. http://www.oceanobs09.net/blog/?p=88

77. Hurrell, J W., Thomas L Delworth, Stephen M Griffies, and Anthony Rosati, et al., September 2010: Decadal Climate Prediction: Opportunities and Challenges, 2010 In OceanObs’09: Sustained Ocean Observations and Information for Society, Vol. 2, ESA Publication, DOI:10.5270/OceanObs09.cwp.45 .
78. Kopp, R E., J X Mitrovica, Stephen M Griffies, Jianjun Yin, C C Hay, and Ronald J Stouffer, et al., December 2010: The impact of Greenland melt on local sea levels: a partially coupled analysis of dynamic and static equilibrium effects in idealized water-hosing experiments. Climatic Change, 103(3-4), DOI:10.1007/s10584-010-9935-1 .
    [ Abstract ]

    Local sea level can deviate from mean global sea level because of both dynamic sea level (DSL) effects, resulting from oceanic and atmospheric circulation and temperature and salinity distributions, and changes in the static equilibrium (SE) sea level configuration, produced by the gravitational, elastic, and rotational effects of mass redistribution. Both effects will contribute to future sea level change. To compare their magnitude, we simulated the effects of Greenland Ice Sheet (GIS) melt by conducting idealized North Atlantic “water-hosing” experiments in a climate model unidirectionally coupled to a SE sea level model. At current rates of GIS melt, we find that geographic SE patterns should be challenging but possible to detect above dynamic variability. At higher melt rates, we find that DSL trends are strongest in the western North Atlantic, while SE effects will dominate in most of the ocean when melt exceeds ~20 cm equivalent sea level.

79. Rienecker, M M., Stephen M Griffies, and Anthony Rosati, et al., September 2010: Synthesis and Assimilation Systems: Essential Adjuncts to the Global Ocean Observing System In OceanObs’09: Sustained Ocean Observations and Information for Society, Vol. 2, ESA Publication, DOI:doi:10.5270/OceanObs09.pp.31 .
80. Yin, Jianjun, Ronald J Stouffer, Michael J Spelman, and Stephen M Griffies, January 2010: Evaluating the uncertainty induced by the virtual salt flux assumption in climate situations and future projections. Journal of Climate, 23(1), DOI:10.1175/2009JCLI3084.1 .
    [ Abstract ]

    The unphysical virtual salt flux (VSF) formulation widely used in the ocean component of climate models has the potential to cause systematic and significant biases in modeling the climate system and projecting its future evolution. Here a freshwater flux (FWF) and a virtual salt flux version of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1) are used to evaluate and quantify the uncertainties induced by the VSF formulation. Both unforced and forced runs with the two model versions are performed and compared in detail. It is found that the differences between the two versions are generally small or statistically insignificant in the unforced control runs and in the runs with a small external forcing. In response to a large external forcing, however, some biases in the VSF version become significant, especially the responses of regional salinity and global sea level. However, many fundamental aspects of the responses differ only quantitatively between the two versions. An unexpected result is the distinctly different ENSO responses. Under a strong external freshwater forcing, the great enhancement of the ENSO variability simulated by the FWF version does not occur in the VSF version and is caused by the overexpansion of the top model layer. In summary, the principle assumption behind using virtual salt flux is not seriously violated and the VSF model has the ability to simulate the current climate and project near-term climate evolution. For some special studies such as a large hosing experiment, however, both the VSF formulation and the use of the FWF in the geopotential coordinate ocean model could have some deficiencies and one should be cautious to avoid them.

81. Yin, Jianjun, Stephen M Griffies, and Ronald J Stouffer, September 2010: Spatial variability of sea-level rise in 21st century projections. Journal of Climate, 23(17), DOI:10.1175/2010JCLI3533.1 .
    [ Abstract ]

    A set of state-of-the-science climate models are used to investigate global sea level rise (SLR) patterns induced by ocean dynamics in twenty-first-century climate projections. The identified robust features include bipolar and bihemisphere seesaws in the basin-wide SLR, dipole patterns in the North Atlantic and North Pacific, and a beltlike pattern in the Southern Ocean. The physical and dynamical mechanisms that cause these patterns are investigated in detail using version 2.1 of the Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model (CM2.1). Under the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario, the steric sea level changes relative to the global mean (the local part) in different ocean basins are attributed to differential heating and salinity changes of various ocean layers and associated physical processes. As a result of these changes, water tends to move from the ocean interior to continental shelves. In the North Atlantic, sea level rises north of the Gulf Stream but falls to the south. The dipole pattern is induced by a weakening of the meridional overturning circulation. This weakening leads to a local steric SLR east of North America, which drives more waters toward the shelf, directly impacting northeastern North America. An opposite dipole occurs in the North Pacific. The dynamic SLR east of Japan is linked to a strong steric effect in the upper ocean and a poleward expansion of the subtropical gyre. In the Southern Ocean, the beltlike pattern is dominated by the baroclinic process during the twenty-first century, while the barotropic response of sea level to wind stress anomalies is significantly delayed.

82. Griffies, Stephen M., Robert Hallberg, A Pirani, Bonita L Samuels, and Michael Winton, et al., January 2009: Coordinated ocean-ice reference experiments (COREs). Ocean Modelling, 26(1-2), DOI:10.1016/j.ocemod.2008.08.007 .
    [ Abstract ]

    Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations. The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development.

83. Griffies, Stephen M., Alistair Adcroft, V Balaji, Robert Hallberg, Sonya Legg, T Martin, and A Pirani, et al., February 2009: Sampling Physical Ocean Field in WCRP CMIP5 Simulations: CLIVAR Working Group on Ocean Model Development (WGOMD) Committee on CMIP5 Ocean Model Output, International CLIVAR Project Office, CLIVAR Publication Series No. 137, 56pp.
    [ PDF ]
84. Griffies, Stephen M., December 2009: Science of ocean climate models In Encyclopedia of Ocean Sciences, 2nd edition, Elsevier, DOI:10.1016/B978-012374473-9.00714-1 .
85. Legg, Sonya, Tal Ezer, Stephen M Griffies, Robert Hallberg, and L Jackson, et al., May 2009: Improving oceanic overflow representation in climate models: The gravity current entrainment climate process team. Bulletin of the American Meteorological Society, 90(5), DOI:10.1175/2008BAMS2667.1 .
    [ Abstract ]

    Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.

86. Griffies, Stephen M., and Alistair Adcroft, 2008: Formulating the equations of ocean models In Ocean Modeling in an Eddying Regime, Geophysical Monograph 177, M. W. Hecht, and H. Hasumi, eds., Washington, DC, American Geophysical Union, 281-318.
    [ Abstract PDF ]

    We formulate mathematical equations describing the thermo-hydrodynamics of the ocean and introduce certain numerical methods employed by models used for ocean simulations.

87. Griffies, Stephen M., H Banks, and A Pirani, 2008: Furthering the science of ocean climate modelling. Clivar Exchanges, 13(1), 281-318.
    [ PDF ]
88. Pirani, A, Stephen M Griffies, and H Banks, 2008: Report from the CLIVAR Working Group on ocean model development (WGOMD). Clivar Exchanges, 13(1), 30-32.
    [ PDF ]
89. Gnanadesikan, Anand, Stephen M Griffies, and Bonita L Samuels, 2007: Effects in a climate model of slope tapering in neutral physics schemes. Ocean Modelling, 16(1-2), DOI:10.1016/j.ocemod.2006.06.004 .
    [ Abstract ]

    In many global ocean climate models, mesoscale eddies are parameterized as along isopycnal diffusion and eddy-induced advection (or equivalently skew-diffusion). The eddy-induced advection flattens isopycnals and acts as a sink of available potential energy, whereas the isopycnal diffusion mixes tracers along neutral directions. While much effort has gone into estimating diffusivities associated with this closure, less attention has been paid to the details of how this closure (which tries to flatten isopycnals) interacts with the mixed layer (in which vertical mixing tries to drive the isopycnals vertical). In order to maintain numerical stability, models often stipulate a maximum slope S_max which in combination with the thickness diffusivity A_gm defines a maximum eddy-induced advective transport A_gm*S_max. In this paper, we examine the impact of changing S_max within the GFDL global coupled climate model. We show that this parameter produces significant changes in wintertime mixed layer depth, with implications for wintertime temperatures in key regions, the distribution of precipitation, and the vertical structure of heat uptake. Smaller changes are seen in details of ventilation and currents, and even smaller changes as regards the overall hydrography. The results suggest that not only the value of the coefficient, but the details of the tapering scheme, need to be considered when comparing isopycnal mixing schemes in models.

90. Griffies, Stephen M., C Böning, and A M Treguier, 2007: Design considerations for coordinated ocean-ice reference experiments. Flux News, 3, 3-5.
    [ PDF ]
91. Griffies, Stephen M., Matthew J Harrison, Ronald C Pacanowski, and Anthony Rosati, 2007: Ocean modelling with MOM. Clivar Exchanges, 12(3), 3-5, 13.
    [ PDF ]
92. Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, V Balaji, J A Beesley, William F Cooke, Keith W Dixon, John P Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen M Griffies, Richard G Gudgel, Matthew J Harrison, Isaac M Held, Richard S Hemler, Larry W Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, Amy R Langenhorst, Hyun-Chul Lee, Shian-Jiann Lin, Jian Lu, Sergey Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), DOI:10.1175/JCLI3629.1 .
    [ Abstract ]

    The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments. The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic. Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/). Manuscript received 8 December 2004, in final form 18 March 2005

93. Gerdes, R, William J Hurlin, and Stephen M Griffies, 2006: Sensitivity of a global ocean model to increased run-off from Greenland. Ocean Modelling, 12(3-4), DOI:10.1016/j.ocemod.2005.08.003 .
    [ Abstract ]

    We study the reaction of a global ocean–sea ice model to an increase of fresh water input into the northern North Atlantic under different surface boundary conditions, ranging from simple restoring of surface salinity to the use of an energy balance model (EBM) for the atmosphere. The anomalous fresh water flux is distributed around Greenland, reflecting increased melting of the Greenland ice sheet and increasing fresh water export from the Arctic Ocean. Depending on the type of surface boundary condition, the large circulation reacts with a slow-down of overturning and gyre circulations. Restoring of the total or mean surface salinity prevents a large scale redistribution of the salinity field that is apparent under mixed boundary conditions and with the EBM. The control run under mixed boundary conditions exhibits large and unrealistic oscillations of the meridional overturning. Although the reaction to the fresh water flux anomaly is similar to the response with the EBM, mixed boundary conditions must thus be considered unreliable. With the EBM, the waters in the deep western boundary current initially become saltier and a new fresh water mass forms in the north-eastern North Atlantic in response to the fresh water flux anomaly around Greenland. After an accumulation period of several decades duration, this new North East Atlantic Intermediate Water spreads towards the western boundary and opens a new southward pathway at intermediate depths along the western boundary for the fresh waters of high northern latitudes.

94. Gnanadesikan, Anand, Keith W Dixon, Stephen M Griffies, V Balaji, M Barreiro, J A Beesley, William F Cooke, Thomas L Delworth, R Gerdes, Matthew J Harrison, Isaac M Held, William J Hurlin, Hyun-Chul Lee, Zhi Liang, G Nong, Ronald C Pacanowski, Anthony Rosati, J L Russell, Bonita L Samuels, Qian Song, Michael J Spelman, Ronald J Stouffer, C Sweeney, Gabriel A Vecchi, Michael Winton, Andrew T Wittenberg, Fanrong Zeng, Rong Zhang, and John P Dunne, 2006: GFDL's CM2 Global Coupled Climate Models. Part II: The baseline ocean simulation. Journal of Climate, 19(5), DOI:10.1175/JCLI3630.1 .
    [ Abstract ]

    The current generation of coupled climate models run at the Geophysical Fluid Dynamics Laboratory (GFDL) as part of the Climate Change Science Program contains ocean components that differ in almost every respect from those contained in previous generations of GFDL climate models. This paper summarizes the new physical features of the models and examines the simulations that they produce. Of the two new coupled climate model versions 2.1 (CM2.1) and 2.0 (CM2.0), the CM2.1 model represents a major improvement over CM2.0 in most of the major oceanic features examined, with strikingly lower drifts in hydrographic fields such as temperature and salinity, more realistic ventilation of the deep ocean, and currents that are closer to their observed values. Regional analysis of the differences between the models highlights the importance of wind stress in determining the circulation, particularly in the Southern Ocean. At present, major errors in both models are associated with Northern Hemisphere Mode Waters and outflows from overflows, particularly the Mediterranean Sea and Red Sea.

95. Jackett, D R., T J McDougall, R Feistel, D Wright, and Stephen M Griffies, 2006: Algorithms for Density, Potential Temperature, Conservative Temperature, and the Freezing Temperature of Seawater. Journal of Atmospheric and Oceanic Technology, 23(12), DOI:10.1175/JTECH1946.1 .
    [ Abstract ]

    Algorithms are presented for density, potential temperature, conservative temperature, and the freezing temperature of seawater. The algorithms for potential temperature and density (in terms of potential temperature) are updates to routines recently published by McDougall et al., while the algorithms involving conservative temperature and the freezing temperatures of seawater are new. The McDougall et al. algorithms were based on the thermodynamic potential of Feistel and Hagen; the algorithms in this study are all based on the “new extended Gibbs thermodynamic potential of seawater” of Feistel. The algorithm for the computation of density in terms of salinity, pressure, and conservative temperature produces errors in density and in the corresponding thermal expansion coefficient of the same order as errors for the density equation using potential temperature, both being twice as accurate as the International Equation of State when compared with Feistel’s new equation of state. An inverse function relating potential temperature to conservative temperature is also provided. The difference between practical salinity and absolute salinity is discussed, and it is shown that the present practice of essentially ignoring the difference between these two different salinities is unlikely to cause significant errors in ocean models.

96. Griffies, Stephen M., 2005: Some ocean model fundamentals In Ocean Weather Forecasting: An Integrated View of Oceanography, Berlin, Germany, Springer, 19-74.
    [ Abstract ]

    The purpose of these lectures is to present elements of the equations and algorithms used in numerical models of the large-scale ocean circulation. Such models generally integrate the ocean's primitive equations, which are based on Newton's Laws applied to a continuum fluid under hydrostatic balance in a spherical geometry, along with linear irreversible thermodynamics and subgrid scale (SGS) parameterizations. During formulations of both the kinematics and dynamics, we highlight issues related to the use of a generalized vertical coordinate. The vertical coordinate is arguably the most critical element determining how a model is designed and applications to which a model is of use.

97. Griffies, Stephen M., Anand Gnanadesikan, Keith W Dixon, John P Dunne, R Gerdes, Matthew J Harrison, Anthony Rosati, J L Russell, Bonita L Samuels, Michael J Spelman, Michael Winton, and Rong Zhang, September 2005: Formulation of an ocean model for global climate simulations. Ocean Science, 1, 45-79.
    [ Abstract PDF ]

    This paper summarizes the formulation of the ocean component to the Geophysical Fluid Dynamics Laboratory's (GFDL) climate model used for the 4th IPCC Assessment (AR4) of global climate change. In particular, it reviews the numerical schemes and physical parameterizations that make up an ocean climate model and how these schemes are pieced together for use in a state-of-the-art climate model. Features of the model described here include the following: (1) tripolar grid to resolve the Arctic Ocean without polar filtering, (2) partial bottom step representation of topography to better represent topographically influenced advective and wave processes, (3) more accurate equation of state, (4) three-dimensional flux limited tracer advection to reduce overshoots and undershoots, (5) incorporation of regional climatological variability in shortwave penetration, (6) neutral physics parameterization for representation of the pathways of tracer transport, (7) staggered time stepping for tracer conservation and numerical efficiency, (8) anisotropic horizontal viscosities for representation of equatorial currents, (9) parameterization of exchange with marginal seas, (10) incorporation of a free surface that accommodates a dynamic ice model and wave propagation, (11) transport of water across the ocean free surface to eliminate unphysical "virtual tracer flux" methods, (12) parameterization of tidal mixing on continental shelves. We also present preliminary analyses of two particularly important sensitivities isolated during the development process, namely the details of how parameterized subgridscale eddies transport momentum and tracers.

98. Sweeney, C, Anand Gnanadesikan, Stephen M Griffies, Matthew J Harrison, Anthony Rosati, and Bonita L Samuels, June 2005: Impacts of shortwave penetration depth on large-scale ocean circulation and heat transport. Journal of Physical Oceanography, 35(6), 1103-1119.
    [ Abstract PDF ]

    The impact of changes in shortwave radiation penetration depth on the global ocean circulation and heat transport is studied using the GFDL Modular Ocean Model (MOM4) with two independent parameterizations that use ocean color to estimate the penetration depth of shortwave radiation. Ten to eighteen percent increases in the depth of 1% downwelling surface irradiance levels results in an increase in mixed layer depths of 3-20 m in the subtropical and tropical regions with no change at higher latitudes. While 1D models have predicted that sea surface temperatures at the equator would decrease with deeper penetration of solar irradiance, this study shows a warming, resulting in a 10% decrease in the required restoring heat flux needed to maintain climatological sea surface temperatures in the eastern equatorial Atlantic and Pacific Oceans. The decrease in the restoring heat flux is attributed to a slowdown in heat transport (5%) from the Tropics and an increase in the temperature of submixed layer waters being transported into the equatorial regions. Calculations were made using a simple relationship between mixed layer depth and meridional mass transport. When compared with model diagnostics, these calculations suggest that the slowdown in heat transport is primarily due to off-equatorial increases in mixed layer depths. At higher latitudes (5°-40°), higher restoring heat fluxes are needed to maintain sea surface temperatures because of deeper mixed layers and an increase in storage of heat below the mixed layer. This study offers a way to evaluate the changes in irradiance penetration depths in coupled ocean-atmosphere GCMs and the potential effect that large-scale changes in chlorophyll a concentrations will have on ocean circulation.

99. Griffies, Stephen M., 2004: Fundamentals of Ocean Climate Models, Princeton, NJ: Princeton University Press, 518 pp.
    [ Abstract ]

    This book sets forth the physical, mathematical, and numerical foundations of computer models used to understand and predict the global ocean climate system. Aimed at students and researchers of ocean and climate science who seek to understand the physical content of ocean model equations and numerical methods for their solution, it is largely general in formulation and employs modern mathematical techniques. It also highlights certain areas of cutting-edge research. Stephen Griffies presents material that spans a broad spectrum of issues critical for modern ocean climate models. Topics are organized into parts consisting of related chapters, with each part largely self-contained. Early chapters focus on the basic equations arising from classical mechanics and thermodynamics used to rationalize ocean fluid dynamics. These equations are then cast into a form appropriate for numerical models of finite grid resolution. Basic discretization methods are described for commonly used classes of ocean climate models. The book proceeds to focus on the parameterization of phenomena occurring at scales unresolved by the ocean model, which represents a large part of modern oceanographic research. The final part provides a tutorial on the tensor methods that are used throughout the book, in a general and elegant fashion, to formulate the equations.

100. Griffies, Stephen M., Matthew J Harrison, Ronald C Pacanowski, and Anthony Rosati, 2004: A Technical Guide to MOM4, GFDL Ocean Group Technical Report No. 5, Princeton, NJ:: NOAA/Geophysical Fluid Dynamics Laboratory, 342 pp.
     [ Abstract PDF ]

     This manual provides a detailed description of the analytical, numerical, and computational aspects of the MOM4 ocean model.

101. Griffies, Stephen M., 2003: An introduction to linear predictability analysis In Global Climate, Rodó, X., and F. A. Comín, eds., Berlin, Springer-Verlag, 80-101.
102. Griffies, Stephen M., 2003: An introduction to ocean climate modeling In Global Climate, Rodó, X., and F. A. Comín, eds., Berlin, Springer-Verlag, 55-79.
103. Griffies, Stephen M., Ronald C Pacanowski, M Schmidt, and V Balaji, 2001: Tracer conservation with an explicit free surface method for z-coordinate ocean models. Monthly Weather Review, 129(5), 1081-1098.
     [ Abstract PDF ]

     This paper details a free surface method using an explicit time stepping scheme for use in z-coordinate ocean models. One key property that makes the method especially suitable for climate simulations is its very stable numerical time stepping scheme, which allows for the use of a long density time step, as commonly employed with coarse-resolution rigid-lid models. Additionally, the effects of the undulating free surface height are directly incorporated into the baroclinic momentum and tracer equations. The novel issues related to local and global tracer conservation when allowing for the top cell to undulate are the focus of this work. The method presented here is quasi-conservative locally and globally of tracer when the baroclinic and tracer time steps are equal. Important issues relevant for using this method in regional as well as large-scale climate models are discussed and illustrated, and examples of scaling achieved on parallel computers provided.

104. Stocker, T F., Thomas L Delworth, Stephen M Griffies, Isaac M Held, V Ramaswamy, and Brian J Soden, et al., 2001: Physical climate processes and feedbacks In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 418-470.
105. Griffies, Stephen M., 2000: Review of "Ocean Modeling and Parameterization," E. P. Chassignet and J. Verron, eds., 1998, Kluwer Academic Publishers. Bulletin of the American Meteorological Society, 81(3), 591-593.
106. Griffies, Stephen M., C Böning, F O Bryan, E P Chassignet, R Gerdes, H Hasumi, A C Hirst, A M Treguier, and D Webb, 2000: Developments in ocean climate modelling. Ocean Modelling, 2, 123-192.
     [ Abstract PDF ]

     This paper presents some research developments in primitive equation ocean models which could impact the ocean component of realistic global climate models aimed at large-scale, low frequency climate simulations and predictions. It is written primarily to an audience of modellers concerned with the ocean component of climate models, although not necessarily experts in the design and implementation of ocean model algorithms.

107. Griffies, Stephen M., and Robert Hallberg, 2000: Biharmonic friction with a Smagorinsky-like viscosity for use in large-scale eddy-permitting ocean models. Monthly Weather Review, 128(8), 2935-2946.
     [ Abstract PDF ]

     This paper discusses a numerical closure, motivated from the ideas of Smagorinsky, for use with a biharmonic operator. The result is a highly scale-selective, state-dependent friction operator for use in eddy-permitting geophysical fluid models. This friction should prove most useful for large-scale ocean models in which there are multiple regimes of geostrophic turbulence. Examples are provided from primitive equation geopotential and isopycnal-coordinate ocean models.

108. Griffies, Stephen M., Ronald C Pacanowski, and Robert Hallberg, 2000: Spurious diapycnal mixing associated with advection in a z-coordinate ocean model. Monthly Weather Review, 128(3), 538-564.
     [ Abstract PDF ]

     This paper discusses spurious diapycnal mixing associated with the transport of density in a z-coordinate ocean model. A general method, based on the work of Winters and collaborators, is employed for empirically diagnosing an effective diapycnal diffusivity corresponding to any numerical transport process. This method is then used to quantify the spurious mixing engendered by various numerical representations of advection. Both coarse and fine resolution examples are provided that illustrate the importance of adequately resolving the admitted scales of motion in order to maintain a small amount of mixing consistent with that measured within the ocean's pycnocline. Such resolution depends on details of the advection scheme, momentum and tracer dissipation, and grid resolution. Vertical transport processes, such as convective adjustment, act as yet another means to increase the spurious mixing introduced by dispersive errors from numerical advective fluxes.

109. Pacanowski, Ronald C., and Stephen M Griffies, 1999: The MOM3 Manual, GFDL Ocean Group Technical Report No. 4, Princeton, NJ: NOAA/Geophysical Fluid Dynamics Laboratory, 680 pp.
110. Schneider, T, and Stephen M Griffies, 1999: A conceptual framework for predictability studies. Journal of Climate, 12(10), 3133-3155.
     [ Abstract PDF ]

     A conceptual framework is presented for a unified treatment of issues arising in a variety of predictability studies. The predictive power (PP), a predictability measure based on information-theoretical principles, lies at the center of the framework. The PP is invarient under linear coordinate transformations and applies to multivariate predictions irrespective of assumptions about the probability distribution of prediction errors. For univariate Gaussian predictions, the PP reduces to conventional predictability measures that are based upon the ratio of the rms error of a model prediction over the rms error of the climatological mean prediction. Since climatic variability on intraseasonal to interdecadal timescales follows an approximately Gaussian distribution, the emphasis of this paper is on multivariate Gaussian random variables. Predictable and unpredictable components of multivariate Gaussian systems can be distinguished by predictable component analysis, a procedure derived from discriminant analysis: seeking components with large PP leads to an eigenvalue problem, whose solution yields uncorrelated components that are ordered by PP from largest to smallest. In a discussion of the application of the PP and the predictable component analysis in different types of predictability studies, studies are considered that use either ensemble integrations of numerical models or autoregressive models fitted to observed or simulated data. An investigation of simulated multidecadal variability of the North Atlantic illustrates the proposed methodology. Reanalyzing an ensemble of integrations of the Geophysical Fluid Dynamics Laboratory coupled general circulation model confirms and refines earlier findings. With an autoregressive model fitted to a single integration of the same model, it is demonstrated that similar conclusions can be reached without resorting to computationally costly ensemble integrations.

111. Griffies, Stephen M., 1998: The Gent-McWilliams skew flux. Journal of Physical Oceanography, 28(5), 831-841.
     [ Abstract PDF ]

     This paper formulates tracer stirring arising from the Gent-McWilliams (GM) eddy-induced transport in terms of a skew-diffusive flux. A skew-diffusive tracer flux is directed normal to the tracer gradient, which is in contrast to a diffusive tracer flux directed down the tracer gradient. Analysis of the GM skew flux provides an understanding of the physical mechanisms prescribed by GM stirring, which is complementary to the more familiar advective flux perspective. Additionally, it unifies the tracer mixing operators arising from Redi isoneutral diffusion and GM stirring. This perspective allows for a computationally efficient and simple manner in which to implement the GM closure in z-coordinate models. With this approach, no more computation is necessary than when using isoneutral diffusion alone. Additionally, the numerical realization of the skew flux is significantly smoother than the advective flux. The reason is that to compute the skew flux, no gradient of the diffusivity or isoneutral slope is taken, whereas such a gradient is needed for computing the advective flux. The skew-flux formulation also exposes a striking cancellation of terms that results when the GM diffusion coefficient is identical to the Redi isoneutral diffusion coefficient. For this case, the horizontal components to the tracer flux are aligned down the horizontal tracer gradient, and the resulting computational cost of Redi diffusion plus GM skew diffusion is roughly half that needed for Redi diffusion alone.

112. Griffies, Stephen M., Anand Gnanadesikan, Ronald C Pacanowski, V D Larichev, J K Dukowicz, and R D Smith, 1998: Isoneutral diffusion in a z-coordinate ocean model. Journal of Physical Oceanography, 28(5), 805-830.
     [ Abstract PDF ]

     This paper considers the requirements that must be satisfied in order to provide a stable and physically based isoneutral tracer diffusion scheme in a z-coordinate ocean model. Two properties are emphasized: 1) downgradient orientation of the diffusive fluxes along the neutral directions and 2) zero isoneutral diffusive flux of locally referenced potential density. It is shown that the Cox diffusion scheme does not respect either of these properties, which provides an explanation for the necessity to add a nontrivial background horizontal diffusion to that scheme. A new isoneutral diffusion scheme is proposed that aims to satisfy the stated properties and is found to require no horizontal background diffusion.

113. Bryan, Kirk, and Stephen M Griffies, 1997: Predictability of North Atlantic climate on decadal times scales estimated using a coupled ocean-atmosphere model. International WOCE Newsletter, 26, 5-9.
114. Griffies, Stephen M., and Kirk Bryan, 1997: Predictability of North Atlantic multidecadal climate variability. Science, 275(5297), 181-184.
     [ Abstract PDF ]

     Atmospheric weather systems become unpredictable beyond a few weeks, but climate variations can be predictable over much longer periods because of the coupling of the ocean and atmosphere. With the use of a global coupled ocean-atmosphere model, it is shown that the North Atlantic may have climatic predictability on the order of a decade or longer. These results suggest that variations of the dominant multidecadal sea surface temperature patterns in the North Atlantic, which have been associated with changes in climate over Eurasia, can be predicted if an adequate and sustainable system for monitoring the Atlantic Ocean exists.

115. Griffies, Stephen M., and Kirk Bryan, 1997: A predictability study of simulated North Atlantic multidecadal variability. Climate Dynamics, 13(7-8), 459-487.
     [ Abstract PDF ]

     The North Atlantic is one of the few places on the globe where the atmosphere is linked to the deep ocean through air-sea interaction. While the internal variability of the atmosphere by itself is only predictable over a period of one to two weeks, climate variations are potentially predictable for much longer periods of months or even years because of coupling with the ocean. This work presents details from the first study to quantify the predictability for simulated multidecadal climate variability over the North Atlantic. The model used for this purpose is the GFDL coupled ocean-atmosphere climate model used extensively for studies of global warming and natural climate variability. This model contains fluctuations of the North Atlantic and high-latitude oceanic circulation with variability concentrated in the 40-60 year range. Oceanic predictability is quantified through analysis of the time-dependent behavior of large-scale empirical orthogonal function (EOF) patterns for the meridional stream function, dynamic topography, 170 m temperature, surface temperature and surface salinity. The results indicate that predictability in the North Atlantic depends on three main physical mechanisms. The first involves the oceanic deep convection in the subpolar region which acts to integrate atmospheric fluctuations, thus providing for a red noise oceanic response as elaborated by Hasselmann. The second involves the large-scale dynamics of the thermohaline circulation, which can cause the oceanic variations to have an oscillatory character on the multidecadal time scale. The third involves non-local effects on the North Atlantic arising from periodic anomalous fresh water transport advecting southward from the polar regions in the East Greenland Current. When the multidecadal oscillatory variations of the thermohaline circulation are active, the first and second EOF patterns for the North Atlantic dynamic topography have predictability time scales on the order of 10-20 y, whereas EOF-1 of SST has predictability time scales of 5-7 y. When the thermohaline variability has weak multidecadal power, the Hasselmann mechanism is dominant and the predictability is reduced by at least a factor of two. When the third mechanism is in an extreme phase, the North Atlantic dynamic topography patterns realize a 10-20 year predictability time scale. Additional analysis of SST in the Greenland Sea, in a region associated with the southward propagating fresh water anomalies, indicates the potential for decadal scale predictability for this high latitude region as well. The model calculations also allow insight into regional variations of predictability, which might be useful information for the design of a monitoring system for the North Atlantic. Predictability appears to break down most rapidly in regions of active convection in the high-latitude regions of the North Atlantic.

116. Toggweiler, J R., E Tziperman, Y Feliks, Kirk Bryan, Stephen M Griffies, and Bonita L Samuels, 1996: Reply. Journal of Physical Oceanography, 26(6), 1106-1110.
     [ Abstract PDF ]

     The comment by Rahmstorf suggests that a numerical problem in Tziperman et al. (1994, TTFB) leads to a noisy E - P field that invalidates TTFB's conclusions. The authors eliminate the noise, caused by the Fourier filtering used in the model, and show that TTFB's conclusions are still valid. Rahmstorf questions whether a critical value in the freshwater forcing separates TTFB's stable and unstable runs. By TTFB's original definition, the unstable runs in both TTFB and in Rahmstorf's comment have most definitely crossed a stability transition point upon switching to mixed boundary conditions. Rahmstorf finally suggests that the instability mechanism active in TTFB is a fast convective mechanism, not the slow advective mechanism proposed in TTFB. The authors show that the timescale of the instability is, in fact, consistent with the advective mechanism

117. Griffies, Stephen M., and E Tziperman, 1995: A linear thermohaline oscillator driven by stochastic atmospheric forcing. Journal of Climate, 8(10), 2440-2453.
     [ Abstract PDF ]

     The interdecadal variability of a stochastically forced four-box model of the oceanic meridional thermohaline circulation (TMC) is described and compared to the THC variability in the coupled ocean-atmosphere GCM of Delworth, Manabe, and Stouffer. The box model is placed in linearly stable thermally dominant mean state under mixed boundary conditions. A linear stability analysis of this state reveals one damped oscillatory THC mode in addition to purely damped modes. The variability of the model under a moderate amount of stochastic forcing, meant to emulate the random variability of the atmosphere affecting the coupled model's interdecadal THC variability, is studied. A linear interpretation, in which the damped oscillatory mode is of primary importance, is sufficient for understanding the mechanism accounting for the stochastically forced variability. Direct comparison of the variability in the box model and coupled GCM reveals common qualitative aspects. Such a comparison supports, although does not verify, the hypothesis that the coupled model's THC variability can be interpreted as the result of atmospheric weather exciting a linear damped oscillatory THC mode.

118. Griffies, Stephen M., and Kirk Bryan, 1994: Predictability of North Atlantic climate variability on multidecadal time scales In The Atlantic Climate Change Program, Proceedings from the principal investigators meeting, NOAA, University Corporation for Atmospheric Research, 77-80.
     [ Abstract ]

     A major goal of the ACCP program is to gain the understanding of North Atlantic climate variability required for making predictions. An essential first step in this direction is to assess the predictability of Atlantic climate variability from models. A methodology for doing this was first proposed by Lorenz (1965) for atmospheric models. Recently, predictability studies have been extended to coupled atmosphere-ocean models in connection with the El Niño/Southern Oscillation phenomenon (e.g., Cane and Zebiak, 1987; Goswami and Shukla, 1991). At present, no operational monitoring system exists to provide proper initial conditions for the ocean on a global basis or even for the North Atlantic. The goal of this study is to use the GFDL climate model to determine the value, in terms of practical prediction of multi-decadal climate variability, of an operational, deep-sea observing system. We present here preliminary results toward this goal.

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