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GFDL Past Events & Seminars - 2015

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Date Speaker Affiliation Title of Presentation
Jan. 7Robbie ToggweilerGFDLA New Element of the Climate System
A New Element of the Climate System
Jan. 14N. Maher (with M. H. England, A. Sen Gupta & S. McGregor) University of New South WalesHiatuses in global warming: the role of volcanic eruptions and Pacific decadal variability
The latest generation of climate model simulations is used to investigate hiatuses in global warming. Large tropical volcanic eruptions are found to cause decade long hiatus periods consistently across the models. These eruptions not only cool the globe to cause hiatus decades, but are also found to influence modes of Indo-Pacific variability. Specifically we find an increased probability of an initial positive Indian Ocean Dipole / El Niño-like response followed by a La Niña-like cooling in the third Southern Hemisphere summer after the eruption, which may increase the persistence of the post-volcanic global cooling anomaly. We further demonstrate that most non-volcanic hiatuses across CMIP5 models are associated with enhanced cooling in the equatorial eastern Pacific, linked to a transition to the negative phase of the Interdecadal Pacific Oscillation. Finally, two future scenarios are investigated to determine the likelihood of hiatus periods occurring under different rates of greenhouse gas emissions. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption.
Jan. 21Honghai ZhangUniversity of MiamiMeridional Mode in an Idealized Aquaplanet Model: an Extratropics-to-Tropics Linkage
Meridional Mode in an Idealized Aquaplanet Model: an Extratropics-to-Tropics Linkage
Jan. 22Anne ThompsonNASA/GSFCStrategic Ozonesonde Networks: Insights from SHADOZ (1998-) and SEACIONS (2013)
Ozonesonde data support satellite validation, model assimilation and evaluation as well as studies of atmospheric dynamics. Strategic ozonesonde networks coordinate and schedule launches in a fixed region to answer specific questions (Thompson et al., 2011)*. We have organized five such networks in the past 15 years. Most of this talk will focus on the Southern Hemisphere Additional Ozonesondes (SHADOZ; ) network that consists of a dozen tropical and subtropical stations, with 2-4 launches monthly. An overview of SHADOZ origins and workings will be given along with illustrative findings in the troposphere and stratosphere. In campaign-class strategic networks daily launches look at ozone short-term variability to complement NASA aircraft missions. Examples from the 2013 SEACIONS (Southeastern American Consortium for Intensive Ozonesonde Network Study), from the SEAC4RS campaign, will be presented.
Jan. 28Poster ExpoGFDLPoster Expo
Feb. 4Cheng-Ta Chen National Taiwan Normal University Assessing Possible Anthropogenic Contributions to the Rainfall Extremes Associated with Typhoon Morakot (2009)
Assessing Possible Anthropogenic Contributions to the Rainfall Extremes Associated with Typhoon Morakot (2009)
Feb. 5Mian ChinNASA Multi-decadal aerosol variations and their effects on solar radiation reaching the surface
will present an investigation on multi-decadal changes of atmospheric aerosols and their effects on surface radiation using a global chemistry transport model along with the near-term to long-term data records from remote sensing and ground-based measurements. I will show the variations and trends of aerosol trends in the past 3 decades (1980-2009) in different regions, link these changes to the change of sources and climate variability, and assess their effects on the multi-decadal change of solar radiation reaching the surface (known as "dimming"or "brightening").
Feb. 11Adrian Matthews University of East AngliaA dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones
The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day-1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation. Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone. Numerical model experiments are carried out to investigate the roles of the underlying SST field and the internal dynamics of the Rossby waves to the existence of the diagonal convergence zones.
Feb. 12Asa RennermalmRutgersGreenland Ice Sheet Meltwater Export and River Discharge
The Greenland ice sheet supplies massive amounts of freshwater and sediments to surrounding oceans, contributing to global sea level rise and influencing marine ecosystems. The majority of this meltwater runoff originates from the ice sheet and reaches the ocean via subglacial conduits at marine terminating outlet glaciers or proglacial rivers. Other sources of Greenland streamflow include runoff from tundra areas, peripheral glaciers and ice caps. Here we present an analysis of meltwater losses from Greenland's ~500 largest drainage basins. Basins were identified by analyzing high-resolution digital elevation models. Discharge from individual basins between 1958 to 2012 were derived by integrating surface runoff simulated with the regional surface mass balance model Modèle Atmosphérique Régional (MAR). We show that proglacial rivers transport about half the meltwater exported to surrounding oceans despite draining less than half the total ice sheet area. This is due to the large river systems in Southwest Greenland where ice sheet surface melting is most pronounced. While the largest meltwater export trends also are found in Southwest Greenland, ice sheet runoff has increased for the majority of ice sheet basins in the 1958 to 2012 time period. This study is one of only few examining spatial patterns in meltwater losses to oceans, and single out David Strait as the ocean most likely to be impacted by ice sheet runoff export.
Feb. 18Liping ZhangAOSAnalysis of the characteristics, mechanism and predictability of the Pacific Decadal Oscillation in a suite of GFDL climate models
Analysis of the characteristics, mechanism and predictability of the Pacific Decadal Oscillation in a suite of GFDL climate models
Feb. 19Eli TzipermanSEAS - HarvardTBA
Feb. 26Janet SprintallU of California - San DiegoThe Indonesian Seas and their role in the coupled ocean-climate system
Mar. 4Alberto Naveira-GarabatoU of Southampton, UKA microscale view of mixing and overturning across the Antarctic Circumpolar Current
The relative roles of isopycnal stirring by mesoscale eddies and diapycnal stirring by small-scale turbulence in setting the large-scale temperature - salinity relation of the Southern Ocean against the action of the overturning circulation are assessed, by analysing a set of shear and temperature microstructure measurements across Drake Passage in a `triple decomposition' framework. It is shown that a full picture of regional mixing and overturning across the Antarctic Circumpolar Current (ACC) may be constructed from a modest number of microstructure profiles. The rates of isopycnal and diapycnal stirring are found to exhibit distinct, characteristic and abrupt variations: most notably, a one-to-two order of magnitude suppression of isopycnal stirring in the upper kilometre of the ACC frontal jets, and an order-of-magnitude intensification of diapycnal stirring in the sub-pycnocline and deepest layers of the ACC. These variations balance an overturning circulation with meridional flows of O(1 mm / s) across the ACC's mean thermohaline structure. Isopycnal and diapycnal stirring play complementary roles in balancing the overturning, with isopycnal processes dominating in intermediate waters and the Upper Circumpolar Deep Water, and diapycnal processes prevailing in lighter and denser layers. The implications of this balance for our present view of the Southern Ocean circulation will be discussed.
Mar. 4Jonas NycanderStockholm UniversityTidal Mixing
Tidal mixing- a key process controlling deap ocean circulation
Mar. 11Dan LiAOSThe role of buoyancy in surface-atmosphere interactions
Abstract: Land-atmosphere interactions and air-sea interactions are largely regulated by turbulence in the atmospheric surface layer, which spans from the surface to about 100 m above the surface. As compared to turbulence in controlled lab experiments that is often only subject to the shear force, turbulence in the atmospheric surface layer is constantly affected by the buoyancy force resulting from surface heating and cooling. As a result, many turbulence phenomenological theories, when applied to the atmospheric surface layer, need to be expanded to consider buoyancy effects. A case in point is the stability correction functions for momentum and heat that are used to account for buoyancy caused distortions to the logarithmic velocity and temperature profiles in the atmospheric surface layer, respectively. These stability correction functions are used in almost all numerical weather and climate models and are only dependent on the stability parameter according to Monin-Obukhov similarity theory (hence they are also called Monin-Obukhov similarity functions). In this presentation, how turbulence and turbulent transports are modulated by buoyancy in the atmospheric surface layer is explored theoretically and experimentally. A spectral and co-spectral budgets model is proposed to link the micro-states of turbulent energy (kinetic and potential) distributions, as measured by their spectra, to the macroscopic relations such as Monin-Obukhov similarity functions that describe the mean flow. It will be shown that the universality of Monin-Obukhov similarity functions hinges on the universality of the Kolmogorov-Obukhov-Corrsin law for turbulent energy spectra in the inertial subrange.
Mar. 12Mat CollinsExeter - United KingdomThe Hiatus, Tropical Climate Feedbacks, Extreme ENSOs and the Role of Model Biases
The recent "hiatus" or "pause" in global mean temperature change has been linked to tropical Pacific decadal variability. Tropical mean water vapor and lapse-rate feedbacks tend to co-vary across models. Recent studies suggest that extreme El Niño and extreme La Niña events will increase in frequency in the future. This talk will review recent research in this area and discuss the use of observations to constrain model results and the role of model biases in determining their credibility.
Mar. 26Richard MossPNNL, Richland, WAMinding the Gap: Ideas for Increasing the Decision Relevance of Climate Assessments
Abstract: People, places, and things are vulnerable to interacting changes in climate and society, and scientific uncertainties can seem to make assessing consequences and planning responses a challenge. However, much is understood about climate change and its potential impacts that can provide a useful input to decision making about avoiding unmanageable levels of change and adapting to changes that can no longer be avoided. This talk will focus on ideas for increasing the utility of climate science assessments for planning adaptation and mitigation. It will review innovations in the Third US National Climate Assessment (NCA), completed in 2014, intended to make climate and impacts research useful to analysts and decision makers. iThese innovations included approaches for assessing and communicating uncertainties and confidence, sea level change scenarios for risk management, and assessment of decision support applications of climate science. It will also discuss the planned "sustained assessment" process for the NCA and continued innovations required to support climate change risk management.
Apr. 2Jeremey FykeLos Alamos Natl. LabModeling land ice in Earth System Models: progress and challenges
Bidirectional coupling of ice sheet models and climate models opens the door to research of ice-sheet/climate interaction at a global scale. However, difficulties encountered in achieving this coupling have proven non-trivial and include both: 1) Technical challenges. Ocean and atmosphere model components cannot easily handle dynamic boundaries; land surface model components cannot easily simulate exposed glacial ice, firn evolution, or evolving land surfaces; coupling infrastructure cannot easily accept new earth system components; and ice sheet models typically operate at an order of magnitude higher spatial resolution than climate models and on regional domains, and require very long integrations to reach an equilibrium state. 2) Scientific challenges. The glaciological modeling and climate modeling communities often work on topics that cover very different spatiotemporal scales; carry out research using very different models and with very different modeling paradigms; and do not consider coupled ice-sheet/climate behavior across the various ice-sheet/climate physical interfaces. These technical and scientific challenges are being tackled within the Community Earth System Model. The resulting coupled architecture ("CESM-CISM") is now capable of simulating the Greenland ice sheet (GrIS) in the climate system. The model includes a relatively sophisticated representation of surface mass balance (SMB), explicit resolution of important ice-sheet climate communication pathways, and prognostic ice dynamics. First results, underpinned by modelled SMB validation, have explored changes in GrIS mean SMB and its variability, partially-coupled GrIS evolution, and emergence of an anthropogenic signal in SMB under RCP8.5 climate forcing. Ongoing work with the evolving CESM-CISM will set the stage for fully-coupled simulations of Greenland in past and future climates, and also, within other DOE projects, for potential integration of the Antarctic ice sheet into a true coupled modeling framework.
Apr. 3Andrew ShaoUnv of Washington Investigating thermocline variability using an offline tracer adjoint model
Abstract: The transport time distribution, the Green's function solution to the transport equation, has been used to estimate the pathways and timescales of tracer transport into the interior and to propagate tracers into the interior. Most modeling studies that integrate forward in time treat the surface as a homogeneous boundary and steady-state flow. Here using an offline tracer model, we propose an adjoint-based method which can be used to relax both of these assumptions and demonstrate its use by identifying the source regions of variability for a point within the Indian Ocean Sub-antarctic Mode Water.
Apr. 9Joanie KleypasNCARThe hunt for reef refugia in the Coral Triangle
Abstract: The Coral Triangle is considered the center of marine biodiversity, and particularly for corals. Coral reefs have already been impacted by climate change, and conservationists are trying to account for climate projections in the design of marine protected area networks. This talk presents how we are using high-resolution oceanographic modeling to better identify potential climate refugia for coral reef ecosystems.
Apr. 13Peter HitchcockDAMTP, University of CambridgeEvaluating Mechanisms for the Downward Influence of Stratospheric Sudden Warmings
There now exists considerable evidence from observations and numerical models that, on average, the tropospheric jet shifts equatorward in the months following the breakdown of the Arctic polar vortex that characterizes stratospheric sudden warmings. Several mechanisms have been proposed to explain this tropospheric response; however, evaluating these mechanisms is made difficult by the large amount of dynamical noise in the observations and the absence of quantitative hypotheses. I present results from a set of numerical experiments with the Canadian Middle Atmosphere Model which demonstrate unambiguously the downward influence of the stratospheric anomalies that occur during sudden warmings on the troposphere below. Consideration of the vertically-averaged angular momentum budget also allows several proposed coupling mechanisms to be quantitatively compared, and points clearly the importance of quasi-stationary planetary waves in the downward coupling.
Apr. 16Robert AllenUC - RiversideA 21st Century Northward Tropical Precipitation Shift Caused by Future Anthropogenic Aerosol Reductions
The tropical rain belt is a narrow band of clouds and showers near the equator, where the most intense rainfall on the planet occurs. On seasonal timescales, the rain moves back and forth across the equator following the sun, resulting in wet and dry seasons in the tropics. The position of the tropical rain belt also varies on longer time scales. Through the latter half of the 20th century, for example, shifts in tropical rainfall have been associated with severe droughts, including the African Sahel and Amazon droughts. Here, I show that climate models project a northward migration of the tropical rain belt through the 21st century, with future anthropogenic aerosol reductions driving the bulk of the shift. Models that include both aerosol indirect effects yield significantly larger northward shifts than models that lack aerosol indirect effects. Moreover, the rate of the shift corresponds to the rate of the decrease of anthropogenic aerosol emissions across different time periods and future emission scenarios. This response is consistent with relative warming of the Northern Hemisphere, and a concomitant northward shift in the Hadley cells, along with weaker northward cross-equatorial energy transport. Although aerosol effects remain uncertain, I conclude that future reductions in anthropogenic aerosol emissions may be the dominant driver of a 21st century northward shift of the tropical rain belt.
Apr. 20 Syukuro ManabeSymposium: Exploring global warming with climate models
In this symposium, talks will examine advances that have been made in understanding climate change and the future challenges that remain in developing a predictive understanding of Earth's climate system. For more information click here.
Apr. 23Tim LentonExeter - UKEarly Warning of Climate Tipping Points
A "tipping point" occurs when a small change in forcing triggers a strongly non-linear response in the internal dynamics of a system, qualitatively changing its future state. Large-scale "tipping elements" have been identified in the Earth's climate system that may pass a tipping point under human-induced global change this century. At the smaller scale of ecosystems, some tipping points have already been observed, and more are anticipated in future. Our capacity to forecast such abrupt, non-linear changes has historically been poor. However, much excitement has recently been generated by the theory that some approaching tipping points carry generic early warning signals. I will critically examine the prospects for gaining early warning of approaching tipping points. Promising methods are based on detecting "critical slowing down" in the rate a system recovers from small perturbations, and on accompanying changes in the statistical distribution of its behaviour. I will show examples of early warning signals in paleo-data approaching past abrupt climate changes, and in models being gradually forced past tipping points. I will also discuss the conditions under which the methods fail. Finally I will show an example of pronounced slowing down in observational climate data, for North Pacific sea surface temperatures, and explore the implications for well-known marine ecosystem "regime shifts".
Apr. 29Karl SmithNOAA/PMEL (Pacific Marine Environmental Laboratory)Get more from your data with PyFerret
Present an introduction and discussion of PyFerret, the new Python-enabled version of Ferret.
Apr. 30Clara DeserNCARUncertainty in climate change projections: the role of internal atmospheric circulation variability
This talk will highlight the relative importance of internally-generated vs. externally-forced climate trends at local and regional scales over North America and Eurasia based on a 30-member ensemble of the Community Earth System Model over the period 1920-2100. Each member is subject to the same historical and future (RCP8.5) radiative forcing scenario, but starts from a slightly different atmospheric state. Unpredictable, intrinsic variability of the large-scale atmospheric circulation strongly influences the pattern, magnitude and polarity of surface air temperature and precipitation trends in any single simulation, despite the common radioactive forcing. Implications for model validation, inter-model comparisons, and interpretation of observed climate trends will be discussed.
May. 6Pu LinAOS/GFDLClimate change in the upper troposphere and lower stratosphere
The upper troposphere and lower stratosphere (UTLS) is a crucial component of the climate system. The atmospheric composition there impacts the radiation balance of the climate system, and may further induce circulation responses in the troposphere and at the surface. Here we examine the UTLS circulation and thermal structure simulated by GFDL GCMs. We find that the strength of the lower stratospheric circulation is tightly coupled with the tropical mean surface temperature regardless of the imposed forcings. The warming of the tropical tropopause under global warming, a robust prediction by many models, is also investigated. Contributions from each radiative, dynamical and thermodynamical processes are quantified. We find that the radiative effect from the elevated CO2 concentration and the warmer troposphere are the major warming factors, while the accelerated stratospheric circulation leads to a cooling at the lower stratosphere. Changes in convection, clouds, ozone and water vapor may further modify the UTLS thermal structure, but their effects are relatively minor.
May. 7Jean-Francois LamarqueNational Center for Atmospheric Research, Boulder, ColoradoLocal and remote climate response to aerosol forcing
This talk aims at addressing the following question: what are the local and remote impacts of regional perturbations in aerosol emissions, as these could be key drivers of regional climate changes over the next several decades? More specifically, using results from chemistry-climate simulations, we will discuss the climate (mostly temperature and precipitations) response to regional perturbations in present-day aerosol (or their precursors) emissions. In another set of experiments and analysis, we will juxtapose the climate response to greenhouse-gas forcing and to aerosol forcing, including extremes. By using large ensembles, we are able to extract the forced response from the noise associated with internal variability.
May. 14Zhuo WangU of IllinoisImpacts of Rossby Wave Breaking on Atlantic Tropical Cyclone Activity
With warm SST anomalies in the tropical Atlantic and cold SST anomalies in the East Pacific, the unusually quite hurricane season in 2013 was a surprise to the hurricane community. Our analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August and early September can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant middle to upper tropospheric dryness and strong vertical wind shear hindered TC development. Using the EOF analysis, we found that the active Rossby wave breaking (RWB) in August 2013 was associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the Main Development Region (MDR) SST, and higher than that with the Niño 3.4 index. Further analysis showed that RWB over the western Atlantic has different impacts on TC activity from that over the eastern basin. RWB over the eastern Atlantic occurs more poleward, and the dry air can be advected equatorward by the subtropical anticyclonic circulation and hinder TC formation over the eastern MDR. Precursor disturbances, however, can still develop later over the western Atlantic if the environmental conditions are favorable there. Active RWB over the eastern basin thus can reduce the accumulative cyclone energy (ACE, a function of TC frequency, intensity and lifetime) but does not affect the basin-wide TC count significantly. Over the West Atlantic, potential vorticity remnants induced by RWB tend to propagate equatorward along the tropical upper tropospheric trough. The associated strong vertical wind shear and dry air over the western Atlantic can significantly reduce both the ACE and the basin-wide TC count. Active RWB occurred over both the western and eastern Atlantic in 2013. The linkages of RWB with different climate factors and the implication for its predictability will also be briefly discussed.
May. 19Eric GalbraithMcGill UniversityDeconstructing Quaternary climate drivers with a library of idealized Earth System Model simulations
It is well-established that global climate variability over the Quaternary ice age cycles was driven by changes in the Earth's orbital parameters, amplified by changes of atmospheric CO2, and modulated by the growth and decay of land-based ice sheets. However, it has proven difficult to directly relate the individual contributions of orbital, CO2 and ice sheet drivers to the overall results observed in paleoclimate archives. I will show results from an array of multi-millennial simulations conducted with CM2Mc, a 3-degree configuration of the GFDL climate model, under idealized changes in CO2, orbital parameters and ice sheet boundary conditions. This matrix of simulations provides a means to isolate the individual effects of the drivers, as well as their interactions. Among the results, the library reveals a strong impact of CO2 on the salinity of the high latitude Southern Ocean, due to changes in atmospheric moisture transport and sea ice cycling, which exerts a major control on modelled deep ocean circulation under low CO2. In addition, the model reveals a swath of boundary conditions under which the Atlantic Meridional Overturning Circulation (AMOC) oscillates between on and off states, on a multi-centennial timescale, in the absence of any external forcing. These spontaneous AMOC oscillations are reminiscent of the abrupt climate change events documented from ice core and marine sediment records. When compared with freshwater hosing experiments carried out in the same model, the unforced AMOC oscillations appear consistent with 'Dansgaard-Oeschger' events, while the freshwater-forced experiments appear more consistent with ‘Heinrich' events.
May. 20Apostolos VoulgarakisImperial CollegeLocal and remote climate effects of regional aerosol emissions
The radiative forcing of short-lived pollutants such as aerosols and tropospheric ozone is highly inhomogeneous and can therefore affect regional temperature, circulation and precipitation in a much more complicated way than the forcing of well-mixed greenhouse gases. However, such effects have not been examined thoroughly and systematically from a global point of view, to understand regional interactions. Here, after outlining some key past work in this area, results from recent simulations with the HadGEM3 global composition-climate model will be presented in which anthropogenic emissions of sulfur dioxide (SO2) and black carbon in key regions (East Asia, South Asia, the US, and the northern mid-latitudes as a whole) have been removed. The linkages between emissions, concentrations, radiative forcing, temperature and precipitation response will be discussed. A particular emphasis will be placed on non-local effects, i.e. how emissions over a certain region can affect other areas remotely. Finally, we will contrast our results with those from similar experiments pursued using the GISS-E2 and the CESM1 models for US and East Asian SO2 emissions, and will discuss the large differences in the models' behaviour.
May. 21Steve KruegerUniversity of UtahSensitivity of Cloud-Resolving Model Simulations to Subgrid-Scale Turbulence Closure, Microphysics, and Resolution
We are using a cloud-resolving model, SAM (System for Atmospheric Modeling) to examine the sensitivity of simulations to subgrid-scale (SGS) turbulence closure, microphysics, and resolution. I will focus on two sets of simulations: One of an evolving mixed-phase cloud-topped boundary layer during a cold-air outbreak over the North Atlantic Ocean, and a second of radiative-convective equilibrium (RCE) at two different sea surface temperatures (in order to assess sensitivity to climate change). Cold-air outbreaks typically produce an evolving cloud-topped boundary layer whose structure is influenced by strong surface fluxes of sensible and latent heat, mixed-phase microphysics, cloud-top radiative cooling, and cloud-top entrainment. By systematically varying the horizontal resolution from 1 to 100 km and comparing the results to a benchmark large-eddy simulation of the case, we have assessed the ability of SHOC (Simplified Higher-Order Closure, Bogenschutz and Krueger 2013) to represent this type of boundary layer. We have also investigated the roles of various physical processes (radiation, precipitation, microphysics). RCE is a prototype for the Earth's climate. The solution is the result of a balance between deep convection, radiative cooling (modulated by both shallow cumulus and upper-tropospheric convectively produced stratiform clouds), surface fluxes, and microphysical processes. Our version of SAM includes SHOC which combines several existing components: A prognostic SGS turbulence kinetic energy (TKE) equation, an assumed double-Gaussian PDF following Golaz et al. (2002), the diagnostic second-moment closure of Redelsperger and Sommeria (1986), the diagnostic closure for the third moment of vertical velocity by Canuto et al. (2001), and a turbulence length scale related to the SGS TKE (Teixeira and Cheinet 2004) and to eddy length scales.
May. 27 Johannes QuaasLeipzig UniversityUse of satellite data to evaluate the aerosol-cloud radiative forcing in climate models
Jun. 3Patrick BrownDuke UniversityUnforced Surface Temperature Variability and its Relationship with the Top-of-Atmosphere Energy Balance in CMIP5 Models
Jun. 10Ronald StoufferNOAA/GFDLNOAA Public Access to Research Results: A discussion of changing NOAA Requirements
The White House is working to re-assert the federal government's copyrights. Anything produced by federal dollars (run on gov't computer, produced by federal employees, produced via grants and proposals, etc.) is owned by the government and the people of the US hold the copyright. As this directive is now being implemented it will result in changes in how we interact with journals and in serving our model data. I will present an overview of the details of the changes to the extent they are known and have a long discussion period to follow.
Jun. 11Michael WolowickColumbia UniversityTraveling Slippery Spots, Travelling Sticky Spots, and Massive Overturned Folds in Ice Sheets
Ice sheets flow by a combination of basal slip and internal deformation. The tradeoff between slip and deformation is recorded in internal ice sheet stratigraphy. Here, I explore the stratigraphic structures produced by sticky and slippery spots at the ice sheet bed that migrate downstream over time. Patterns of basal slip that migrate downstream at the column-average ice velocity produce the most deformation of the overlying ice column. Sticky spots that move slower than the column velocity produce overturned folds in the lower part of the ice sheet, similar to structures observed in ice-penetrating radar data. Alternating sequences of traveling slippery and sticky spots are produced naturally by a thermomechanical ice flow model forced by variability in upstream water supply. Slippery and sticky spots at the ice sheet bed migrate downstream because of a feedback between ice flow, deformation, and temperature. Following a single sticky spot in a moving reference frame reveals that cold sticky spots are kept cold by thermal overturn. A temperature inversion forms in the lower part of the ice sheet when stratigraphy overturns more rapidly than diffusion can erase the thermal structure of the ice. Cold temperatures near the bed result in freezing and loss of basal water, producing a positive feedback that reinforces the original moving sticky spot. As a proof of concept, three example overturned folds from northern Greenland demonstrate how measurements of stratigraphic geometry can be used to constrain the dynamic properties of basal slip.
Jun. 17Andrew BartonGFDL/CICSAnthropogenic climate change drives shift and shuffle in marine phytoplankton communities
Anthropogenic climate change drives shift and shuffle in marine phytoplankton communities
Jun. 18Susan LozierDuke UniversityOverturning in the North Atlantic: old paradigms, new observations and lingering questions
Jun. 19Lei YinUniveristy Texas, AustinUnderstanding Rainfall Variability and Extremes over the Amazon to Improve Their Future Projection
Jun. 19Bing PuUniversity of Texas at AustinHydrological changes in the climate system from leaf responses to increasing CO2
Vegetation is a major component of the climate system because of its controls on the energy and water balance over land. This functioning changes because of the physiological response of leaves to increased carbon dioxide (CO2). The Community Earth System Model is used to compare these changes with the climate changes from radiative forcing by greenhouse gases. Ensemble integrations are done for current and doubled CO2 concentrations. The consequent reduction of transpiration and net increase of surface radiative heating from reduction in cloudiness increases the temperature over land by a significant fraction of that directly from the radiative warming by CO2. Large-scale atmospheric circulation adjustments result. In particular, over the tropics, a low-level westerly wind anomaly develops associated with reduced geopotential height over land, enhancing moisture transport and convergence, and precipitation increases over the western Amazon, the Congo basin, South Africa, and Indonesia, while over mid-latitudes, land precipitation decreases from reduced evapotranspiration. On average, land precipitation is enhanced by 0.03 mm day-1 (about 19 % of the CO2 radiative forcing induced increase). This increase of land precipitation with decreased evapotranspiration is an apparent negative feedback, i.e., less evapotranspiration makes more precipitation. Global precipitation is slightly reduced. Runoff increases associated with both the increased land precipitation and reduced evapotranspiration. Examining the consistency of the variations among ensemble members shows that vegetation feedbacks on precipitation are more robust over the tropics and in mid to high latitudes than over the subtropics where vegetation is sparse and the internal climate variability has a larger influence.
Jun. 22Adrienne WoottenNorth Carolina State UniversityCreating High Resolution Projections for Puerto Rico: Downscaling with Ecosystems in Mind
Abstract: The twin goals of advancing climate science and promoting the appropriate use of climate information are both key to improving a wide range of applied research and decision making efforts, with the treatment of uncertainties surrounding future climate projections presenting notable challenges. Topics in this talk include (a) preliminary results from the production of high resolution projections of climate change in Puerto Rico using regional climate modeling, (b) insights into the propagation and characterization of uncertainty in downscaled climate projections, and (c) the potential influence of such uncertainty on the appropriate use / incorporation of downscaled projections in decision making and vulnerability assessments.
Jun. 22Jie HeRosenstiel School of Marine and Atmospheric Science, University of MiamiMechanisms of regional precipitation change from anthropogenic forcing
The growing demand by the scientific community, policy makers and the public for realistic projections of regional precipitation change has aroused considerable interest in model simulations of anthropogenic precipitation change. However, the current projections of anthropogenic precipitation change are facing great challenges, including inadequate model resolution, uncertainties in SST changes and biases in climatology. This seminar addresses these challenges and discusses some practical ways forward. To improve model resolutions, the idea of using high-resolution time-slice experiments has been brought forth. On the other hand, AGCMs are often criticized for the lack of consistent surface energetics. This seminar addresses the utility of AGCMs for anthropogenic climate change simulations and the impact of surface energetic inconsistency, which is also a concern for flux adjusted simulations. Another challenge for the simulation of precipitation change results from the uncertainty in the pattern of SST change, which accounts for a large portion of the uncertainty in precipitation projections over tropical oceans. Here, it is shown that the pattern of SST change has little impact on precipitation changes over land. In comparison, land precipitation changes are more susceptible to biases in climatology. Models show substantial improvements in the simulation of anthropogenic climate change over land when biases in the climatological SST are corrected
Jun. 22Christopher AllenAtmospheric mechanisms of dust storm formation in the central Sahara: results from new ground-based observations and implications.
In boreal summer, satellite measurements show that the central Sahara is the dustiest region of the planet. However, until the Fennec Project, ground-based observations of the central Sahara were limited to its outer edges, leaving a void in observations approximately 1 million km2 in area. Analysis of the new Fennec observations shows that four mechanisms are responsible for dust storm formation in the central Sahara: cold pool outflows, monsoon surges, low-level jets and dry convective plumes. In both field seasons, cold pool outflows cause approximately 70% of the total dust loading. Dynamical models which parameterise convection typically fail to generate cold pool outflows. These models, which are commonly used to generate dust emission estimates, are therefore missing the prime cause of dust emission in the global dust maximum region. Furthermore, since cold pools frequently occur at night they are missed by polar orbiting satellites that pass during the day (e.g. the A-train). Many cold pools are also missed by geostationary satellites which cannot retrieve under cloud or under conditions of relatively high water vapour. Finally, the sun photometer, a benchmark ground instrument, misses cold pools that occur at night or under cloud. Almost all of the techniques currently used to study dust in the region are systematically biased to result in underestimates of dust burden in summer. The implications are twofold: (i) caution should be used when validating satellite retrieval/model output/sun photometer data against each other, since they share the same bias (ii) the Sahara is likely much more dusty than previously thought.
Jun. 25Richard RotunnoUCAR, Boulder, COSevere Convection: From Theory to Applications
In this lecture I will survey developments in numerical models of severe convection (supercell thunderstorms, squall lines, etc.). From their primitive beginnings in the 1970's, numerical models have been able to capture some of the very basic features of severe convection as a function of the imposed environmental conditions of thermodynamic instability and vertical wind shear. In the following decades these models, together with advances in radar meteorology, provided a useful basis for forecasters trying to predict severe convective weather in the 0-6h time frame. Over the past decade the growth in computer power has allowed numerical prediction models with explicitly simulated convection to be run in real time over large domains; moreover real data initial conditions have provided realistic environments for the numerically simulated convection. Although the predictability horizon for convective cells may be at most only a few hours, these forecast models present the possibility of forecasting larger-scale aspects of convective weather over the longer 6-24h time frame. In an attempt to cope with the inherent limits of predictability, I will discuss how ensembles of real-time high-resolution predictions are currently being tested.
Jul. 1Neil BanasUniversity of Strathclyde, Glasgow, UKFrom climate to top predators via a diverse, adapatable plankton community: New model approaches in arctic and subarctic seas
Jul. 8Tom DelworthNOAA/GFDLThe impact of the North Atlantic Oscillation on climate through its influence on the Atlantic Meridional Overturning Circulation (AMOC)
Jul. 9Usama AnberDepartment of Earth and Environmental Sciences/Columbia University. Idealized Cloud-Resolving Modeling for Tropical Climate Studies
Many studies have utilized cloud-resolving models (CRMs) to study the characteristics of tropical moist convection in the state of radiative-convective equilibrium (RCE). However, RCE cannot be used to represent the tropical atmosphere locally because it lacks large-scale mean circulation. For this matter, I will use a limited domain model in idealized configuration that explicitly resolves convection and parameterizes the large scale vertical motion. This method allows a two-way interaction between the large scale dynamics and cumulus convection when the model domain is too small to resolve the large scale circulation, allowing the model itself to determine the occurrence and intensity of deep convection. This is as opposed to more traditional methods, in which otherwise similar numerical experiments are performed with specified large-scale vertical motion, strongly constraining the bulk properties of convection a priori. It is also computationally inexpensive and can be used to explore the effect of a wide range of parameters. In this talk I will introduce two methods used to represent the large-scale motion in CRMs. Then, utilizing a theory for tropical mean precipitation based on conserved variables of the dry and moist static energy, I will examine some parameters controlling mean precipitation: surface fluxes vs. radiative heating, vertical wind shear, and interactive radiation. Finally, I will use the model in its idealized settings to test its predictive power to simulate the seasonal and diurnal cycle in the Amazon basin.
Jul. 10Xiaoming ShiDepartment of Atmospheric Sciences/University of WashingtonDifferent Response of Extreme Precipitation to Warming over Mountains, Plains, and Oceans
Climate simulations predict an intensification of extreme precipitation in almost all areas of the world under global warming. The rate of this intensification depends on the change in temperature and vertical velocity in extreme events, because temperature determines the amount of moisture available to condense for a given adiabatic ascent, and vertical velocity determines the timescale of that condensation. Over mountains, the response of vertical motion to warming was recently found controlled by gravity wave dynamics. Here we show that, in idealized climate simulations where orography can be modestly resolved, precipitation extremes over mid-latitude at areas and idealized mountains respond to climate change differently, due to distinct dynamical forcings to their vertical motion. In both cases, the increase in the intensity of precipitation extremes caused by temperature rise is around 6% per K of surface warming, yet the ascent in the extreme events over oceans and plains is enhanced due to stronger diabatic forcing from moist processes, whereas the ascent in orographic extremes mostly weakens. This makes the precipitation extremes over plains and oceans increase about 3%/K faster than orographic extremes, suggesting that the population in mid-latitude at regions might be more susceptible to the impacts of global warming.
Jul. 15Thomas KnutsonNOAA/GFDLClimate trends analysis updates: Heat stress and the global warming hiatus
In this two-part talk, I will discuss results from two projects looking at different aspects of historical climate trends: summertime heat stress and the global warming hiatus. We examine an observed heat stress metric, a simplified form of wet bulb globe temperature (WBGT), over land regions during summer (1973-2012). Observed trends in WBGT are compared to trends from CMIP5 historical simulations (eight model ensemble) using either All Forcings (anthropogenic and natural forcing agents combined) or Natural Forcings only. Our study suggests that there has been a detectable anthropogenic increase in this summertime heat stress metric since 1973, both globally and in most land regions analyzed. Notably, summertime WBGT over land has continued increasing in recent years--consistent with climate models--despite the apparent hiatus' in global mean temperature and despite a decreasing tendency in observed relative humidity over land since the late 1990s. Our ongoing work on the global warming hiatus compares maps of trends (observed and modeled) for five key periods of evolution of global mean temperature since the late 1800s: the late 1800s decline; the early 20th century warming; the 1940-1070 hiatus'; the late 20th century warming; and the global warming ‘hiatus' since about 2000. We compare trends from the CMIP5 multi-model ensemble (All-Forcing) runs and internal climate variability as simulated by the GFDL CM3 preindustrial control run. We also explore how long the current hiatus' could potentially extend according to internal variability as simulated by the CM3 model.
Jul. 22Xianglei HuangUniversity of MichiganIncorporating the realistic surface spectral emissivity in the climate models: motivation, treatment, and preliminary results
While atmospheric general circulation models (GCMs) still treat the surface as blackbody in their longwave radiation scheme, recent studies suggest the need of taking realistic surface spectral emissivity into account, especially for the simulation of polar climates. I will review such studies first, focusing on the impact on the polar radiation budgets. Such studies motivated us to develop a global surface spectral emissivity database suitable for the use in climate model. However, few observation or laboratory measurements are available for the surface emissivity in the far IR (
Jul. 23 Peter BogenschutzUCAROn the Assumed PDF-Based Cloud Parameterization for Climate Simulations
Recent years has seen the advent of the assumed probability density function (PDF) parameterizations implemented into atmospheric general circulation models (GCMs). These parameterizations are centered around a trivariate assumed double Gaussian distribution to close turbulence and cloud macrophysics. In addition, these PDF-based parameterizations are "unified"in the sense that they replace the conventional planetary boundary layer, shallow convection, and cloud macrophysics schemes with one equation set. This presentation will focus on the successes and challenges on coupling NCAR's Community Atmosphere Model (CAM) with such a scheme, known as Cloud Layers Unified By Binormals (CLUBB). Results will focus on the simulated mean state climate, tropical variability, and coupled modes of variability for both prescribed sea surface temperature and fully coupled simulations. The final part of this presentation will discuss how these assumed PDF parameterizations could be potentially beneficial to the high-resolution GCMs of the future.
Jul. 27Laurie MenvielUniversity of New South Wales, Sydney, AustraliaAntarctic Bottom Water influence on the last glacial and deglaciation
Jul. 29Paul Spence University of New South Wales, Sydney, AustraliaRapid subsurface warming of West Antarctic Peninsula waters by East Antarctic coastal winds
Massive mass loss and accelerated Antarctic glacial flows in recent decades are generally linked with warmer ocean water at the base of the floating ice shelves. However, the physical mechanisms responsible for changing the subsurface ocean heat content on the Antarctic continental shelf remain largely unknown. This talk explores the magnitude and causes of subsurface Antarctic coastal ocean temperature variability on annual to decadal time scales. Ocean model simulations using the GFDL MOM5 ¼ degree model reveal that intense subsurface ocean warming (>2C at 200-700m depth) can rapidly (10,000km away) Antarctic coastal winds. The local wind response is driven by changes in near-shore Ekman pumping, while the distant wind response is driven by Antarctic coastal Kelvin waves. The Bellingshausen and Amundsen seas and the western side of the Antarctica Peninsula are particularly susceptible to Southern Annular Mode like wind changes in East Antarctic coastal winds.
Aug. 5Robin WordsworthHarvard UniversityDeciphering the Climate of Noachian Mars through 3D Climate Modeling
The early Martian climate is a long-standing problem in planetary science. Evidence for flowing liquid water on the surface 3-4 Ga is conclusive, but Mars' orbital distance the faintness of the young Sun make warming the early climate exceedingly difficult. Various solutions have been proposed over the years, but most previous modelling of early Mars has been limited to simple 1D climate models. Here I describe recent 3D climate simulations with realistic radiative transfer that have shed new light on this problem. In particular, I show how careful comparison between simulation results and the geologic evidence allows us to distinguish "warm and wet" and "cold and icy" scenarios for the early climate. I also discuss the implications of the results for the history of Gale crater and the probability of ancient Martian life.
Aug. 6Matthew HuberUniversity of New HampshireThe Next HyperThermal
After some discussion of the history and application of models in greenhouse paleoclimate contexts, I step through a set of case studies in paleoclimate from Eocene through Miocene in which I assess the ability of models to capture the essence of paleoclimate proxy data interpretations. I present results on both greenhouse gas and non-greenhouse gas climate forcings and discuss some of the opportunities and pitfalls associated focusing only on one or the other potential type of forcing. Along the way, I will cover the topics such as how tectonics and ice sheets affect climate, the stability of the tropical ocean atmosphere circulation structure, the conceptual underpinnings of climate sensitivity, and the ability of models to inform interpretations of paleoclimate information and vice versa. Finally, implications are drawn for the future with an emphasis on heat stress.
Aug. 7Natalie BurlsGeorge Mason UniversityContrasting the Hydrological Cycle in Past and Future Warm Climates - with implications for Ocean Overturning Circulation
The Pliocene Epoch, ~3-5 million years ago, is a particularly interesting period in Earth's history. With a continental configuration similar to present-day, it is the most recent period during which atmospheric CO2 levels are estimated to have been as high as today's anthropogenically-forced levels. SST reconstructions from locations around the globe reveal that the Pliocene was characterized by weak meridional and zonal gradients. This talk briefly reviews the available data together with theory and sensitivity experiments suggesting that changes in cloud radiative forcing played a crucial role in maintaining weak Pliocene gradients. These weak gradients have important implications for the hydrological cycle as illustrated by contrasting a typical quadrupling-of-CO2 experiment against a simulation in which modified cloud properties maintain reduced Pliocene-like SST gradients. Supporting weaker atmospheric circulation, the Pliocene-like simulation is more consistent with reconstructions of Pliocene vegetation suggesting that subtropical regions had enough precipitation to support rich vegetation and fauna. Only with global cooling and the onset of glacial cycles some 3 million years ago did the broad pattern of arid and semi-arid subtropical regions become fully established. Moreover, the associated changes in ocean freshwater forcing give rise to North Pacific deep-water formation and meridional overturning in the Pliocene-like experiment - a result in consistent with calcium carbonate accumulation rates in the subarctic North Pacific.
Aug. 19Malte StueckerUniversity of Hawai`i at MÄnoaENSO/Annual Cycle interactions and their impact on Indo-Pacific climate
Nonlinear interactions between the El Niño-Southern Oscillation (ENSO) and the Western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of atmospheric circulation variability. We demonstrate that C-mode dynamics are responsible for the development of an anomalous low-level North-West Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally anti-symmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on the Asian Monsoon system. In contrast to previous studies, we find the role of air-sea coupling in the Indian Ocean and North-West Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain by near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies. Furthermore, we hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions towards a developing La Niña phase terminates the low-level cyclonic circulation response in the Central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (such as the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally anti-symmetric annual cycle modulated ENSO response (C-mode). We emphasize that it is not adequate to assess ENSO impacts by considering only interannual timescales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability. A general framework for this frequency cascade will be discussed.
Aug. 26Ron StoufferThe History of Computer Users Advisory Board (CUAB) at GFDL
History of CUAB By Ronald J Stouffer The Computer Users Advisory Board (CUAB) was formed on October 20, 1976 by the GFDL Director, Dr. Joseph Smagorinsky. Its charge was to help enhance the utility of the computer facility at GFDL. Functionally CUAB acts as a focal point for user concerns and problems with the computing environment. CUAB helps communicate those issues to people in the Systems Group and to the Front Office (the Director and Deputy Director). This talk will review CUAB's history and outlook for the future. There will also be a discussion time with the two founding members of CUAB.
Sep. 2Alessandra GianniniIRI, Palisades, NYTBD
Sep. 9Meiyun LinNOAA/GFDLThe role of climate variability and large-scale transport on western US surface ozone means and extremes: Implications for seasonal prediction of air quality
I will give an overview of our recent publications on the key drivers of western US surface ozone means and extremes, with a particular focus on how climate variability (e.g. ENSO) modulates deep stratospheric ozone intrusions [Lin M. et al., Nature Communications, 2015] and long-range transport of Asian pollution [Lin M. et al., Nature Geoscience,2014]. Advancing this knowledge is directly relevant for an effective implementation of the lowered US ozone standard - A decision by the EPA Administrator is expected in October 2015. Recognizing the link between known modes of climate variability and regional air quality offers an opportunity to develop seasonal forecast, which could allow public education to reduce health effects.
Sep. 10Steve WofsyHarvard UniversityCase Studies in Regional Scale Inverse Modeling: Surface fluxes of CH4 and CO2 from Arctic and Urban/Industrial Sources
We examine the current state-of-the-art for determining regional scale emissions of greenhouse gases and pollutants, using the "top down" approach that combines high resolution assimilated meteorology, spatial information on sources, and atmospheric measurements, using a Lagrangian model framework. We discuss regional to continental (100 -- 1000 km) scale inverse model results for CH4 emissions over North America, including the Northeast, major natural gas production areas, and Alaska, and also CO2 net fluxes over Alaska. Recent advances promise simulations of observed atmospheric concentrations with higher fidelity than possible hitherto. The emerging analytical framework shjould enable us to deliver strong constraints on the loss rates of natural gas to the atmosphere, emissions of greenhouse gases from the Arctic as climate warms, and a range of other scientific questions with strong societal interest.
Sep. 14Jong-Yeon ParkMax Plank Institute for MeteorologyImpact of bio-geophysical feedback on present climate variability and future climate projection
Phytoplankton, single-celled marine organisms, have attracted increasing attention in climate science due to their geochemical and geophysical impacts on climate systems, such as regulating atmospheric carbon dioxide and modifying oceanic radiant heating. A new generation of climate models can now simulate present and future climate, considering such biological feedbacks through the development of the coupled physical-ecosystem model. Modelling results using both an ocean circulation model and a fully-coupled ocean-atmosphere circulation model, which interact with a biogeochemical model, show that the geophysical feedback of phytoplankton can have significant impacts on the characteristics of the El Niño-Southern Oscillation (ENSO), including its amplitude and asymmetry, as well as on the mean state in the equatorial Pacific. That is, phytoplankton generally decreases mean sea surface temperature (SST) in the eastern equatorial Pacific due to the predominance of indirect dynamical response over direct biological heating. The shoaling of the mean thermocline depth accompanied with the decreased SST amplifies ENSO variability, whereas the interactively-varying phytoplankton slightly dampen ENSO. The skewness of ENSO is also enhanced by mean phytoplankton due to the phase dependency of thermocline feedback. A suite of future warming experiments show that the future phytoplankton change influenced by greenhouse warming can also significantly modify future climate projections, particularly in the Arctic. The warming-induced sea ice melting, and the corresponding increase in shortwave radiation penetrating into the ocean, both result in an increase in phytoplankton in the Arctic. In turn, the phytoplankton increase warms the ocean surface layer through a direct biological heating, triggering additional positive feedbacks in the Arctic, and consequently intensifying the Arctic warming further.
Sep. 16Trevor McDougallUniversity of New South Wales, Sydney, AUSHow does the deep ocean manage to achieve upwelling, and on the thermodynamics of seawater and frazil ice
The upwelling of Bottom Water through density surfaces in the deep ocean is only possible because of the sloping nature of the sea floor. The bottom-intensified mixing activity arising from the interaction of internal tides with bottom topography implies that the dianeutral advection in the ocean interior is downwards, rather than upwards as is required by continuity. A toy model of the abyss will be described that shows that while the upwelling of Bottom Water might be ~25 Sv, this is achieved by very strong upwelling in the bottom turbulent boundary layer of ~125 Sv and strong downwelling in the ocean interior of ~100 Sv. This downwelling occurs within 10 degrees of longitude of the continental boundaries. This clearly has implications for the Stommel-Arons circulation. The second part of the talk discusses the thermodynamically correct ways in which ice and seawater interact based on the 2010 standard of TEOS-10 for seawater ice Ih and humid air:- in particular, how frazil ice should be represented in ocean/ice models.
Sep. 17Lantau SunNOAA CIRES Univeristy of Colorado/ESRLModeling the impact of current and future Arctic sea ice loss on the atmosphere
Arctic sea ice extent is declining at an accelerating pace, and climate models project a seasonally ice-free Arctic by the middle of this century in response to increasing greenhouse gas (GHG) concentrations. The impact of Arctic sea ice loss on lower latitudes in a changing climate is a topic of current research and highly debated. First, we use a multi-model approach to explore the impact of recent sea ice loss on the winter atmosphere, and specifically, test the hypothesis that "warm Arctic, cold continents"is a symptomatic pattern of climate change. The atmospheric circulation response to the recent sea ice loss alone is found to be non-robust across models. Any surface cooling associated with sea ice loss is appreciably smaller than the warming caused by external radiative forcing. Therefore, the paradigm of climate change is better expressed as "warm Arctic, warm continents"for the Northern Hemisphere during winter. The observed recent cooling trend over Central Asia has likely been a low probability state of internal variability, not a fingerprint of forced response to Arctic sea ice loss. By contrast, the atmospheric response to late 21st century Arctic sea ice loss is larger, and robust across models. The mechanism of the stratospheric and tropospheric circulation response will be explored using NCAR's "high-top"climate model, Whole Atmosphere Community Climate Model (WACCM). It is also shown that there is a tug of war in winter between the future Arctic sea ice loss and the GHG effect in causing tropospheric circulation changes, consistent with other modeling studies.
Sep. 17Doug MartinsonColumbia UniversityWarming of Upper Circumpolar Deep Water and Climate Change Impact in West Antarctic Peninsula
Sep. 23Gustavo MarquesGFDLOn the Process Controlling Antarctic Dense Shelf Warters Outflow
Formation of intermediate and abyssal water masses as dense water flows off continental shelves contributes to the lower limb of the meridional overturning circulation. This talk will be focused on two previously unknown processes that may influence the volume flux and physical properties of dense shelf water outflows around Antarctica. First, idealized and realistic numerical simulations will be used to show that Antarctic outflows can excite topographic vorticity waves (TVWs). The modeled waves are sufficiently energetic to play an important role in cross-slope water mass exchanges and Antarctic Bottom Water production. The wave frequency depends on the amount of stretching in the ambient fluid over the outflow and on the background along-slope mean flow. Frequency is higher for steeper bottom slope, larger outflow density anomaly, and stronger westward mean flow. For weak stratification and weak westward along-slope flows typical of the Antarctic slope, wave energy propagates eastward, in the opposite direction from phase velocity. These findings are consistent with recent observations of TVWs in the southern Weddell Sea and with a realistic simulation of the Ross Sea. Second, high-resolution numerical simulations that reveal the formation of a double plume pattern in oceanic outflows will be presented. Double plumes, previously observed in laboratory studies, carry water properties from the shelf into the deep ocean at two distinct depths. An important characteristic of the double plume regime is the flow transition from a supercritical condition, where the Froude number (Fr) is greater than one, to a slower and more uniform subcritical condition (Fr < 1). This transition is associated with an internal hydraulic jump and consequent mixing enhancement. The parameters needed to identify this flow regime will be discussed. These results are the first evidence that the double plume pattern may occur in oceanic environments, such as the Antarctic outflows.
Sep. 24Aiguo Dai University at AlbanyWhat Has Caused the Global Warming Hiatus Since Year 2000?
Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming. The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific, intensifying trade winds, changes in El Niño activity, increasing volcanic activity and decreasing solar irradiance. Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood. Here we analyze observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the difference between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called ‘hiatus' period since about2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns different from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.
Sep. 29Dr. Hailey ShinNCARTBDToward Scale-Aware Parameterization for the Convective Boundary Layer: From an LES Analysis to a Simple Model
The gray zone for parameterizing a physical process in atmospheric numerical models is defined as the range of model resolution, in which the physical process is partly resolved but yet needs to be parameterized at the same time. In the past few years, running mesoscale atmospheric models with horizontal grid spacing of a few kilometers or finer has been becoming practicable and its advantages have been reported. Meanwhile, the gray-zone problem in parameterizing subgrid-scale (SGS) turbulent transport in the convective boundary layer (CBL) has remained at the kilometer- and sub-kilometer grid sizes, leading a number of studies to develop scale-aware parameterizations. This talk introduces one of the attempts. In the first half of this talk, a reference resolution dependency is suggested for the SGS vertical transport in the CBL. The reference is obtained by spatially filtering large-eddy simulation (LES) output, and it is considered as a true solution in the gray zone. Then, traditional CBL parameterizations are evaluated against the reference. In the second half of this talk, a simple model for representing the SGS transport in the gray zone grid is introduced. The model uses a prescribed vertical transport profile and resolution dependency functions, which are fit to the LES and reference.
Sep. 30Bill AndereggPrinceton UniversityDrought impacts on forests and the carbon cycle
The impacts of climate extremes on terrestrial ecosystems are poorly understood but central for predicting carbon cycle feedbacks to climate change. Coupled climate-carbon cycle models typically assume that vegetation recovery from extreme drought is immediate and complete, which conflicts with basic plant physiological understanding. We examine the recovery of tree stem growth after severe drought at 1,338 forest sites globally comprising 49,339 site-years and compare it to simulated recovery in climate-vegetation models. We find pervasive and substantial "legacy effects"of reduced growth and incomplete recovery for 1-4 years after severe drought, and that legacy effects are most prevalent in dry ecosystems, Pinaceae, and species with low hydraulic safety margins. In contrast, no or limited legacy effects are simulated in current climate-vegetation models after drought. Our results highlight hysteresis in ecosystem carbon cycling and delayed recovery from climate extremes.
Oct. 1Yi DengGeorgia Institute of TechnologyLinking Weather and Climate
Problems of weather and climate are historically treated separately in the field of atmospheric sciences despite that the phenomena of weather and climate span continuous temporal and spatial scales. The interactions among atmospheric disturbances residing in different frequency bands form the foundation of a weather-climate continuum. To further understand the connection between long-term climate variations and short-term, highimpact weather, we investigate the role of sub-monthly-scale atmospheric disturbances in driving subseasonal to decadal variability in regional water and energy cycle including the formation of extreme events. In the context of the weather-climate continuum, I will present and discuss recent diagnostic and modeling results related to 1) Atmospheric Rivers (AR) near the west coast of North America; 2) decadal-scale variability in the Arctic precipitation, 3) temperature footprints of El Niño in the northern extratropical atmosphere, and 4) the "weather-connections"among extratropical teleconnection patterns.
Oct. 7Ben-Jei TsuangNOAAOcean warm layer and MJO resolved coupled model for extended-weather forecasting
Here we show that coupling a high-resolution one-column ocean model to an atmospheric general circulation model dramatically improves simulation of the Madden-Julian oscillation (MJO) to have realistic strength, period, and propagation speed. The mechanism for the simulated MJO involves both frictional wave-convective conditional instability of the second kind (Frictional wave-CISK) and air-sea convective intraseasonal interaction (ASCII). In particular, better resolving the fine structure of upper ocean temperature, especially the warm layer, produces more vigorous atmosphere-ocean interaction and strengthens intraseasonal variations in both SST and atmospheric circulation. This helps organize and strengthen deep convection, inducing a stronger Kelvin-wave like perturbation and frictional near-surface convergence to the east. In addition, the warmer SST ahead of the MJO also acts to destabilize the boundary layer and enhance frictional convergence. These lead to a more realistic eastward-propagating MJO. A suite of sensitivity experiments were performed to show the robustness of the mechanisms and to demonstrate: (1) that mean state differences are not the main contributors to the improved simulation of our coupled model; (2) the role of SST variability in enhancing frictional convergence and intraseasonal variations in precipitation, and (3) that the simulation is significantly degraded when the first ocean model layer is thicker than 10 m. Our coupled model results are consistent with observations and demonstrate a simple but effective means to significantly improve MJO simulation and potentially also forecasts.
Oct. 8Amy ClementRSMAS - MiamiThe Atlantic Multidecadal Oscillation Without a Role for Ocean Circulation
The Atlantic Multi-decadal Oscillation (AMO) is a major mode of climate variability with important societal impacts. Most previous explanations identify the driver of the AMO as the ocean circulation, specifically the Atlantic Meridional Overturning Circulation (AMOC). Here we show that the main features of the observed AMO are reproduced in models where the ocean heat transport is prescribed and thus cannot be the driver. Allowing the ocean circulation to interact with the atmosphere does not significantly alter the characteristics of the AMO in current-generation climate models. These results suggest that the AMO is the response to stochastic forcing from the mid-latitude atmospheric circulation, with thermal coupling playing a role in the tropics. In this view, the AMOC and other ocean circulation changes would be largely a response, not a cause of the AMO.
Oct. 14Dr. Victor BrovkinMax-Planck-Institute of MeteorologyGlobal-scale interactions between climate and vegetation cover
Processes in terrestrial ecosystems, to large extent, are controlled by climate and CO2 concentration. In turn, geographical distribution of vegetation cover strongly affects heat, moisture, and momentum fluxes between land surface and atmosphere. These interactions form different feedback loops between terrestrial biosphere and climate, which modulate substantially the climate system dynamics on different time scales. Research on climate-vegetation interactions is mostly concentrated on "hot spots"where the interaction is the most significant: boreal forests, North Africa, and Amazon forest. Boreal forests, even deciduous ones, significantly reduce the albedo of snow-covered surfaces. Simulations with different climate models reveal that boreal deforestation cools the climate, but positive feedbacks between forest and surface air temperature in the boreal region are not strong enough to establish multiple steady states. In general, the climate models agree that tropical deforestation exerts a net regional warming while a global effect on climate is less certain. In the Sahel/Sahara region, several models are able to simulate „green Sahara" phenomenon during the mid-Holocene. Some models reveal multiple steady states in the region due to strong interaction between vegetation and monsoon precipitation. Sensitivity simulations show that some expansion of vegetation cover into the Sahara is possible under CO2-induced climate changes. Other examples of vegetation-atmosphere interactions in the most recent climate model intercomparison (CMIP5) and plans for the next model intercomparison (CMIP6) will be presented.
Oct. 21Till WagnerScripps Institute of OceanographyHow climate model complexity influences sea ice stability, and how not to predict a tipping point
Abstract: The change in albedo when sea ice is replaced by open water introduces a nonlinearity that has sparked an ongoing debate about the stability of the Arctic sea ice cover and the possibility of Arctic ``tipping points''. Studies using idealized climate models often find such instabilities. GCMs, on the other, typically do not simulate them. To help bridge the gap between idealized models and GCMs, I will present an idealized model that includes both latitudinal and seasonal variations. We find that the stability of the ice cover vastly increases with the inclusion of meridional heat transport or a solar seasonal cycle, being most stable when both of these are included. If we set the associated parameters to values that correspond to the current climate, the ice retreat is reversible and there is no instability when the climate is warmed. Furthermore, it has been suggested that "critical slowing down"(and specifically, rising autocorrelation) may be used as an early warning signal for a sea ice tipping point. Using our model, I will argue that a rise in autocorrelation is ill-suited to predict sea ice instabilities. In fact, it could raise a false alarm under sea ice retreat - warning of a tipping point that is not actually there.
Oct. 22Mary-Louise TimmermansYale UniversityArctic Ocean Scales of Variability and Change
Abstract: This talk will review highlights of recent Arctic Ocean measurements, encompassing a wide range of temporal and spatial scales, in an exploration of ocean drivers of sea ice and climate change. Arctic freshwater dynamics, ocean heat and mixing processes, circulation and eddies, and atmosphere-ice-ocean interactions and their interrelationships will be surveyed. Observations indicate apparently rapid changes in the basin-scale freshwater distribution that have marked effects on Arctic stratification. Recent measurements support the idea that a strengthened stratification limits the vertical flux of deep-ocean heat. All ocean layers exhibit a rich mesoscale eddy field. Measurements further reveal an active submesoscale flow field that modifies heat, salt, and momentum fluxes between the ocean and adjacent sea-ice cover. Pervasive double-diffusive structures link the smallest to the largest spatial scales of variability in the Arctic, and allow for constraints to be placed on the regional and temporal changes in the transfer of heat and salt.
Oct. 28Jingqiu MaoGFDLThe role of biogenic volatile organic compounds (VOCs) in climate system
Oct. 29Jadwiga RichterNCAR, Boulder COWhy do GCMs need a well-resolved stratosphere to get El Nino impacts right?
The effects of the tropical Pacific El Nino-Southern Oscillation (ENSO) phenomenon are communicated to the rest of the globe via atmospheric teleconnections. Traditionally, ENSO teleconnections have been viewed as tropospheric phenomena, propagating to higher latitudes as Rossby waves. Recent studies, however, suggest an influence of the stratosphere on extra-tropical ENSO teleconnections. The primary modes of variability in the stratosphere are sudden stratospheric warmings (SSWs) and the tropical quasi-biennial oscillation (QBO). Here, we will show results from a) a 10-member ensemble of AMIP-type simulations (1952 - 2001) and b) 40-member seasonal forecast ensembles for selected El Nino years, and demonstrate the effects of SSWs and the QBO on the extra-tropical El Nino response. We compare simulations with the default Community Atmosphere Model, version 5 (CAM5) as well as one with a better-resolved stratosphere and increased model lid. We show that a well-resolved stratosphere, especially SSWs, have a significant influence on El Nino impacts in climate as well as seasonal forecasting simulations.
Nov. 4Sarah PurkeyColumbia UniversityA decline in strength of the bottom limb of the Meridional Overturning Circulation and implications for the global heat and sea level rise budgets
> Over the past three decades, the bottom limb of the Meridional Overturning Circulation (MOC) has shown significant variability; with large implications for the global heat and sea level rise (SLR) budgets. Here, I give an overview of the variability and recent trends observed in the abyssal oceans and present results suggesting the recent trends are being driven by a decrease in deep water production in the Southern Ocean. Using both direct measurements of recent warming and by coupling in-situ ocean data with satellite altimetry to preform regional and global SLR budgets, we show the global abyssal ocean is warming and contributing roughly 5% to the global SLR budget through thermal expansion. This global scale warming is associated with a decline in volume of the cold Antarctic Bottom Water (AABW) that feeds the bottom limb of the MOC. To further explore the recent volume loss of AABW in the interior ocean, we use conservative tracers collected along a repeat section in the Pacific directly down stream from one of the AABW formation sites to quantify a decrease in the ventilation rate of the AABW and show the deep waters are 5-10 years older in 2011 than in 1992. This analysis provides strong evidence that the rate of ventilation and circulation within the abyssal Pacific sector of the Southern Ocean has significantly decreased over the past 20 years, driving the observed global scale warming of the abyssal ocean.
Nov. 4John MitchellChief Scientist, UKMET (Hadley Center)Detection/Attribution does it really work?
Nov. 10Rong WangLaboratoire des Sciences du Climat et de l’Environnement (LSCE), FranceUnderstanding the role of aerosols in the Earth System - knowledge from a preliminary research
Aerosols are important in the Earth System for a dual role in climate change and air pollution. Since my PhD study, I have been working on understanding the role of aerosols in the Earth System under supervision by prestigious scientists in China and France. My work can be divided into 2 topics related to aerosol effects on the Earth System: 1) the effects of black carbon (BC) on radiative forcing and human health and 2) of anthropogenic aerosols on ocean productivity. In the first part of my presentation, I will talk about the radiative forcing of BC. It is well known that BC contributes to global warming by absorbing sunlight. However, the size of this contribution, namely the direct radiative forcing (RF), ranges from +0.1 to +1.0 W m-2, largely due to differences between bottom-up and observation-based estimates. Current global models systematically underestimate BC radiation absorption relative to observations, which was mainly attributed to the underestimation of BC emissions. Adjusting the emissions to correct this misfit of global models resulted in a revised upward estimate of the BC RF (+0.7 to +0.9 W m-2). However, the observation-constrained BC RF as well as its uncertainty was not estimated in a rigours way in previous studies. Here we simulated the BC radiation absorption at 10-km resolution by developing a new emission inventory with a high-resolution AGCM. As a result, the underestimation in BC radiation absorption was reduc ed by 90% in Asia and by 46% globally. Furthermore, we developed a Bayesian method that fully accounts for the modeling uncertainties and observational representativeness error to estimate the BC radiation absorption and RF. As a result, the uncertainty in BC RF got reduced by 62% over Asia and by 40% over other continental regions. Finally, we derived an observation-constrained BC RF of 0.37 Wm-2 (0.10 to 0.83 as 90% confidence) as our best estimate, half of previous estimates, with the uncertainty reduced by 40%. Our estimate implies that reduction in BC emission would contribute less than previously thought to slow down global warming. In addition, our study confirms a very high level of human exposure to BC in Asia, indicating a severe health risk. In the second part, I will talk about biogeochemical effects of anthropogenic aerosols on ocean biogeochemistry. Nutrient limitation in the ecosystems is increasingly concerned in modeling the carbon cycle in the Earth System. Increasing atmospheric deposition of nutrients interacts with the biogeochemical cycle, but the impact on ocean productivity under changing climate is poorly understood. Here, under a Marie Curie Project funded by European Commission, we systematically studied the sources, transport and deposition of anthropogenic nutrients, including nitrogen (N), phosphorus (P) and iron (Fe) as well as the biogeochemical effect on ocean primary productivity (NPP) using the IPSL's ESM. In particular, we provided a new estimation of the budget of P in the atmosphere. Moreover, in the modeling of oceans, the fertilizing effect of anthropogenic aerosol deposition on ocean NPP was considered under changing climate. As a result, we found global warming was mainly responsibl e for decline of oceanic NPP due to enhanced ocean stratification from 1948 to 2008. We also found that, interestingly, the sensitivity of NPP to warming was significantly altered by anthropogenic N, P and Fe deposition, which partly offsets the decline of NPP in the oceans. We predicted that decline of ocean NPP due to warming might be accelerated if aerosol emissions are mitigated in the future.
Nov. 12INFORMAL SEMINAR - Marjolein van HuijgevoorUniversity of Aberdeen-United KingdomHydrological drought - Linking models and observations across scales and climates
Drought can have large environmental and socio-economic impacts across the globe. Knowledge about the development and recovery of hydrological drought (groundwater, rivers) is a prerequisite to safeguard water availability in the future. Catchment features, in particular subsurface storage, are important for drought propagation through the earth system, but are not always well represented in global hydrological models. I will present results from a multi-model analysis aimed at the correct identification of past drought events and predictions of changes in drought events in the future at different scales (global, continental, river basin).
Nov. 13Tim PalmerOxford UniversityMore accuracy with less precision: a stochastic paradigm for weather and climate prediction
Weather and climate prediction involves solving nonlinear partial differential equations whose natural truncation scales are many orders of magnitude smaller than can be achieved in practice. This raises the theoretical question at the interface between PDE theory and computational science: What is the real information content in the variables we seek to predict, as a function of scale? In turn this raises the practical question: Are we squandering valuable resources (energy and hence computational) by representing these variables computationally with more precision than is justified theoretically? How much more accurately could we predict weather and climate if we could utilise these resources more efficiently?
Nov. 13Nadir JeevanjeeUniversity of California (Berkeley)Clouds, from the bottom-up and top-down
Progress in understanding and parameterizing clouds will likely require both a `bottom-up' approach of studying their basic physics, as well as a `top-down' approach of parsing uncertainties in the relevant model parameterizations (e.g. turbulent, convective, microphysical). In the `bottom-up' category I will present recent work analyzing the acceleration of buoyant air parcels, and how this acceleration depends on a parcel's aspect ratio and surface proximity. In the `top-down' category I will discuss a proposal to use the modular parameterization schemes of some cloud-resolving models to run `multi-model' climate change experiments analogous to those of CMIP, but with resolved clouds and a clear understanding of exactly how ensemble members differ. Such experiments could identify dominant sources of uncertainty in climate sensitivity, and thus guide our `bottom-up' investigations. Furthermore, both studies mentioned here have implications for our current transition from hydrostatic to non-hydrostatic global modeling.
Nov. 16Eric GuilyardiInstitut Pierre Simon Laplace (IPSL) in FranceReconstructing extreme AMOC events through nudging of the ocean surface
Rapid and large changes in the Atlantic meridional overturning circulation (AMOC) can strongly impact climate at the global scale. These rapid fluctuations emerge frequently in models as a result of internal climate variability, but in the real world, the lack of long-enough continuous observations has prevented their identification. Indirect estimates of past AMOC variability can be obtained through the use of climate models, following different assimilation techniques. However, the fidelity of these products can only be partially validated with the limited in situ measurements available. To overcome some of these limitations, we here follow a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several nudging techniques towards different sets of surface variables (i.e. sea surface temperature and salinity relaxation, wind stress restoring) in reconstructing the simulated AMOC variability. The motivation to use only surface nudging comes from the longer well-observed surface ocean variables when compared to the sub-surface. We first use the standard 2-months relaxation time scale for surface restoring, classically used for ocean-only simulations. A specific focus is made on the representation of an extreme positive peak in the target control simulation used as "surrogate reality". Our analysis highlights the sensitivity to the initial conditions, and recommends the use of an ensemble of nudged simulations to guarantee a correct estimate of uncertainty. All the standard nudging approaches used here succeed in reproducing the timing of the extreme AMOC peak, but underestimate its amplitude. A careful analysis of the AMOC precursors reveals that this underestimation comes from a deficit in the formation of the dense water masses in the main regions of convection. This issue is largely corrected in an improved nudged simulation that uses a varying relaxation term, proportional to the mixed layer depth. This development improves the restoring of surface temperature and salinity in the regions of convection, and eventually the representation of AMOC variability, preventing unphysical restoring fluxes elsewhere. This is therefore a promising nudging strategy that applied to the real world can help to better constrain the recent AMOC variability over the last few decades.
Nov. 17Dr. Brandon ReichlUniversity of Rhode IslandSurface wave impacts on air-sea momentum flux and upper ocean turbulence under tropical cyclones
Under tropical cyclones the air-sea interface is characterized by complex wave fields that influence air-sea interaction and near surface physics. An overview of two processes that are modified by surface waves under tropical cyclones will be presented. The air-sea momentum flux (or wind stress) varies due to the surface waves because of the complexity of the wave form stress under tropical cyclone winds. The magnitude of the wind stress varies by greater than a factor of 2 due to different parameterizations of the high frequency component of the wave spectrum (wavelength less than ~20 m). The direction of the wind stress varies by more than 10 degrees in certain conditions due to assumptions about the wave growth-rate and the mean wind profile. The surface waves also drive a Lagrangian current (the Stokes drift) that interacts with background turbulence in the near-surface ocean to increase mixing (the Langmuir turbulence, LT). An upper-ocean turbulence closure model (the K-Profile Parameterization, KPP) is modified to account for LT under tropical cyclones by adding an enhancement to the mixing coefficient and a Stokes-gradient mixing contribution. One-dimensional column model simulations with KPP are compared to parallel Large Eddy Simulations (that resolve the turbulence) to evaluate the KPP model. The KPP with explicit LT is then implemented in a basin-scale three-dimensional ocean model to examine the impact of LT on the ocean response to a tropical cyclone. It is found that LT modifies the three-dimensional dynamics including reducing the storm-induced upwelling and reducing the horizontal advection in the cold-wake. Models that use a version of KPP tuned to implicitly include the mean impact of LT on vertical mixing are not sufficient for predicting upper ocean temperature and currents compared to the explicit LT model.
Nov. 18Yohan Ruprich-RobertPacific impacts of the Atlantic Multidecadal Vartiability
The North Atlantic sea surface temperature (SST) anomalies are characterized by a marked low frequency variability known as the Atlantic Multidecadal Variability (AMV). The AMV has been shown to modulate climate over many areas of the globe including Europe, North America, Africa, South East Asia or South America, but the physical processes behind several of these teleconnections are not well understood. In this study we largely focus on the link between the AMV and the Pacific variability. We present experiment based on the GFDL CM2.1 and NCAR CESM1 models in which SSTs in the North Atlantic sector are restored to the observed AMV pattern, while other basins are left fully coupled. In order to isolate the AMV impacts and maximize the signal-to-noise ratio, we use large ensemble simulation run for 10 years (100 and 30 members for CM2.1 and CESM1, respectively). We show that a positive AMV phase leads to the set-up of a negative Interdecadal Pacific Variability pattern characterized by cold anomalies in the tropical Pacific and warm anomalies in the western Pacific mid-latitudes. The response over the tropical Pacific Ocean to a positive phase of the AMV is similar to that observed during a La Niña event. However, the AMV-induced anomalies are not stationary. We investigate the frequency of occurrence of El Niño and La Niña events in response to a shift of the AMV from a negative to a positive phase and found that the probability of La Niña events increase by a factor 2 during the first 3 years succeeding this shift. The response tends then to show more El Niño events during a period of about 3 years before to show again an increase of La Niña events. We propose a physical mechanism to explain this modulation of ENSO by the North Atlantic variability.
Nov. 19Marty SinghHarvard UniversityRevisiting the role of eddies in determining the strength of the Hadley circulation
Large-scale eddies originating in midlatitudes have been shown to play an important role in determining the strength of the Hadley circulation. Previous work has focused primarily on the atmospheric angular-momentum budget; in a flow regime where non-linear advection of angular momentum by the mean circulation is weak, eddy momentum fluxes strongly constrain the circulation strength. However, depending on the time of year, Earth's Hadley circulation may be relatively far from the linear limit. Here, we use idealized simulations on an equatorial beta-plane to explore the response of the Hadley circulation to large-scale eddies in a regime where non-linear momentum advection is important. In simulations run with a fixed distribution of sea-surface temperature (SST), the Hadley circulation strength is relatively insensitive to the presence of large-scale eddies despite their strong influence on the angular-momentum budget. In slab-ocean simulations, on the other hand, the eddies act to amplify the Hadley circulation substantially. The importance of the SST distribution for the Hadley circulation strength in our simulations suggests that one should consider the energy budget as well as the angular-momentum budget in order to fully understand the influence of large-scale eddies on the tropical-mean circulation.
Dec. 4Dr. Justin E. StopaIFREMER Labortarie d'Oceangraphie Spatiale, Plouzane, FranceWave Attenuation in Sea Ice using Synthetic Aperture Radar and Waves in the Arctic Ocean 1992-2014
Sea ice has important implications in the climate system yet relatively little is known about wave-ice interaction. Swell evolution from the open ocean into sea ice is poorly understood, in particular the amplitude attenuation expected from scattering and dissipation. Synthetic aperture radar (SAR) data from Sentinel-1A's wave mode reveal interesting patterns of bright oscillating lines. These patterns are interpreted as distortions created by the SAR processing of two different swell systems. These deviations mirror the response to the orbital velocity toward the satellite and through this process we are able to estimate swell attenuation through sea ice. The decay is consistent with an exponential attenuation on a length scale of 200 km. The calculated attenuation rates are directly applicable to wave-ice modeling. In the past decade the Arctic Ocean has experienced increasing losses of sea ice which has a direct impact on the wave field. The sea state of the Arctic is largely dependent on the ice-free area and wind. Due to the lack of detailed wave information in the Arctic Ocean, we use a numerical wave model and altimetry data to describe the climate for 1992-2014. The wave model is driven by winds from the Climate Forecast System Reanalysis and ice concentrations are derived from satellites on a 12.5 km grid sufficiently able to resolve important features in the marginal ice zone. The seasonal advance and retreat of the ice largely dictates the characteristics of the waves. This creates distinct features in the wind seas and swell fields. The Nordic and Greenland Seas are mostly dominated by swell from the North Atlantic while the coastal regions and semi-enclosed seas of the Kara, Laptev, Chukchi, and Beaufort have more of an equal split between wind waves and swell. Trends in the altimeters and model are in agreement and show increasing wave activities in the Beaufort Sea, Chukchi Sea, Laptev Sea, Kara Sea, and Baffin Bay. It is confirmed that the trends are largely related to the loss of ice rather than the change in winds. In the Nordic and Greenland Seas, there are decreasing trends mainly related to the changes in the local wind forcing. The waves also directly influence the ice sheet and storms are expected to impact the marginal ice zone. The crux of the research identifies the need for continued study and improvement of wave-ice interaction.
Dec. 9Laura WilcoxUniversity of Reading, UKThe role of anthropogenic aerosol in global and regional change
There are many species of anthropogenic aerosol. The first part of this talk focuses only on sulphate aerosol. Sulphate aerosol can interact with the radiation balance directly by scattering radiation, or indirectly through their interactions with clouds. Overall, sulphate produces a negative radiative forcing of climate, causing a cooler surface and decreases in precipitation. The magnitude of the indirect effect is currently uncertain, and its representation in models is diverse. I will explore the global response to the indirect effects of sulphate changes in CMIP5, and quantify the contributions to the uncertainty in this response. In the second half of the talk, I will quantify the role of all species of anthropogenic aerosol in historical circulation changes, with a particular focus on the position of the jets and the width of the tropics. I will ask whether the magnitude of the circulation response to aerosol changes can be related to aerosol radiative forcing, or to the diversity in the representation of sulphate indirect effects. Moving to sub-continental scales, I will identify geographical regions that are particularly sensitive to aerosol forcing, and look at how the distribution of these regions changes with time. Using these regions as case studies, I examine the feasibility of characterising the aerosol signature in regional climate change.
Dec. 10Leif ThomasStanford UniversityEnergy exchange between near-inertial waves and balanced currents at ocean fronts
Wind-generated kinetic energy (KE) manifests itself primarily in balanced currents and near-inertial waves. The dynamics of these flows is strongly constrained by the Earth's rotation, causing the KE in balanced currents to follow an inverse cascade but also preventing wave-wave interactions from fluxing energy in the near-inertial band to lower frequencies and higher vertical wavenumbers. How wind-generated KE is transferred to small-scale turbulence and dissipated is thus a non-trivial problem. In this talk I will describe theoretical calculations and numerical simulations that demonstrate how bizarre modifications to internal wave physics by lateral density gradients present at ocean fronts, allow for strong interactions between balanced currents and near-inertial waves that ultimately result in energy loss for both types of motion.
Dec. 11Dr. Paulo NobreInstitute Nacional De Pesquisas Espaciais- BrazilThe development of the Brazilian ESM - BESM for CMIP simulations
In addition, the following will be accompanying Dr. Nobre: Vinicius Capistrano: Results of coupling MOM5 into BESM Demerval Moreira: Coupling INLAND surface model into CM2.1: preliminary results. Arildo Silva: The development of BESM runtime environment. The development of the Brazilian Earth System Model for CMIP simulations. Paulo Nobre and collaborators; INPE, Brazil. The strategy adopted toward the development of the Brazilian ESM (BESM) is presented. The model uses NOAA/GFDL's FMS coupler with CPTEC's spectral AGCM (at resolution T062L28 and SSIB vegetation model) and NOAA/GFDL MOM4p1 (at ~1 degree lat-lon and 50 z-levels, with marine ice - SIS). BESM2.3.1 was used to generate Brazil's contribution to CMIP5, with 10,000+ years of global simulations. Distinctive features of BESM2.3.1 are presented, namely those related to a better representation of cloud cover parameterization and its impacts on Amazon rainfall and global atmospheric circulation patterns. An improved version of BESM2.5 is under development for CMIP6, which incorporates several enhancements on the model physics, including but not limited to: a moist PBL; atmospheric aerosols and chemistry - MOZART; dynamical vegetation with surface hydrology and forest fire - INLAND.