Bibliography - Thomas L Delworth

1. Chang, You-Soon, Shaoqing Zhang, Anthony Rosati, Thomas L Delworth, and William F Stern, February 2013: An assessment of oceanic variability for 1960–2010 from the GFDL ensemble coupled data assimilation. Climate Dynamics, 40(3-4), doi:10.1007/s00382-012-1412-2.
   [ Abstract ]

   The Geophysical Fluid Dynamics Laboratory has developed an ensemble coupled data assimilation (ECDA) system based on the fully coupled climate model, CM2.1, in order to provide reanalyzed coupled initial conditions that are balanced with the climate prediction model. Here, we conduct a comprehensive assessment for the oceanic variability from the latest version of the ECDA analyzed for 51 years, 1960–2010. Meridional oceanic heat transport, net ocean surface heat flux, wind stress, sea surface height, top 300 m heat content, tropical temperature, salinity and currents are compared with various in situ observations and reanalyses by employing similar configurations with the assessment of the NCEP’s climate forecast system reanalysis (Xue et al. in Clim Dyn 37(11):2511–2539, 2011). Results show that the ECDA agrees well with observations in both climatology and variability for 51 years. For the simulation of the Tropical Atlantic Ocean and global salinity variability, the ECDA shows a good performance compared to existing reanalyses. The ECDA also shows no significant drift in the deep ocean temperature and salinity. While systematic model biases are mostly corrected with the coupled data assimilation, some biases (e.g., strong trade winds, weak westerly winds and warm SST in the southern oceans, subsurface temperature and salinity biases along the equatorial western Pacific boundary, overestimating the mixed layer depth around the subpolar Atlantic and high-latitude southern oceans in the winter seasons) are not completely eliminated. Mean biases such as strong South Equatorial Current, weak Equatorial Under Current, and weak Atlantic overturning transport are generated during the assimilation procedure, but their variabilities are well simulated. In terms of climate variability, the ECDA provides good simulations of the dominant oceanic signals associated with El Nino and Southern Oscillation, Indian Ocean Dipole, Pacific Decadal Oscillation, and Atlantic Meridional Overturning Circulation during the whole analyzed period, 1960–2010.

2. Doi, Takeshi, Gabriel A Vecchi, Anthony Rosati, and Thomas L Delworth, in press: Response to CO2 doubling of the Atlantic Hurricane Main Development Region in a High-Resolution Climate Model. Journal of Climate. doi:10.1175/JCLI-D-12-00110.1. 2/13.
   [ Abstract ]

   Response of climate conditions in the Atlantic Hurricane Main Development Region (MDR) to doubling of atmospheric CO2 has been explored, using the new high-resolution coupled Climate Model version 2.5 developed at the Geophysical Fluid Dynamics Laboratory (GFDL-CM2.5). In the annual mean, the SST in the MDR warms by about 2°C in the CO2 doubling run relative to the Control run, the trade winds become weaker in the northern tropical Atlantic, and the rainfall increases over the ITCZ and its northern region. The amplitude of the annual cycle of the SST over the MDR is not significantly changed by CO2 doubling. However, we find that the interannual variations show significant responses to CO2 doubling: the seasonal maximum peak of the interannual variations of the SST over the MDR is about 25% stronger than in the Control run. The enhancement of the interannual variations of the SST in the MDR is due to changes in effectiveness of the Wind-Evaporation-SST (WES) positive feedback: WES remains a positive feedback until boreal early summer in the CO2 doubling run. The enhancement of the interannual variability of the SST over the MDR in boreal early summer due to CO2 doubling could lead to serious damages associated with the Atlantic Hurricane count and drought (or flood) in the Sahel and South America in a future climate.

3. Goddard, L M., Rym Msadek, and Thomas L Delworth, et al., January 2013: A verification framework for interannual-to-decadal predictions experiments. Climate Dynamics, 40(1-2), doi:10.1007/s00382-012-1481-2.
   [ Abstract ]

   Decadal predictions have a high profile in the climate science community and beyond, yet very little is known about their skill. Nor is there any agreed protocol for estimating their skill. This paper proposes a sound and coordinated framework for verification of decadal hindcast experiments. The framework is illustrated for decadal hindcasts tailored to meet the requirements and specifications of CMIP5 (Coupled Model Intercomparison Project phase 5). The chosen metrics address key questions about the information content in initialized decadal hindcasts. These questions are: (1) Do the initial conditions in the hindcasts lead to more accurate predictions of the climate, compared to un-initialized climate change projections? and (2) Is the prediction model’s ensemble spread an appropriate representation of forecast uncertainty on average? The first question is addressed through deterministic metrics that compare the initialized and uninitialized hindcasts. The second question is addressed through a probabilistic metric applied to the initialized hindcasts and comparing different ways to ascribe forecast uncertainty. Verification is advocated at smoothed regional scales that can illuminate broad areas of predictability, as well as at the grid scale, since many users of the decadal prediction experiments who feed the climate data into applications or decision models will use the data at grid scale, or downscale it to even higher resolution. An overall statement on skill of CMIP5 decadal hindcasts is not the aim of this paper. The results presented are only illustrative of the framework, which would enable such studies. However, broad conclusions that are beginning to emerge from the CMIP5 results include (1) Most predictability at the interannual-to-decadal scale, relative to climatological averages, comes from external forcing, particularly for temperature; (2) though moderate, additional skill is added by the initial conditions over what is imparted by external forcing alone; however, the impact of initialization may result in overall worse predictions in some regions than provided by uninitialized climate change projections; (3) limited hindcast records and the dearth of climate-quality observational data impede our ability to quantify expected skill as well as model biases; and (4) as is common to seasonal-to-interannual model predictions, the spread of the ensemble members is not necessarily a good representation of forecast uncertainty. The authors recommend that this framework be adopted to serve as a starting point to compare prediction quality across prediction systems. The framework can provide a baseline against which future improvements can be quantified. The framework also provides guidance on the use of these model predictions, which differ in fundamental ways from the climate change projections that much of the community has become familiar with, including adjustment of mean and conditional biases, and consideration of how to best approach forecast uncertainty.

4. Kapnick, S B., and Thomas L Delworth, in press: Controls of Global Snow Under a Changed Climate. Journal of Climate. doi:10.1175/JCLI-D-12-00528.1. 2/13.
   [ Abstract ]

   This study assesses the ability of a newly developed high-resolution coupled model from the Geophysical Fluid Dynamics Laboratory to simulate the cold-season hydroclimate in the present climate, and examines its response to climate change forcing. Output is assessed from a 280-yr control simulation based on 1990 atmospheric composition and an idealized 140-yr future simulation where atmospheric CO2 increases at 1% yr−1 until doubling in year 70 and then remains constant. When compared to a low-resolution model, the high-resolution model is found to better represent the geographic distribution of snow variables in the present climate. In response to idealized radiative forcing changes, both models produce similar global-scale responses where global-mean temperature and total precipitation increase while snowfall decreases. Zonally, snowfall tends to decrease in the low to mid latitudes and increase in the mid to high latitudes. At the regional scale, the high and low-resolution models sometimes diverge in the sign of projected snowfall changes; the high-resolution model exhibits future increases in a few select high altitude regions, notably the northwestern Himalaya region and small regions in the Andes and southwestern Yukon. Despite such local signals, there is an almost universal reduction in snowfall as a percent of total precipitation in both models. Using a simple multivariate model, temperature is shown to drive these trends by decreasing snowfall almost everywhere while precipitation increases snowfall in the high altitudes and mid to high latitudes. Mountainous regions of snowfall increases in the high-resolution model exhibit a unique dominance of the positive contribution from precipitation over temperature.

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

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

6. Msadek, Rym, W E Johns, S G Yeager, G Danabasoglu, Thomas L Delworth, and Anthony Rosati, in press: The Atlantic Meridional Heat transport at 26.5° N and its relationship with the MOC in the RAPID array and the GFDL and NCAR coupled models. Journal of Climate. doi:10.1175/JCLI-D-12-00081.1. 1/13.
   [ Abstract ]

   The link at 26.5° N between the Atlantic meridional heat transport (MHT) and the Atlantic meridional overturning circulation (MOC) is investigated in two climate models, GFDL CM2.1 and NCAR CCSM4, and compared with the recent observational estimates from the RAPID-MOCHA array. Despite a stronger than observed MOC magnitude, both models underestimate the mean MHT at 26.5° N due to an overly diffuse thermocline. Biases result from errors in both overturning and gyre components of the MHT. The observed linear relationship between MHT and MOC at 26.5° N is realistically simulated by the two models and is mainly due to the overturning component of the MHT. Fluctuations in overturning MHT are dominated by Ekman transport variability in CM2.1 and CCSM4, whereas baroclinic geostrophic transport variability plays a larger role in RAPID. CCSM4 which has a parameterization of Nordic Sea overflows and thus a more realistic North Atlantic Deep Water (NADW) penetration shows smaller biases in the overturning heat transport than CM2.1 due to deeper NADW at colder temperatures. The horizontal gyre heat transport and its sensitivity to the MOC are poorly represented in both models. The wind-driven gyre heat transport is northward in observations at 26.5° N whereas it is weakly southward in both models, reducing the total MHT. We emphasize model biases that are responsible for the too weak MHT, particularly at the western boundary. The use of direct MHT observations through RAPID allows us to identify the source of the too weak MHT in the two models, a bias shared by a number of CMIP5 coupled models.

7. Vecchi, Gabriel A., Rym Msadek, Whit G Anderson, You-Soon Chang, Thomas L Delworth, Keith W Dixon, Rich Gudgel, Anthony Rosati, William F Stern, G Villarini, Andrew T Wittenberg, Xiaosong Yang, Fanrong Zeng, Rong Zhang, and Shaoqing Zhang, in press: Multi-year Predictions of North Atlantic Hurricane Frequency: Promise and limitations. Journal of Climate. doi:10.1175/JCLI-D-12-00464.1. 2/13.
   [ Abstract ]

   Retrospective predictions of multi-year North Atlantic hurricane frequency are explored, by applying a hybrid statistical-dynamical forecast system to initialized and non-initialized multi-year forecasts of tropical Atlantic and tropical mean sea surface temperatures (SSTs) from two global climate model forecast systems. By accounting for impacts of initialization and radiative forcing, retrospective predictions of five-year mean and nine-year mean tropical Atlantic hurricane frequency show significant correlation relative to a null hypothesis of zero correlation. The retrospective correlations are increased in a two-model average forecast and by using a lagged-ensemble approach, with the two-model ensemble decadal forecasts hurricane frequency over 1961-2011 yielding correlation coefficients that approach 0.9. These encouraging retrospective multi-year hurricane predictions, however, should be interpreted with care: although initialized forecasts have higher nominal skill than uninitialized ones, the relatively short record and large autocorrelation of the time series limits our confidence in distinguishing between the skill due to external forcing and that added by initialization. The nominal increase in correlation in the initialized forecasts relative to the uninitialized experiments is due to improved representation of the multi-year tropical Atlantic SST anomalies. The skill in the initialized forecasts comes in large part from the persistence of a mid-1990s shift by the initialized forecasts, rather than from predicting its evolution. Predicting shifts like that observed in 1994-1995 remains a critical issue for the success of multi-year forecasts of Atlantic hurricane frequency. The retrospective forecasts highlight the possibility that changes in observing system impact forecast performance.

8. Wu, X, Shaoqing Zhang, Z Liu, Anthony Rosati, and Thomas L Delworth, April 2013: A study of impact of the geographic dependence of observing system on parameter estimation with an intermediate coupled model. Climate Dynamics, 40(7-8), doi:10.1007/s00382-012-1385-1.
   [ Abstract ]

   Observational information has a strong geographic dependence that may directly influence the quality of parameter estimation in a coupled climate system. Using an intermediate atmosphere-ocean-land coupled model, the impact of geographic dependent observing system on parameter estimation is explored within a ‘‘twin’’ experiment framework. The ‘‘observations’’ produced by a ‘‘truth’’ model are assimilated into an assimilation model in which the most sensitive model parameter has a different geographic structure from the ‘‘truth’’, for retrieving the ‘‘truth’’ geographic structure of the parameter. To examine the influence of data-sparse areas on parameter estimation, the twin experiment is also performed with an observing system in which the observations in some area are removed. Results show that traditional single-valued parameter estimation (SPE) attains a global mean of the ‘‘truth’’, while geographic dependent parameter optimization (GPO) can retrieve the ‘‘truth’’ structure of the parameter and therefore significantly improves estimated states and model predictability. This is especially true when an observing system with data-void areas is applied, where the error of state estimate is reduced by 31 % and the corresponding forecast skill is doubled by GPO compared with SPE.

9. Yang, Xiaosong, Anthony Rosati, Shaoqing Zhang, Thomas L Delworth, Rich Gudgel, Rong Zhang, Gabriel A Vecchi, Whit G Anderson, You-Soon Chang, T DelSole, Keith W Dixon, Rym Msadek, William F Stern, Andrew T Wittenberg, and Fanrong Zeng, January 2013: A predictable AMO-like pattern in GFDL's fully-coupled ensemble initialization and decadal forecasting system. Journal of Climate, 26(2), doi:10.1175/JCLI-D-12-00231.1.
   [ Abstract ]

   The decadal predictability of sea surface temperature (SST) and 2m air temperature (T2m) in Geophysical Fluid Dynamics Laboratory (GFDL)'s decadal hindcasts, which are part of the Fifth Coupled Model Intercomparison Project experiments, has been investigated using an average predictability time (APT) analysis. Comparison of retrospective forecasts initialized using the GFDL's Ensemble Coupled Data Assimilation system with uninitialized historical forcing simulations using the same model, allows identification of internal multidecadal pattern (IMP) for SST and T2m. The IMP of SST is characterized by an inter-hemisphere dipole, with warm anomalies centered in the North Atlantic subpolar gyre region and North Pacific subpolar gyre region, and cold anomalies centered in the Antarctic Circumpolar Current region. The IMP of T2m is characterized by a general bi-polar seesaw, with warm anomalies centered in Greenland, and cold anomalies centered in Antarctica. The retrospective prediction skill of the initialized system, verified against independent observations, indicates that the IMP of SST may be predictable up to 4 (10) year lead time at 95% (90%) significance level, and the IMP of T2m may be predictable up to 2 (10) years at 95% (90%) significance level. The initialization of multidecadal variations of northward oceanic heat transport in the North Atlantic significantly improves the predictive skill of the IMP. The dominant roles of oceanic internal dynamics in decadal prediction are further elucidated by fixed-forcing experiments, in which radiative forcing is returned to 1961 values. These results point towards the possibility of meaningful decadal climate outlooks using dynamical coupled models, if they are appropriately initialized from a sustained climate observing system.

10. Zhang, Shaoqing, Michael Winton, Anthony Rosati, Thomas L Delworth, and B Huang, April 2013: Impact of Enthalpy-Based Ensemble Filtering Sea-Ice Data Assimilation on Decadal Predictions: Simulation with a Conceptual Pycnocline Prediction Model. Journal of Climate, 26(7), doi:10.1175/JCLI-D-11-00714.1.
    [ Abstract ]

    The non-Gaussian probability distribution of sea-ice concentration makes difficulties for directly assimilating sea-ice observations into a climate model. Because of the strong impact of the atmospheric and oceanic forcing on the sea-ice state, any direct assimilation adjustment on sea-ice states is easily overridden by model physics.A new approach implements sea-ice data assimilation in enthalpy space where a sea-ice model represents a nonlinear function that transforms a positive-definite space into the sea-ice concentration subspace.Results from observation-assimilation experiments using a conceptual pycnocline prediction model that characterizes the influences of sea-ice on the decadal variability of the climate system show that the new scheme efficiently assimilates “sea-ice observations” into the model – while improving “sea-ice” variability itself, it consistently improves the estimates of all “climate” components.The resulted coupled initialization that is physically consistent among all coupled components significantly improves decadal-scale predictability of the coupled model.

11. Zhang, Rong, Thomas L Delworth, R Sutton, D Hodson, Keith W Dixon, Isaac M Held, Y Kushnir, D Marshall, Yi Ming, Rym Msadek, J Robson, Anthony Rosati, Mingfang Ting, and Gabriel A Vecchi, April 2013: Have Aerosols Caused the Observed Atlantic Multidecadal Variability? Journal of the Atmospheric Sciences, 70(4), doi:10.1175/JAS-D-12-0331.1.
    [ Abstract ]

    Identifying the prime drivers of the twentieth-century multidecadal variability in the Atlantic Ocean is crucial for predicting how the Atlantic will evolve in the coming decades and the resulting broad impacts on weather and precipitation patterns around the globe. Recently Booth et al (2012) showed that the HadGEM2-ES climate model closely reproduces the observed multidecadal variations of area-averaged North Atlantic sea surface temperature in the 20th century. The multidecadal variations simulated in HadGEM2-ES are primarily driven by aerosol indirect effects that modify net surface shortwave radiation. On the basis of these results, Booth et al (2012) concluded that aerosols are a prime driver of twentieth-century North Atlantic climate variability. However, here it is shown that there are major discrepancies between the HadGEM2-ES simulations and observations in the North Atlantic upper ocean heat content, in the spatial pattern of multidecadal SST changes within and outside the North Atlantic, and in the subpolar North Atlantic sea surface salinity. These discrepancies may be strongly influenced by, and indeed in large part caused by, aerosol effects. It is also shown that the aerosol effects simulated in HadGEM2-ES cannot account for the observed anti-correlation between detrended multidecadal surface and subsurface temperature variations in the tropical North Atlantic. These discrepancies cast considerable doubt on the claim that aerosol forcing drives the bulk of this multidecadal variability.

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

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

13. Delworth, Thomas L., and Fanrong Zeng, July 2012: Multicentennial variability of the Atlantic Meridional Overturning Circulation and its climatic influence in a 4000 year simulation of the GFDL CM2.1 climate model. Geophysical Research Letters, 39, L13702, doi:10.1029/2012GL052107.
    [ Abstract ]

    We investigate decadal to multicentennial variability of Northern Hemisphere surface air temperature in a 4000-year control simulation of the GFDL CM2.1 climate model. Spectral analysis shows the presence of a distinct multicentennial timescle of temperature variability. The associated spatial pattern is broad, covering the entire Northern Hemisphere extratropics, but with enhanced amplitude in the Atlantic and Arctic sectors. This variability appears to be driven by interhemispheric fluctuations in oceanic heat transport associated with the Atlantic Meridional Overturning Circulation (AMOC). The AMOC variability is associated with century-scale propagation of salinity anomalies from the Southern Ocean to the subpolar North Atlantic, with out of phase transport variations between the upper ocean and deeper layers of the Atlantic. When positive (negative) upper ocean salinity anomalies reach the subpolar North Atlantic they strengthen (weaken) the AMOC by modulating upper ocean density and vertical stratification. The large-scale warming also appears to be enhanced by reductions in surface albedo associated with reduced sea-ice and low-level cloudiness, thereby increasing the absorption of shortwave radiation and amplifying the warming from AMOC changes. We speculate that such multicentennial variations in the AMOC could contribute to long-time scale climate fluctuations in the observed paleo record. This could arise purely as internal variability of the climate system, or through radiatively-induced changes to atmospheric circulation patterns, such as the NAO, that would in turn influence the AMOC.

14. Doi, Takeshi, Gabriel A Vecchi, Anthony Rosati, and Thomas L Delworth, August 2012: Biases in the Atlantic ITCZ in seasonal-interannual variations for a coarse and a high resolution coupled climate model. Journal of Climate, 25(16), doi:10.1175/JCLI-D-11-00360.1.
    [ Abstract ]

    Using two fully coupled ocean-atmosphere models of CM2.1 (the Climate Model version 2.1 developed at the Geophysical Fluid Dynamics Laboratory) and CM2.5 (a new high-resolution climate model based on CM2.1), the characteristics and sources of SST and precipitation biases associated with the Atlantic ITCZ have been investigated. CM2.5 has an improved simulation of the annual mean and the annual cycle of the rainfall over the Sahel and the northern South America, while CM2.1 shows excessive Sahel rainfall and lack of northern South America rainfall in boreal summer. This marked improvement in CM2.5 is due to not only high-resolved orography, but also a significant reduction of biases in the seasonal meridional migration of the ITCZ. In particular, the seasonal northward migration of the ITCZ in boreal summer is coupled to the seasonal variation of the SST and a subsurface doming of the thermocline in the northeastern tropical Atlantic, known as the Guinea Dome. Improvements in the ITCZ allow for better representation of the coupled processes that are important for an abrupt seasonally phase-locked decay of the interannual SST anomaly in the northern tropical Atlantic. Nevertheless, the differences between CM2.5 and CM2.1 were not sufficient to reduce the warm SST biases in the eastern equatorial region and Angola-Benguela Area. The weak bias of southerly winds along the southwestern African coast associated with the excessive southward migration bias of the ITCZ may be a key to improve the warm SST biases there.

15. Srokosz, M, and Thomas L Delworth, et al., November 2012: Past, present and future change in the Atlantic meridional overturning circulation. Bulletin of the American Meteorological Society, 93(11), doi:10.1175/BAMS-D-11-00151.1.
    [ Abstract ]

    Observations and numerical modelling experiments provide evidence for links between variability in the Atlantic Meridional Overturning Circulation (AMOC) and global climate patterns. Reduction in the strength of the overturning circulation is thought to have played a key role in rapid climate change in the past and may have the potential to significantly influence climate change in the future, as noted in the last two IPCC assessment reports (2001, 2007). Both IPCC reports also highlighted the significant uncertainties that exist regarding the future behaviour of the AMOC under global warming. Model results suggest that changes in the AMOC can impact surface air temperature, precipitation patterns and sea level, particularly in areas bordering the North Atlantic, thus affecting human populations. Here current understanding of past, present and future change in the AMOC and the effects of such changes on climate are reviewed. The focus is on observations of the AMOC, how the AMOC influences climate and in what way the AMOC is likely to change over the next few decades and the 21st century. The potential for decadal prediction of the AMOC is also discussed. Finally, the outstanding challenges and possible future directions for AMOC research are outlined.

16. Vecchi, Gabriel A., Rym Msadek, Thomas L Delworth, Keith W Dixon, E Guilyardi, E Hawkins, A R Karspeck, J Mignot, J Robson, Anthony Rosati, and Rong Zhang, November 2012: Comment on "Multiyear Prediction of Monthly Mean Atlantic Meridional Overturning Circulation at 26.5�N". Science, 338(6107), doi:10.1126/science.1222566.
    [ Abstract ]

    Matei et al. (Reports, 6 January 2012, p. 76) claim to show skillful multiyear predictions of the Atlantic Meridional Overturning Circulation (AMOC). However, these claims are not justified, primarily because the predictions of AMOC transport do not outperform simple reference forecasts based on climatological annual cycles. Accordingly, there is no justification for the "confident" prediction of a stable AMOC through 2014.

17. Wu, X, Shaoqing Zhang, Z Liu, Anthony Rosati, Thomas L Delworth, and Y Liu, December 2012: Impact of Geographic Dependent Parameter Optimization on Climate Estimation and Prediction: Simulation with an Intermediate Coupled Model. Monthly Weather Review, 140(12), doi:10.1175/MWR-D-11-00298.1.
    [ Abstract ]

    Due to the geographic dependence of model sensitivities and observing systems, allowing optimized parameter values to vary geographically may significantly enhance the signal in parameter estimation. Using an intermediate atmosphere-ocean-land coupled model, the impact of geographic dependence of model sensitivities on parameter optimization is explored within a twin experiment framework. The coupled model consists of a 1-layer global barotropic atmosphere model, a 1.5-layer baroclinic ocean including a slab mixed layer with simulated upwelling by a streamfunction equation and a simple land model. The assimilation model is biased by erroneously setting the values of all model parameters. Four most sensitive parameters identified by sensitivity studies are used to perform traditional single-value parameter estimation and new geographic dependent parameter optimization. Results show that the new parameter optimization significantly improves the quality of state estimates compared to the traditional scheme, with reductions of root mean square errors as 41%, 23%, 62% and 59% for the atmospheric streamfunction, the oceanic streamfunction, sea surface temperature and land surface temperature respectively. Consistently, the new parameter optimization greatly improves the model predictability due to the improvement of initial conditions and the enhancement of observational signals in optimized parameters. These results suggest that the proposed geographic dependent parameter optimization scheme may provide a new perspective when a coupled general circulation model is used for climate estimation and prediction.

18. Zhang, Shaoqing, Z Liu, Anthony Rosati, and Thomas L Delworth, January 2012: A study of enhancive parameter correction with coupled data assimilation for climate estimation and prediction using a simple coupled model. Tellus A, 64, 10963, doi:10.3402/tellusa.v64i0.10963.
    [ Abstract ]

    Uncertainties in physical parameters of coupled models are an important source of model bias and adversely impact initialisation for climate prediction. Data assimilation using error covariances derived from model dynamics to extract observational information provides a promising approach to optimise parameter values so as to reduce such bias. However, effective parameter estimation in a coupled model is usually difficult because the error covariance between a parameter and the model state tends to be noisy due to multiple sources of model uncertainties. Using a simple coupled model consisting of the 3-variable Lorenz model and a slowly varying slab ‘ocean’, this study first investigated how to enhance the signal-to-noise ratio in covariances between model states and parameters, and then designed a data assimilation scheme for enhancive parameter correction (DAEPC). In DAEPC, parameter estimation is facilitated after state estimation reaches a ‘quasiequilibrium’ where the uncertainty of coupled model states is sufficiently constrained by observations so that the covariance between a parameter and the model state is signal dominant. The observation-updated parameters are applied to improving the next cycle of state estimation and the refined covariance of parameter and model state further improves parameter correction. Performing dynamically adaptive state and parameter estimations with speedy convergence, DAEPC provides a systematic way to estimate the whole array of coupled model parameters using observations, and produces more accurate state estimates. Forecast experiments show that the DAEPC initialisation with observation-estimated parameters greatly improves the model predictability - while valid ‘atmospheric’ forecasts are extended two times longer, the ‘oceanic’ predictability is almost tripled. The simple model results here provide some insights for improving climate estimation and prediction with a coupled general circulation model.

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

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

20. Mahajan, S, Rong Zhang, and Thomas L Delworth, December 2011: Impact of the Atlantic Meridional Overturning Circulation (AMOC) on Arctic surface air temperature and sea-ice variability. Journal of Climate, 24(24), doi:10.1175/2011JCLI4002.1.
    [ Abstract ]

    The simulated impact of the Atlantic Meridional Overturning Circulation (AMOC) on the low frequency variability of the Arctic Surface Air temperature (SAT) and sea-ice extent is studied with a 1000 year-long segment of a control simulation of GFDL CM2.1 climate model. The simulated AMOC variations in the control simulation are found to be significantly anti-correlated with the Arctic sea-ice extent anomalies and significantly correlated with the Arctic SAT anomalies on decadal timescales in the Atlantic sector of the Arctic. The maximum anti-correlation with the Arctic sea-ice extent and the maximum correlation with the Arctic SAT occur when the AMOC Index leads by one year. An intensification of the AMOC is associated with a sea-ice decline in the Labrador, Greenland and Barents Seas in the control simulation, with the largest change occurring in the winter. The recent declining trend in the satellite observed sea-ice extent also shows a similar pattern in the Atlantic sector of the Arctic in the winter, suggesting the possibility of a role of the AMOC in the recent Arctic sea-ice decline in addition to anthropogenic greenhouse gas induced warming. However, in the summer, the simulated sea-ice response to the AMOC in the Pacific sector of the Arctic is much weaker than the observed declining trend, indicating a stronger role for other climate forcings or variability in the recently observed summer sea-ice decline in the Chukchi, Beaufort, East Siberian and Laptev Seas.

21. Mahajan, S, Rong Zhang, Thomas L Delworth, Shaoqing Zhang, Anthony Rosati, and You-Soon Chang, September 2011: Predicting Atlantic meridional overturning circulation (AMOC) variations using subsurface and surface fingerprints. Deep-Sea Research, Part II, 58(17-18), doi:10.1016/j.dsr2.2010.10.067.
    [ Abstract ]

    Recent studies have suggested that the leading modes of North Atlantic subsurface temperature (T_sub) and sea surface height (SSH) anomalies are induced by Atlantic meridional overturning circulation (AMOC) variations and can be used as fingerprints of AMOC variability. Based on these fingerprints of the AMOC in the GFDL CM2.1 coupled climate model, a linear statistical predictive model of observed fingerprints of AMOC variability is developed in this study. The statistical model predicts a weakening of AMOC strength in a few years after its peak around 2005. Here, we show that in the GFDL coupled climate model assimilated with observed subsurface temperature data, including recent Argo network data (2003–2008), the leading mode of the North Atlantic T_sub anomalies is similar to that found with the objectively analyzed T_sub data and highly correlated with the leading mode of altimetry SSH anomalies for the period 1993–2008. A statistical auto-regressive (AR) model is fit to the time-series of the leading mode of objectively analyzed detrended North Atlantic T_sub anomalies (1955–2003) and is applied to assimilated T_sub and altimetry SSH anomalies to make predictions. A similar statistical AR model, fit to the time-series of the leading mode of modeled T_sub anomalies from the 1000-year GFDL CM2.1 control simulation, is applied to predict modeled T_sub, SSH, and AMOC anomalies. The two AR models show comparable skills in predicting observed T_sub and modeled T_sub, SSH and AMOC variations.

22. Solomon, A, and Thomas L Delworth, et al., February 2011: Distinguishing the roles of natural and anthropogenically forced decadal climate variability: Implications for prediction US CLIVAR Decadal Predictability Working Group. Bulletin of the American Meteorological Society, 92(2), doi:10.1175/2010BAMS2962.1.
    [ Abstract ]

    Given that over the course of the next 10–30 years the magnitude of natural decadal variations may rival that of anthropogenically forced climate change on regional scales, it is envisioned that initialized decadal predictions will provide important information for climate-related management and adaptation decisions. Such predictions are presently one of the grand challenges for the climate community. This requires identifying those physical phenomena—and their model equivalents—that may provide additional predictability on decadal time scales, including an assessment of the physical processes through which anthropogenic forcing may interact with or project upon natural variability. Such a physical framework is necessary to provide a consistent assessment (and insight into potential improvement) of the decadal prediction experiments planned to be assessed as part of the IPCC's Fifth Assessment Report.

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

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

24. Wu, S, Z Liu, Rong Zhang, and Thomas L Delworth, February 2011: On the observed relationship between the Pacific Decadal Oscillation and the Atlantic Multi-decadal Oscillation. Journal of Oceanography, 67(1), doi:10.1007/s10872-011-0003-x.
    [ Abstract ]

    We studied the relationship between the dominant patterns of sea surface temperature (SST) variability in the North Pacific and the North Atlantic. The patterns are known as the Pacific Decadal Oscillation (PDO) and the Atlantic Multi-decadal Oscillation (AMO). In the analysis we used two different observational data sets for SST. Due to the high degree of serial correlation in the PDO and AMO time series, various tests were carried out to assess the significance of the correlations. The results demonstrated that the correlations are significant when the PDO leads the AMO by 1 year and when the AMO leads the PDO by 11–12 years. The possible physical processes involved are discussed, along with their potential implication for decadal prediction.

25. Zhang, D, Rym Msadek, M J McPhaden, and Thomas L Delworth, April 2011: Multidecadal variability of the North Brazil Current and its connection to the Atlantic Meridional Overturning Circulation. Journal of Geophysical Research, 116, C04012, doi:10.1029/2010JC006812.
    [ Abstract ]

    The North Brazil Current (NBC) connects the North and South Atlantic and is the major pathway for the surface return flow of the Atlantic meridional overturning circulation (AMOC). Here, we calculate the NBC geostrophic transport time series based on 5 decades of observations near the western boundary off the coast of Brazil. Results reveal a multidecadal NBC variability that lags Labrador Sea deep convection by a few years. The NBC transport time series is coherent with the Atlantic Multidecadal Oscillation in sea surface temperature, which also has been widely linked to AMOC fluctuations in previous modeling studies. Our results thus suggest that the observed multidecadal NBC transport variability is a useful indicator for AMOC variations. The suggested connection between the NBC and AMOC is assessed in a 700 year control simulation of the Geophysical Fluid Dynamics Laboratory's CM2.1 coupled climate model. The model results are in agreement with observations and further demonstrate that the variability of NBC transport is a good index for tracking AMOC variations. Concerning the debate about whether a slowdown of AMOC has already occurred under global warming, the observed NBC transport time series suggests strong multidecadal variability but no significant trend.

26. Zhang, Rong, Thomas L Delworth, Anthony Rosati, Whit G Anderson, Keith W Dixon, Hyun-Chul Lee, and Fanrong Zeng, December 2011: Sensitivity of the North Atlantic Ocean circulation to an abrupt change in the Nordic Sea overflow in a high resolution global coupled climate model. Journal of Geophysical Research, 116, C12024, doi:10.1029/2011JC007240.
    [ Abstract ]

    The sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow is investigated for the first time using a high resolution eddy-permitting global coupled ocean-atmosphere model (GFDL CM2.5). The Nordic Sea overflow is perturbed through the change of the bathymetry in GFDL CM2.5. We analyze the Atlantic Meridional Overturning Circulation (AMOC) adjustment process and the downstream oceanic response to the perturbation. The results suggest that north of 34N, AMOC changes induced by changes in the Nordic Sea overflow propagate on the slow tracer advection time scale, instead of the fast Kelvin wave time scale, resulting in a time lead of several years between subpolar and subtropical AMOC changes. The results also show that a stronger and deeper-penetrating Nordic Sea overflow leads to stronger and deeper AMOC, stronger northward ocean heat transport, reduced Labrador Sea deep convection, stronger cyclonic Northern Recirculation Gyre (NRG), westward shift of the North Atlantic Current (NAC) and southward shift of the Gulf Stream, warmer sea surface temperature (SST) east of Newfoundland and colder SST south of the Grand Banks, stronger and deeper NAC and Gulf Stream, and stronger oceanic eddy activities along the NAC and the Gulf Stream paths. A stronger/weaker Nordic Sea overflow also leads to a contracted/expanded subpolar gyre (SPG). This sensitivity study points to the important role of the Nordic Sea overflow in the large scale North Atlantic ocean circulation, and it is crucial for climate models to have a correct representation of the Nordic Sea overflow.

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

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

28. Farneti, Riccardo, and Thomas L Delworth, October 2010: The role of mesoscale eddies in the remote oceanic response to altered Southern Hemisphere winds. Journal of Physical Oceanography, 40(10), doi:10.1175/2010JPO4480.1.
    [ Abstract ]

    It has been suggested that a strengthening of the Southern Hemisphere winds would induce a more vigorous overturning through an increased northward Ekman flux, bringing more light waters into the oceanic basins and enhancing the upwelling of North Atlantic Deep Water in the Southern Ocean, thereby increasing ocean ventilation. We present here simulations from a coarse and a fine resolution version of a coupled model subject to idealized wind stress changes in the Southern Ocean. In the fine resolution eddy-permitting model, we find that changes in poleward eddy fluxes largely compensate for the enhanced equatorward Ekman transport in the Southern Ocean. As a consequence, northward transport of light waters, pycnocline depth, Northern Hemisphere overturning and Southern Ocean upwelling anomalies are much reduced compared with simulations in the coarse resolution model with parameterized eddies. These results point to a relatively weak sensitivity of present-day global ocean

29. Findell, Kirsten L., and Thomas L Delworth, February 2010: Impact of common sea surface temperature anomalies on global drought and pluvial frequency. Journal of Climate, 23(3), doi:10.1175/2009JCLI3153.1.
    [ Abstract ]

    Climate model simulations run as part of the Climate Variability and Predictability (CLIVAR) Drought Working Group initiative were analyzed to determine the impact of three patterns of sea surface temperature (SST) anomalies on drought and pluvial frequency and intensity around the world. The three SST forcing patterns include a global pattern similar to the background warming trend, a pattern in the Pacific, and a pattern in the Atlantic. Five different global atmospheric models were forced by fixed SSTs to test the impact of these SST anomalies on droughts and pluvials relative to a climatologically forced control run. The five models generally yield similar results in the locations of drought and pluvial frequency changes throughout the annual cycle in response to each given SST pattern. In all of the simulations, areas with an increase in the mean drought (pluvial) conditions tend to also show an increase in the frequency of drought (pluvial) events. Additionally, areas with more frequent extreme events also tend to show higher intensity extremes. The cold Pacific anomaly increases drought occurrence in the United States and southern South America and increases pluvials in Central America and northern and central South America. The cold Atlantic anomaly increases drought occurrence in southern Central America, northern South America, and central Africa and increases pluvials in central South America. The warm Pacific and Atlantic anomalies generally lead to reversals of the drought and pluvial increases described with the corresponding cold anomalies. More modest impacts are seen in other parts of the world. The impact of the trend pattern is generally more modest than that of the two other anomaly patterns.

30. Held, Isaac M., Michael Winton, K Takahashi, Thomas L Delworth, Fanrong Zeng, and Geoffrey K Vallis, May 2010: Probing the fast and slow components of global warming by returning abruptly to pre-industrial forcing. Journal of Climate, 23(9), doi:10.1175/2009JCLI3466.1.
    [ Abstract ]

    The fast and slow components of global warming in a comprehensive climate model are isolated by examining the response to an instantaneous return to pre-industrial forcing. The response is characterized by an initial fast exponential decay with an e-folding time smaller than 5 years, leaving behind a remnant that evolves more slowly. The slow component is estimated to be small at present, as measured by the global mean near-surface air temperature, and, in the model examined, grows to 0.4C by 2100 in the A1B SRES scenario and then to 1.4C by 2300 if one holds radiative forcing fixed after 2100. The dominance of the fast component at present is supported by examining the response to an instantaneous doubling of CO₂ and by the excellent fit to the model's ensemble mean 20th century evolution with a simple one-box model with no long times scales.

31. Hurrell, J W., Thomas L Delworth, Stephen M Griffies, and Anthony Rosati, et al., September 2010: Decadal Climate Prediction: Opportunities and Challenges, 2010 In OceanObs’09: Sustained Ocean Observations and Information for Society, Vol. 2, ESA Publication, doi:10.5270/OceanObs09.cwp.45.
32. Hurrell, J W., G A Meehl, D Bader, Thomas L Delworth, B P Kirtman, and B A Wielicki, December 2010: Reply to Comments on “A Unified Modeling Approach to Climate System Prediction”. Bulletin of the American Meteorological Society, 91(12), doi:10.1175/2010BAMS3118.1.
33. Li, F, V Ramaswamy, Paul Ginoux, Anthony J Broccoli, Thomas L Delworth, and Fanrong Zeng, December 2010: Toward understanding the dust deposition in Antarctica during the Last Glacial Maximum: Sensitivity studies on plausible causes. Journal of Geophysical Research, 115, D24120, doi:10.1029/2010JD014791.
    [ Abstract ]

    Understanding the plausible causes for the observed high dust concentrations in Antarctic ice cores during the Last Glacial Maximum (LGM) is crucial for interpreting the Antarctic dust records in the past climates and could provide insights into dust variability in future climates. Using the Geophysical Fluid Dynamics Laboratory (GFDL) General Circulation Models, we conduct an investigation into the various factors modulating dust emission, transport and deposition, with a view towards an improved quantification of the LGM dust enhancements in the Antarctic ice cores. The model simulations show that the expansion of source areas and changes in the Antarctic ice accumulation rates together can account for most of the observed increase of dust concentrations in the Vostok, Dome C and Taylor Dome cores, but there is an overestimate of the LGM-to-present ratio in the case of the Byrd core. The source expansion due to the lowering of sea level yields a factor of 2–3 higher contribution than that due to the reduction of continental vegetation. The changes in other climate parameters (e.g., SH precipitation change) are estimated to be relatively less important within the context of this sensitivity study, while the model-simulated LGM surface winds yield a 20–30 % reduction rather than an increase in dust deposition in Antarctica. This research yields insights towards a fundamental understanding of the causes for the significant enhancement of the dust deposition in the Antarctic ice cores during the LGM.

34. Msadek, Rym, Keith W Dixon, Thomas L Delworth, and William J Hurlin, October 2010: Assessing the predictability of the Atlantic meridional overturning circulation and associated fingerprints. Geophysical Research Letters, 37, L19608, doi:10.1029/2010GL044517.
    [ Abstract ]

    The North Atlantic is among the few places where decadal climate variations are considered potentially predictable. The physical mechanisms of the decadal variability are hypothesized to be associated with fluctuations of the Atlantic meridional overturning circulation (AMOC). Perfect model predictability experiments using the GFDL CM2.1 climate model are analyzed to investigate the potential predictability of the AMOC. Results indicate that the AMOC is predictable up to 20 years. We further connect AMOC predictability to readily observable fields. We show that modeled surface and subsurface signatures of AMOC variations defined by characteristic patterns of sea surface height, subsurface temperature, and upper ocean heat content anomalies, have a potential predictability similar to the AMOC's. Since we have longer observational records for these quantities than for direct measurements of the AMOC, our study highlights a potentially new promising method for monitoring AMOC variations, and hence assessing the predictability of the real climate system.

35. Zhang, Shaoqing, Anthony Rosati, and Thomas L Delworth, October 2010: The adequacy of observing systems in monitoring AMOC and North Atlantic climate. Journal of Climate, 23(19), doi:10.1175/2010JCLI3677.1.
    [ Abstract ]

    The Atlantic Meridional Overturning Circulation (AMOC) has an important influence on climate, and yet we lack adequate observations of this circulation. Here we assess the adequacy of past and current widely deployed routine observing systems for monitoring the AMOC and associated North Atlantic climate. To do so we draw on two independent simulations of the 20th century using an IPCC AR4 coupled climate model. We treat one simulation as “truth” and sample it according to the observing system we are evaluating. We then assimilate these synthetic “observations” into the second simulation within a fully-coupled system that instantaneously exchanges information among all coupled components and produces a nearly balanced and coherent estimate for global climate states including the North Atlantic climate system. The degree to which the assimilation recovers the “truth” is an assessment of the adequacy of the observing system being evaluated. As the coupled system responds to the constraint of the atmosphere or ocean, the assessment of the recovery for climate quantities such as Labrador Sea Water (LSW) and the North Atlantic Oscillation increases our understanding for the factors that determine AMOC variability. For example, we found the low-frequency sea-surface forcings provided by the atmospheric and sea-surface temperature observations can excite a LSW variation that governs the long time scale variability of the AMOC. When we use the most complete modern observing system consisting of atmospheric winds and temperature, along with Argo ocean temperature and salinity down to 2000 meters, a skill estimate of AMOC reconstruction is 90% (out of 100% maximum). Similarly encouraging results hold for other quantities, such as LSW. The past XBT observing system, in which deep ocean temperature and salinity were not available, has a lesser ability to recover the “truth” AMOC (the skill is reduced to 52%). While these results raise concerns about our ability to properly characterize past variations of the AMOC, they also hold promise for future monitoring of the AMOC and for initializing prediction models.

36. Hurrell, J W., G A Meehl, D Bader, Thomas L Delworth, B P Kirtman, and B A Wielicki, December 2009: A unified modeling approach to climate system prediction. Bulletin of the American Meteorological Society, 90(12), doi:10.1175/2009BAMS2752.1.
    [ Abstract ]

    There is a new perspective of a continuum of prediction problems, with a blurring of the distinction between short-term predictions and long-term climate projections. At the heart of this new perspective is the realization that all climate system predictions, regardless of time scale, share common processes and mechanisms; moreover, interactions across time and space scales are fundamental to the climate system itself. Further, just as seasonal-to-interannual predictions start from an estimate of the state of the climate system, there is a growing realization that decadal and longer-term climate predictions could be initialized with estimates of the current observed state of the atmosphere, oceans, cryosphere, and land surface. Even though the prediction problem itself is seamless, the best practical approach to it may be described as unified: models aimed at different time scales and phenomena may have large commonality but place emphasis on different aspects of the system. The potential benefits of this commonality are significant and include improved predictions on all time scales and stronger collaboration and shared knowledge, infrastructure, and technical capabilities among those in the weather and climate prediction communities.

37. Schubert, S D., Thomas L Delworth, and Kirsten L Findell, et al., October 2009: A US CLIVAR project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results. Journal of Climate, 22(19), doi:10.1175/2009JCLI3060.1.
    [ Abstract ]

    The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niño–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern. One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the United States tends to occur when the two oceans have anomalies of opposite signs. Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world’s land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models. It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

38. Stenchikov, G, Thomas L Delworth, V Ramaswamy, Ronald J Stouffer, Andrew T Wittenberg, and Fanrong Zeng, August 2009: Volcanic signals in oceans. Journal of Geophysical Research, 114, D16104, doi:10.1029/2008JD011673.
    [ Abstract ]

    Sulfate aerosols resulting from strong volcanic explosions last for 2–3 years in the lower stratosphere. Therefore it was traditionally believed that volcanic impacts produce mainly short-term, transient climate perturbations. However, the ocean integrates volcanic radiative cooling and responds over a wide range of time scales. The associated processes, especially ocean heat uptake, play a key role in ongoing climate change. However, they are not well constrained by observations, and attempts to simulate them in current climate models used for climate predictions yield a range of uncertainty. Volcanic impacts on the ocean provide an independent means of assessing these processes. This study focuses on quantification of the seasonal to multidecadal time scale response of the ocean to explosive volcanism. It employs the coupled climate model CM2.1, developed recently at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, to simulate the response to the 1991 Pinatubo and the 1815 Tambora eruptions, which were the largest in the 20th and 19th centuries, respectively. The simulated climate perturbations compare well with available observations for the Pinatubo period. The stronger Tambora forcing produces responses with higher signal-to-noise ratio. Volcanic cooling tends to strengthen the Atlantic meridional overturning circulation. Sea ice extent appears to be sensitive to volcanic forcing, especially during the warm season. Because of the extremely long relaxation time of ocean subsurface temperature and sea level, the perturbations caused by the Tambora eruption could have lasted well into the 20th century.nd sea level, the perturbations caused by the Tambora eruption could last well into the 20th century.

39. Zhang, Rong, and Thomas L Delworth, March 2009: A new method for attributing climate variations over the Atlantic Hurricane Basin's main development region. Geophysical Research Letters, 36, L06701, doi:10.1029/2009GL037260.
    [ Abstract ]

    We propose a new approach to decompose observed climate variations over the Atlantic Hurricane Basin's main development region (MDR) into components attributable to radiative forcing changes and to internal oceanic variability. Our attribution suggests that the observed multidecadal anomalies of vertical shear (Uz) and a simple index of maximum potential intensity (SIMPI) for tropical cyclones are both dominated by internal variability, consistent with multidecadal variations of Atlantic Hurricane activity; changes in radiative forcing led to increasing Uz and decreasing SIMPI since the late 50's, unfavorable for Atlantic Hurricane activity. Physically, at least for the GFDL model, sea surface temperature (SST) anomalies induced by ocean heat transport variations are more efficient in producing negative Uz anomalies than that induced by altered radiative forcing.

40. Clark, P U., Thomas L Delworth, and A J Weaver, April 2008: Freshwater Forcing:Will History Repeat Itself? Science, 320(5874), 316.
41. Delworth, Thomas L., and Fanrong Zeng, October 2008: Simulated impact of altered Southern Hemisphere winds on the Atlantic Meridional Overturning Circulation. Geophysical Research Letters, 35, L20708, doi:10.1029/2008GL035166.
    [ Abstract PDF ]

    Previous work has suggested that the strength and latitudinal position of the Southern Hemisphere (SH) mid-latitude westerly winds has an important impact on climate and the Atlantic Meridional Overturning Circulation (AMOC). We probe this hypothesis by conducting ensembles of experiments using the GFDL CM2.1 coupled ocean-atmosphere model with altered SH wind stress. We find, consistent with previous work, that enhanced (reduced) and poleward (equatorward) displaced SH westerly winds lead to an AMOC intensification (weakening). While the AMOC takes more than a century to respond fully to the altered SH winds, initial effects in the North Atlantic can occur within a few decades. The AMOC changes generate SST and surface air temperature responses in the North Atlantic and adjacent continental regions. In the Southern Hemisphere, the atmosphere responds to the altered ocean circulation with a further strengthening and poleward movement of the SH winds, thereby constituting a modest positive feedback.

42. Delworth, Thomas L., and Rong Zhang, et al., December 2008: The potential for abrupt change in the Atlantic Meridional Overturning Circulation In Abrupt Climate Change: Final Report, Synthesis & Assessment Product 3.4, CSSP, Reston, VA, U.S. Geological Survey, 258-359.
    [ PDF ]
43. Delworth, Thomas L., Rong Zhang, and M E Mann, 2007: Decadal to centennial variability of the Atlantic from observations and models In Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series 173, Washington, DC, American Geophysical Union, 131-148.
    [ Abstract PDF ]

    Some aspects of multidecadal Atlantic climate variability, and its impact on regional and hemispheric scale climate, are reviewed. Observational analyses have documented distinct patterns of Atlantic variability with decadal (8-12 years) and multidecadal (30-80 years) time scales. Numerical models have succeeded in capturing some aspects of this observed variability, but much work remains to understand the mechanisms of the observed variability. The impacts of the variability — particularly on the multidecadal time scale — are striking, including modulation of African and Indian summer monsoon rainfall, summer climate over North America and Europe, and a potential influence on Atlantic hurricane activity. Some of the observed variability, particularly in recent decades, is likely influenced by changing radiative forcings, of both anthropogenic and natural origin. This poses an important challenge for the detection, attribution and prediction of climate change.

44. Delworth, Thomas L., and Kirsten L Findell, 2007: Decadal to centennial scale changes in summer continental hydrology In Climate Variability and Change: Past, Present, and Future, John E. Kutzbach Symposium, Gisela Kutzbach, Ed., Madison, WI, Ctr. of Climatic Research, U. Wisconsin-Madison, 49-56.
    [ Abstract ]

    Past studies have suggested that increasing atmospheric CO₂ will lead to a substantial reduction of soil moisture during summer in the extratropics. We revisit this topic using a new climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory. The new model has a horizontal resolution of 2.5° longitude by 2.0° latitude, with 24 vertical levels, and has both a diurnal and seasonal cycle of insolation. The model incorporates substantially updated physics relative to previous versions. Results from earlier studies showed, among other things, an increase in wintertime rainfall over most mid-latitude continental regions when CO₂ is doubled, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer in mid-latitude in doubled-CO₂ experiments. The new model shows similar results, and the processes discussed above are important in this model as well. In addition, we find that changes in atmospheric circulation play an important role in regional hydrologic changes. Additional experiments have been run to probe the causes of the circulation changes. These simulations show that global scale sea surface temperature increases caused by the CO₂ doubling explain the majority of the atmospheric circulation changes, while positive feedbacks from the land surface have a secondary impact. These results highlight the importance of global scale sea surface temperature changes for future regional hydrology changes.

45. Little, C M., Ventakramani Balaji, Thomas L Delworth, Robert W Hallberg, Hiram Levy II, Ronald J Stouffer, and Michael Winton, et al., 2007: Toward a new generation of ice sheet models. EOS, 88(52), 578-579.
    [ PDF ]
46. Meehl, G A., C Covey, Thomas L Delworth, M Latif, B McAveney, J F B Mitchell, Ronald J Stouffer, and K E Taylor, 2007: The WCRP CMIP3 multimodel dataset: A new era in climate change research. Bulletin of the American Meteorological Society, 88(9), doi:10.1175/BAMS-88-9-1383.
    [ Abstract ]

    A coordinated set of global coupled climate model [atmosphere–ocean general circulation model (AOGCM)] experiments for twentieth- and twenty-first-century climate, as well as several climate change commitment and other experiments, was run by 16 modeling groups from 11 countries with 23 models for assessment in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Since the assessment was completed, output from another model has been added to the dataset, so the participation is now 17 groups from 12 countries with 24 models. This effort, as well as the subsequent analysis phase, was organized by the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Working Group on Coupled Models (WGCM) Climate Simulation Panel, and constitutes the third phase of the Coupled Model Intercomparison Project (CMIP3). The dataset is called the WCRP CMIP3 multimodel dataset, and represents the largest and most comprehensive international global coupled climate model experiment and multimodel analysis effort ever attempted. As of March 2007, the Program for Climate Model Diagnostics and Intercomparison (PCMDI) has collected, archived, and served roughly 32 TB of model data. With oversight from the panel, the multimodel data were made openly available from PCMDI for analysis and academic applications. Over 171 TB of data had been downloaded among the more than 1000 registered users to date. Over 200 journal articles, based in part on the dataset, have been published so far. Though initially aimed at the IPCC AR4, this unique and valuable resource will continue to be maintained for at least the next several years. Never before has such an extensive set of climate model simulations been made available to the international climate science community for study. The ready access to the multimodel dataset opens up these types of model analyses to researchers, including students, who previously could not obtain state-of-the-art climate model output, and thus represents a new era in climate change research. As a direct consequence, these ongoing studies are increasing the body of knowledge regarding our understanding of how the climate system currently works, and how it may change in the future.

47. Zhang, Rong, Thomas L Delworth, and Isaac M Held, 2007: Can the Atlantic Ocean drive the observed multidecadal variability in Northern Hemisphere mean temperature? Geophysical Research Letters, 34, L02709, doi:10.1029/2006GL028683.
    [ Abstract PDF ]

    While the Northern Hemisphere mean surface temperature has clearly warmed over the 20th century due in large part to increasing greenhouse gases, this warming has not been monotonic. The departures from steady warming on multidecadal timescales might be associated in part with radiative forcing, especially solar irradiance, volcanoes, and anthropogenic aerosols. It is also possible that internal oceanic variability explains a part of this variation. We report here on simulations with a climate model in which the Atlantic Ocean is constrained to produce multidecadal fluctuations similar to observations by redistributing heat within the Atlantic, with other oceans left free to adjust to these Atlantic perturbations. The model generates multidecadal variability in Northern Hemisphere mean temperatures similar in phase and magnitude to detrended observations. The results suggest that variability in the Atlantic is a viable explanation for a portion of the multidecadal variability in the Northern Hemisphere mean temperature record.

48. Zhang, Rong, and Thomas L Delworth, December 2007: Impact of the Atlantic Multidecadal Oscillation on North Pacific climate variability. Geophysical Research Letters, 34, L23708, doi:10.1029/2007GL031601.
    [ Abstract PDF ]

    In this paper, we found that the Atlantic Multidecadal Oscillation (AMO) can contribute to the Pacific Decadal Oscillation (PDO), especially the component of the PDO that is linearly independent of El Niño and the Southern Oscillation (ENSO), i.e. the North Pacific Multidecadal Oscillation (NPMO), and the associated Pacific/North America (PNA) pattern. Using a hybrid version of the GFDL CM2.1 climate model, we show that the AMO provides a source of multidecadal variability to the North Pacific, and needs to be considered along with other forcings for North Pacific climate change. The lagged North Pacific response to the North Atlantic forcing is through atmospheric teleconnections and reinforced by oceanic dynamics and positive air-sea feedback over the North Pacific. The results indicate that a North Pacific regime shift, opposite to the 1976–77 shift, might occur now a decade after the switch of the observed AMO to a positive phase around 1995.

49. Delworth, Thomas L., and Keith W Dixon, 2006: Have anthropogenic aerosols delayed a greenhouse gas-induced weakening of the North Atlantic thermohaline circulation? Geophysical Research Letters, 33(2), L02606, doi:10.1029/2005GL024980.
    [ Abstract ]

    In many climate model simulations using realistic, time-varying climate change forcing agents for the 20th and 21st centuries, the North Atlantic thermohaline circulation (THC) weakens in the 21st century, with little change in the 20th century. Here we use a comprehensive climate model to explore the impact of various climate change forcing agents on the THC. We conduct ensembles of integrations with subsets of climate change forcing agents. Increasing greenhouse gases – in isolation – produce a significant THC weakening in the late 20th century, but this change is partially offset by increasing anthropogenic aerosols, which tend to strengthen the THC. The competition between increasing greenhouse gases and anthropogenic aerosols thus produces no significant THC change in our 20th century simulations when all climate forcings are included. The THC weakening becomes significant several decades into the 21st century, when the effects of increasing greenhouse gases overwhelm the aerosol effects.

50. Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, Ventakramani Balaji, J A Beesley, William F Cooke, Keith W Dixon, John P Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen M Griffies, Rich Gudgel, Matthew J Harrison, Isaac M Held, Richard S Hemler, Larry W Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, Amy R Langenhorst, Hyun-Chul Lee, Shian-Jiann Lin, Jian Lu, Sergey Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), doi:10.1175/JCLI3629.1.
    [ Abstract ]

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

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

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

52. Hurrell, J W., M Visbeck, A Busalacchi, R A Clarke, Thomas L Delworth, R R Dickson, W E Johns, K P Koltermann, P J Kushner, D Marshall, C Mauritzen, M S McCartney, A Piola, C Reason, G Reverdin, F Schott, R Sutton, I Wainer, and D Wright, 2006: Atlantic Climate Variability and Predictability: A CLIVAR Perspective. Journal of Climate, 19(20), doi:10.1175/JCLI3902.1.
    [ Abstract ]

    Three interrelated climate phenomena are at the center of the Climate Variability and Predictability (CLIVAR) Atlantic research: tropical Atlantic variability (TAV), the North Atlantic Oscillation (NAO), and the Atlantic meridional overturning circulation (MOC). These phenomena produce a myriad of impacts on society and the environment on seasonal, interannual, and longer time scales through variability manifest as coherent fluctuations in ocean and land temperature, rainfall, and extreme events. Improved understanding of this variability is essential for assessing the likely range of future climate fluctuations and the extent to which they may be predictable, as well as understanding the potential impact of human-induced climate change. CLIVAR is addressing these issues through prioritized and integrated plans for short-term and sustained observations, basin-scale reanalysis, and modeling and theoretical investigations of the coupled Atlantic climate system and its links to remote regions. In this paper, a brief review of the state of understanding of Atlantic climate variability and achievements to date is provided. Considerable discussion is given to future challenges related to building and sustaining observing systems, developing synthesis strategies to support understanding and attribution of observed change, understanding sources of predictability, and developing prediction systems in order to meet the scientific objectives of the CLIVAR Atlantic program.

53. Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, Isaac M Held, Jian Lu, V Ramaswamy, M Daniel Schwarzkopf, G Stenchikov, and Ronald J Stouffer, 2006: Assessment of Twentieth-Century regional surface temperature trends using the GFDL CM2 coupled models. Journal of Climate, 19(9), doi:10.1175/JCLI3709.1.
    [ Abstract ]

    Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing. Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observations contribute substantially to an underprediction of the observed warming over northern Asia. In the all-forcing and natural-only forcing runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s. The simulations support previous findings that twentieth-century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-twentieth-century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/western Pacific warm pool during the past half-century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multidecadal variability.

54. Stott, P, J F B Mitchell, M R Allen, Thomas L Delworth, J M Gregory, G A Meehl, and B D Santer, 2006: Observational Constraints on Past Attributable Warming and Predictions of Future Global Warming. Journal of Climate, 19(13), doi:10.1175/JCLI3802.1.
    [ Abstract ]

    This paper investigates the impact of aerosol forcing uncertainty on the robustness of estimates of the twentieth-century warming attributable to anthropogenic greenhouse gas emissions. Attribution analyses on three coupled climate models with very different sensitivities and aerosol forcing are carried out. The Third Hadley Centre Coupled Ocean–Atmosphere GCM (HadCM3), Parallel Climate Model (PCM), and GFDL R30 models all provide good simulations of twentieth-century global mean temperature changes when they include both anthropogenic and natural forcings. Such good agreement could result from a fortuitous cancellation of errors, for example, by balancing too much (or too little) greenhouse warming by too much (or too little) aerosol cooling. Despite a very large uncertainty for estimates of the possible range of sulfate aerosol forcing obtained from measurement campaigns, results show that the spatial and temporal nature of observed twentieth-century temperature change constrains the component of past warming attributable to anthropogenic greenhouse gases to be significantly greater (at the 5% level) than the observed warming over the twentieth century. The cooling effects of aerosols are detected in all three models. Both spatial and temporal aspects of observed temperature change are responsible for constraining the relative roles of greenhouse warming and sulfate cooling over the twentieth century. This is because there are distinctive temporal structures in differential warming rates between the hemispheres, between land and ocean, and between mid- and low latitudes. As a result, consistent estimates of warming attributable to greenhouse gas emissions are obtained from all three models, and predictions are relatively robust to the use of more or less sensitive models. The transient climate response following a 1% yr−1 increase in CO2 is estimated to lie between 2.2 and 4 K century−1 (5–95 percentiles).

55. Stouffer, Ronald J., Thomas L Delworth, Keith W Dixon, Rich Gudgel, Isaac M Held, Richard S Hemler, Thomas R Knutson, M Daniel Schwarzkopf, Michael J Spelman, Michael Winton, Anthony J Broccoli, Hyun-Chul Lee, Fanrong Zeng, and Brian J Soden, 2006: GFDL's CM2 Global Coupled Climate Models. Part IV: Idealized Climate Response. Journal of Climate, 19(5), doi:10.1175/JCLI3632.1.
    [ Abstract ]

    The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented. Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR). The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).

56. Zhang, Rong, and Thomas L Delworth, 2006: Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes. Geophysical Research Letters, 33, L17712, doi:10.1029/2006GL026267.
    [ Abstract ]

    Prominent multidecadal fluctuations of India summer rainfall, Sahel summer rainfall, and Atlantic Hurricane activity have been observed during the 20th century. Understanding their mechanism(s) will have enormous social and economic implications. We first use statistical analyses to show that these climate phenomena are coherently linked. Next, we use the GFDL CM2.1 climate model to show that the multidecadal variability in the Atlantic ocean can cause the observed multidecadal variations of India summer rainfall, Sahel summer rainfall and Atlantic Hurricane activity (as inferred from vertical wind shear changes). These results suggest that to interpret recent climate change we cannot ignore the important role of Atlantic multidecadal variability.

57. Delworth, Thomas L., V Ramaswamy, and G Stenchikov, 2005: The impact of aerosols on simulated ocean temperature and heat content in the 20th century. Geophysical Research Letters, 32, L24709, doi:10.1029/2005GL024457.
    [ Abstract ]

    Observational analyses have documented increases in global ocean temperature, heat content, and sea level in the 20th century. Previous studies argued that the observed ocean warming is a response to increasing greenhouse gases. We use a new climate model to decompose simulated ocean temperature changes into components attributable to subsets of anthropogenic and natural influences. The model simulates a positive trend in global ocean volume mean temperature from the mid 1950s to 2000, consistent with observational estimates. We show that for the period 1861–2000 aerosols have delayed the onset of ocean warming by several decades and reduced the magnitude of the transient warming by approximately two-thirds when compared to the response that arises solely from increasing greenhouse gases. The simulated cooling signature from large volcanic eruptions in the late 19th and early 20th centuries is clearly visible in the subsurface ocean well into the middle part of the 20th century.

58. Findell, Kirsten L., and Thomas L Delworth, 2005: A modeling study of dynamic and thermodynamic mechanisms for summer drying in response to global warming. Geophysical Research Letters, 32, L16702, doi:10.1029/2005GL023414.
    [ Abstract PDF ]

    Past studies have suggested that increasing atmospheric CO2 will lead to a significant reduction of soil moisture during summer in the extratropics. These studies showed an increase in wintertime rainfall over most mid-latitude continental regions when CO2 is doubled, an earlier snowmelt season and onset of springtime evaporation, and a higher ratio of evaporation to precipitation in summer. These factors led to large-scale increases in soil moisture in winter and decreases in summer. We find that the above processes are important in simulated summer drying in a newly developed climate model. In addition to these thermodynamic processes, we find that changes in atmospheric circulation play an important role in regional hydroclimatic changes. Additional experiments show that the atmospheric circulation changes are forced by the CO2-induced warming of the ocean, particularly the tropical ocean. These results highlight the importance of sea surface temperature changes for regional hydroclimatic changes.

59. Held, Isaac M., Thomas L Delworth, Jian Lu, Kirsten L Findell, and Thomas R Knutson, 2005: Simulation of Sahel drought in the 20th and 21st centuries. Proceedings of the National Academy of Sciences, 102(50), doi:10.1073/pnas.0509057102.
    [ Abstract ]

    The Sahel, the transition zone between the Saharan desert and the rainforests of Central Africa and the Guinean Coast, experienced a severe drying trend from the 1950s to the 1980s, from which there has been partial recovery. Continuation of either the drying trend or the more recent ameliorating trend would have far-ranging implications for the economy and ecology of the region. Coupled atmosphere/ocean climate models being used to simulate the future climate have had difficulty simulating Sahel rainfall variations comparable to those observed, thus calling into question their ability to predict future climate change in this region. We describe simulations using a new global climate model that capture several aspects of the 20th century rainfall record in the Sahel. An ensemble mean over eight realizations shows a drying trend in the second half of the century of nearly half of the observed amplitude. Individual realizations can be found that display striking similarity to the observed time series and drying pattern, consistent with the hypothesis that the observations are a superposition of an externally forced trend and internal variability. The drying trend in the ensemble mean of the model simulations is attributable to anthropogenic forcing, partly to an increase in aerosol loading and partly to an increase in greenhouse gases. The model projects a drier Sahel in the future, due primarily to increasing greenhouse gases.

60. Lu, Jian, and Thomas L Delworth, 2005: Oceanic forcing of the late 20th century Sahel drought. Geophysical Research Letters, 32, L22706, doi:10.1029/2005GL023316.
    [ Abstract ]

    The Sahel region of Africa underwent a pronounced interdecadal drying trend in the latter half of the 20th century. In order to investigate this drying trend, several ensembles of numerical experiments are conducted using a recently developed atmospheric general circulation model (AM2, developed at NOAA's Geophysical Fluid Dynamics Laboratory). When the model is forced with the time series of observed SSTs and sea ice from 1950 to 2000, it successfully reproduces the observed interdecadal variability of Sahelian rainfall. Additional experiments are used to estimate the separate contributions to Sahel drought from SST anomalies in various ocean basins. In these, SST anomalies are applied only in the tropics, or only in the Atlantic, Indian and Pacific oceans separately. Forcing from the tropical oceans is dominant in driving the Sahelian rainfall trend. The response of Sahel rainfall to a general warming of the tropical oceans suggests a possible link to greenhouse gas-induced climate change.

61. Zhang, Rong, and Thomas L Delworth, 2005: Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation. Journal of Climate, 18(12), doi:10.1175/JCLI3460.1.
    [ Abstract ]

    In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.

62. Anderson, Jeffrey L., Ventakramani Balaji, Anthony J Broccoli, William F Cooke, Thomas L Delworth, Keith W Dixon, Leo J Donner, Krista A Dunne, Stuart Freidenreich, Stephen T Garner, Rich Gudgel, C Tony Gordon, Isaac M Held, Richard S Hemler, Larry W Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, Amy R Langenhorst, Ngar-Cheung Lau, Zhi Liang, Sergey Malyshev, P C D Milly, Mary Jo Nath, Jeff J Ploshay, V Ramaswamy, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Brian J Soden, William F Stern, Lori T Sentman, R John Wilson, Andrew T Wittenberg, and Bruce Wyman, December 2004: The New GFDL global atmosphere and land model AM2–LM2: Evaluation with prescribed SST simulations. Journal of Climate, 17(24), 4641-4673.
    [ Abstract PDF ]

    for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO). One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation. An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.

63. Manabe, Syukuro, Richard T Wetherald, P C D Milly, Thomas L Delworth, and Ronald J Stouffer, 2004: Century-scale change in water availability: CO₂-quadrupling experiment. Climatic Change, 64(1-2), 59-76.
    [ Abstract PDF ]

    It has been suggested that, unless a major effort is made, the atmospheric concentration of carbon dioxide may rise above four times the pre-industrial level in a few centuries. Here we use a coupled atmosphere-ocean-land model to explore the response of the global water cycle to such a large increase in carbon dioxide, focusing on river discharge and soil moisture. Our results suggest that water is going to be more plentiful in those regions of the world that are already `water-rich'. However, water stresses will increase significantly in regions and seasons that are already relatively dry. This could pose a very challenging problem for water-resource management around the world. For soil moisture, our results indicate reductions during much of the year in many semi-arid regions of the world, such as the southwestern region of North America, the northeastern region of China, the Mediterranean coast of Europe, and the grasslands of Australia and Africa. In some of these regions, soil moisture values are reduced by almost a factor of two during the dry season. The drying in semi-arid regions is likely to induce the outward expansion of deserts to the surrounding regions. Over extensive regions of both the Eurasian and North American continents in high and middle latitudes, soil moisture decreases in summer but increases in winter, in contrast to the situation in semi-arid regions. For river discharge, our results indicate an average increase of ~ 15% during the next few centuries. The discharges from Arctic rivers such as the Mackenzie and Ob' increase by much larger fractions. In the tropics, the discharges from the Amazonas and Ganga-Brahmaputra also increase considerably. However, the percentage changes in runoff from other tropical and many mid-latitude rivers are smaller.

64. Broccoli, Anthony J., Keith W Dixon, Thomas L Delworth, Thomas R Knutson, Ronald J Stouffer, and Fanrong Zeng, 2003: Twentieth-century temperature and precipitation trends in ensemble climate simulations including natural and anthropogenic forcing. Journal of Geophysical Research, 108(D24), 4798, doi:10.1029/2003JD003812.
    [ Abstract PDF ]

    We present results from a series of ensemble integrations of a global coupled atmosphere-ocean model for the period 1865-1997. Each ensemble consists of three integrations initialized from different points in a long-running GFDL R30 coupled model control simulation. The first ensemble includes time-varying forcing from greenhouse gases only. In the remaining three ensembles, forcings from anthropogenic sulfate aerosols, solar variability, and volcanic aerosols in the stratosphere are added progressively, such that the fourth ensemble uses all four of these forcings. The effects of anthropogenic sulfate aerosols are represented by changes in surface albedo, and the effects of volcanic aerosols are represented by latitude-dependent perturbations in incident solar radiation. Comparisons with observations reveal that the addition of the natural forcings (solar and volcanic) improves the simulation of global multidecadal trends in temperature, precipitation, and ocean heat content. Solar and volcanic forcings are important contributors to early twentieth century warming. Volcanic forcing reduces the warming simulated for the late twentieth century. Interdecadal variations in global mean surface air temperature from the ensemble of experiments with all four forcings are very similar to observed variations during most of the twentieth century. The improved agreement of simulated and observed temperature trends when natural climate forcings are included supports the climatic importance of variations in radiative forcing during the twentieth century.

65. Dixon, Keith W., Thomas L Delworth, Thomas R Knutson, Michael J Spelman, and Ronald J Stouffer, 2003: A comparison of climate change simulations produced by two GFDL coupled climate models. Global & Planetary Change, 37(1-2), 81-102.
    [ Abstract PDF ]

    The transient responses of two versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model to a climate change forcing scenario are examined. The same computer codes were used to construct the atmosphere, ocean, sea ice and land surface components of the two models, and they employ the same types of sub-grid-scale parameterization schemes. The two model versions differ primarily, but not solely, in their spatial resolution. Comparisons are made of results from six coarse-resolution R15 climate change experiments and three medium-resolution R30 experiments in which levels of greenhouse gases (GHGs) and sulfate aerosols are specified to change over time. The two model versions yield similar global mean surface air temperature responses until the second half of the 21st century, after which the R15 model exhibits a somewhat larger response. Polar amplification of the Northern Hemisphere's warming signal is more pronounced in the R15 model, in part due to the R15's cooler control climate, which allows for larger snow and ice albedo positive feedbacks. Both models project a substantial weakening of the North Atlantic overturning circulation and a large reduction in the volume of Arctic sea ice to occur in the 21st century. Relative to their respective control integrations, there is a greater reduction of Arctic sea ice in the R15 experiments than in the R30 simulations as the climate system warms. The globally averaged annual mean precipitation rate is simulated to increase over time, with both model versions projecting an increase of about 8% to occur by the decade of the 2080s. While the global mean precipitation response is quite similar in the two models, regional differences exist, with the R30 model displaying larger increases in equatorial regions.

66. Rutherford, S, M E Mann, Thomas L Delworth, and Ronald J Stouffer, 2003: Climate field reconstruction under stationary and nonstationary forcing. Journal of Climate, 16(3), 462-479.
    [ Abstract PDF ]

    The fidelity of climate reconstructions employing covariance-based calibration techniques is tested with varying levels of sparseness of available data during intervals of relatively constant (stationary) and increasing (nonstationary) forcing. These tests employ a regularized expectation-maximization algorithm using surface temperature data from both the instrumental record and coupled ocean-atmosphere model integrations. The results indicate that if radiative forcing is relatively constant over a data-rich calibration period and increases over a data-sparse reconstruction period, the imputed temperatures in the reconstruction period may be biased and may underestimate the true temperature trend. However, if radiative forcing is stationary over a data-sparse reconstruction period and increases over a data-rich calibration period, the imputed values in the reconstruction period are nearly unbiased. These results indicate that using the data-rich part of the twentieth-century instrumental record (which contains an increasing temperature trend plausibly associated with increasing radiative forcing) for calibration does not significantly bias reconstructions of prior climate.

67. Visbeck, M, E P Chassignet, R G Curry, Thomas L Delworth, R R Dickson, and G Krahmann, 2003: The ocean's response to North Atlantic Oscillation variability In The North Atlantic Oscillation: Climatic Significance and Environmental Impact, Washington, DC, American Geophysical Union, 113-145.
    [ Abstract ]

    The North Atlantic Oscillation (NAO) is the dominant mode of atmospheric variability in the North Atlantic Sector. Basin scale changes in the atmospheric forcing significantly affect properties and circulation of the ocean. Part of the response is local and rapid (surface temperature, mixed-layer depth, upper ocean heat content, surface Ekman transport, sea ice cover). However, the geostrophically balanced large-scale horizontal and overturning circulation can take several years to adjust to changes in the forcing. The delayed response is non-local in the sense that waves and the mean circulation communicate perturbations at the air-sea interface to other parts of the Atlantic basin. A delayed and non-local response can potentially give rise to oscillatory behavior if there is significant feedback from the ocean to the atmosphere. We conjecture that, on decadal and longer time scales, changes in the ocean's heat storage and transport should have an increasingly important impact on the climate. Finally, changes in the ocean circulation and distribution of heat and freshwater will also alter ventilation rates and pathways. Thus we expect a change in the net uptake of gases (e.g., O₂, CO₂), altered nutrient balance, and changes in the dispersion of marine life. We review what is known about the oceanic response to changes in NAO-induced forcing from combined theoretical, numerical experimentation and observational perspectives.

68. Delworth, Thomas L., Ronald J Stouffer, Keith W Dixon, Michael J Spelman, Thomas R Knutson, Anthony J Broccoli, P J Kushner, and Richard T Wetherald, 2002: Review of simulations of climate variability and change with the GFDL R30 coupled climate model. Climate Dynamics, 19(7), 555-574.
    [ Abstract PDF ]

    A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near milennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.

69. Milly, P C., Richard T Wetherald, Krista A Dunne, and Thomas L Delworth, 2002: Increasing risk of great floods in a changing climate. Nature, 415(6871), 514-517.
    [ Abstract PDF ]

    Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle with a consequent increase in flood risk. But the detection of anthropogenically forced changes in flooding is difficult because of the substantial natural variability; the dependence of streamflow trends on flow regime further complicates the issue. Here we investigate the changes in risk of great floods- that is, floods with discharges exceeding 100-year levels from basins larger than 200,000 km2- using both streamflow measurements and numerical simulations of the anthropogenic climate change associated with greenhouse gases and direct radiative effects of sulphate aerosols. We find that the frequency of great floods increased substantially during the twentieth century. The recent emergence of a statistically significant positive trend in risk of great floods is consistent with results from the climate model, and the model suggests that the trend will continue.

70. Broccoli, Anthony J., Thomas L Delworth, and Ngar-Cheung Lau, 2001: The effect of changes in observational coverage on the association between surface temperature and the Arctic Oscillation. Journal of Climate, 14(11), 2481-2485.
    [ Abstract PDF ]

    The effect of changes in observational coverage on the association between the Arctic oscillation (AO) and extratropical Northern Hemisphere surface temperature is examined. A coupled atmosphere-ocean model, which produces a realistic simulation of the circulation and temperature patterns associated with the AO, is used as a surrogate for the real-climate system. The association between the AO and spatial mean-temperature, as quantified by regressing the latter on the AO index, is subject to a positive bias due to the incomplete spatial coverage of the observational network. The bias is largest during the early part of the twentieth century and decreases, but does not vanish, thereafter.

71. Kushner, P J., Isaac M Held, and Thomas L Delworth, 2001: Southern Hemisphere atmospheric circulation response to global warming. Journal of Climate, 14(10), 2238-2249.
    [ Abstract PDF ]

    The response of the Southern Hemisphere (SH), extratropical, atmospheric general circulation to transient, anthropogenic, greenhouse warming is investigated in a coupled climate model. The extratropical circulation response consists of a SH summer half-year poleward shift of the westerly jet and a year-round positive wind anomaly in the stratosphere and the tropical upper troposphere. Along with the poleward shift of the jet, there is a poleward shift of several related fields, including the belt of eddy momentum-flux convergence and the mean meridional overturning in the atmosphere and in the ocean. The tropospheric wind response projects strongly onto the model's "Southern Annular Mode" (also known as the "Antarctic oscillation"), which is the leading pattern of variability of the extratropical zonal winds.

72. Levitus, S, J I Antonov, J Wang, Thomas L Delworth, M R Dix, and Anthony J Broccoli, 2001: Anthropogenic warming of Earth's climate system. Science, 292(5515), 267-270.
    [ Abstract PDF ]

    We compared the temporal variability of the heat content of the world ocean, of the global atmosphere, and of components of Earth's cryosphere during the latter half of the 20th century. Each component has increased its heat content (the atmosphere and the ocean) or exhibited melting (the cryosphere). The estimated increase of observed global ocean heat content (over the depth range from 0 to 3000 meters) between the 1950s and 1990s is at least one order of magnitude larger than the increase in heat content of any other component. Simulation results using an atmosphere-ocean general circulation model that includes estimates of the radiative effects of observed temporal variations in greenhouse gases, sulfate aerosols, solar irradiance, and volcanic aerosols over the past century agree with our observation-based estimate of the increase in ocean heat content. The results we present suggest that the observed increase in ocean heat content may largely be due to the increase of anthropogenic gases in Earth's atmosphere.

73. Manabe, Syukuro, Thomas R Knutson, Ronald J Stouffer, and Thomas L Delworth, 2001: Exploring natural and anthropogenic variation of climate. Quarterly Journal of the Royal Meteorological Society, 127(571), 1-24.
    [ Abstract PDF ]

    This lecture discusses the low-frequency variability of surface temperature using a coupled ocean-atmosphere-land-surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the coupled model.

74. Stocker, T F., Thomas L Delworth, Stephen M Griffies, Isaac M Held, V Ramaswamy, and Brian J Soden, et al., 2001: Physical climate processes and feedbacks In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 418-470.
75. Allen, M R., P Stott, J F B Mitchell, R Schnur, and Thomas L Delworth, 2000: Quantifying the uncertainty in forecasts of anthropogenic climate change. Nature, 407(6804), 617-620.
    [ Abstract PDF ]

    Forecasts of climate change are inevitably uncertain. It is therefore essential to quantify the risk of significant departures from the predicted response to a given emission scenario. Previous analyses of this risk have been based either on expert opinion, perturbation analysis of simplified climate models or the comparison of predictions from general circulation models. Recent observed changes that appear to be attributable to human influence provide a powerful constraint on the uncertainties in multi-decadal forecasts. Here we assess the range of warming rates over the coming 50 years that are consistent with the observed near-surface temperature record as well as with the overall patterns of response predicted by several general circulation models. We expect global mean temperatures in the decade 2036-46 to 1-2.5 K warmer than in pre-industrial times under a 'business as usual' emission scenario. This range is relatively robust to errors in the models' climate sensitivity, rate of oceanic heat uptake or global response to sulphate aerosols as long as these errors are persistent over time. Substantial changes in the current balance of greenhouse warming and sulphate aerosol cooling would, however, increase the uncertainty. Unlike 50-year warming rates, the final equilibrium warming after the atmospheric composition stabilizes remains very uncertain, despite the evidence provided by the emerging signal.

76. Delworth, Thomas L., and Keith W Dixon, 2000: Implications of the recent trend in the Arctic/North Atlantic Oscillation for the North Atlantic Thermohaline Circulation. Journal of Climate, 13(21), 3721-3727.
    [ Abstract PDF ]

    Most projections of greenhouse gas-induced climate change indicate a weakening of the thermohaline circulation (THC) in the North Atlantic in response to increased freshening and warming in the subpolar region. These changes reduce high-latitude upper-ocean density and therefore weaken the THC. Using ensembles of numerical experiments with a coupled ocean-atmosphere model, it is found that this weakening could be delayed by several decades in response to a sustained upward trend in the Arctic/North Atlantic oscillation during winter, such as has been observed over the last 30 years. The stronger winds over the North Atlantic associated with this trend extract more heat from the ocean, thereby cooling and increasing the density of the upper ocean and thus opposing the previously described weakening of the THC. This result is of particular importance if the positive trend in the Arctic/North Atlantic oscillation is a response to increasing greenhouse gases, as has been recently suggested.

77. Delworth, Thomas L., and R J Greatbatch, 2000: Multidecadal thermohaline circulation variability driven by atmospheric surface flux forcing. Journal of Climate, 13(9), 1481-1495.
    [ Abstract PDF ]

    Previous analyses of an extended integration of the Geophysical Fluid Dynamics Laboratory coupled climate model have revealed pronounced multidecadal variations of the thermohaline circulation (THC) in the North Atlantic. The purpose of the current work is to assess whether those fluctuations can be viewed as a coupled air-sea mode (in the sense of ENSO), or as an oceanic response to forcing from the atmosphere model, in which large-scale feedbacks from the ocean to the atmospheric circulation are not critical. A series of integrations using the ocean component of the coupled model are performed to address the above question. The ocean model is forced by suitably chosen time series of surface fluxes from either the coupled model or a companion integration of an atmosphere-only model run with a prescribed seasonal cycle of SSTs and sea-ice thickness. These experiments reveal that 1) the previously identified multidecadal THC variations can be largely viewed as an oceanic response to surface flux forcing from the atmosphere model, although air-sea coupling through the thermodynamics appears to modify the amplitude of the variability, and 2) variations in heat flux are the dominant term (relative to the freshwater and momentum fluxes) in driving the THC variability. Experiments driving the ocean model using either high- or low-pass-filtered heat fluxes, with a cutoff period of 20 yrs. show that the multidecadal THC variability is driven by the low-frequency portion of the spectrum of atmospheric flux forcing. Analyses have also revealed that the multidecadal THC fluctuations are driven by a spatial pattern of surface heat flux variations that bears a strong resemblance to the North Atlantic oscillation. No conclusive evidence is found that the THC variability is part of a dynamically coupled mode of the atmosphere and ocean models.

78. Delworth, Thomas L., and Thomas R Knutson, 2000: Simulation of early 20th Century global warming. Science, 287(5461), 2246-2250.
    [ Abstract PDF ]

    The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model's climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.

79. Delworth, Thomas L., and M E Mann, 2000: Observed and simulated multidecadal variability in the Northern Hemisphere. Climate Dynamics, 16(9), 661-676.
    [ Abstract PDF ]

    Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct oscillatory mode of variability with an approximate time scale of 70 years. This variability is also seen in instrumental records, although the oscillatory nature of the variability is difficult to assess due to the short length of the instrumental record. The spatial pattern of this variability is hemispheric or perhaps even global in scale, but with particular emphasis on the Atlantic region. Independent analyses of multi-century integrations of two versions of the GFDL coupled atmosphere-ocean model also show the existence of distinct multidecadal variability in the North Atlantic region which resembles the observed pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic. It is our intent here to provide a direct comparison of the observed variability to that simulated in a coupled ocean-atmosphere model, making use of both existing instrumental analyses and newly available proxy based multi-century surface temperature estimates. The analyses demonstrate a substantial agreement between the simulated and observed patterns of multidecadal variability in sea surface temperature (SST) over the North Atlantic. There is much less agreement between the model and observations for sea level pressure. Seasonal analyses of the variability demonstrate that for both the model and observations SST appears to be the primary carrier of the multidecadal signal.

80. Mehta, V M., E Lindstrom, A Busalacchi, J E Hansen, K M Lau, J Susskind, Thomas L Delworth, C Deser, G A Meehl, L-L Fu, and S Levitus, et al., 2000: Proceedings of the NASA Workshop on Decadal Climate Variability. Bulletin of the American Meteorological Society, 81(12), 2983-2986.
    [ PDF ]
81. Mehta, V M., M J Suarez, J Y Manganello, and Thomas L Delworth, 2000: Oceanic influence on the North Atlantic Oscillation and associated Northern Hemisphere climate variations: 1959-1993. Geophysical Research Letters, 27(1), 121-124.
    [ Abstract PDF ]

    The North Atlantic Oscillation (NAO) exhibits variations at interannual to multidecadal time scales and is associated with climate variations over eastern North America, the North Atlantic, Europe, and North Africa. Therefore, it is very important to understand causes of these NAO variations and assess their predictability. It has been hypothesized, based on observations, that sea surface temperature (SST) and sea-ice variations in the North Atlantic Ocean influence the NAO. We describe results of an ensemble of sixteen experiments with an atmospheric general circulation model in which we used observed SST and sea-ice boundary conditions globally during 1949-1993. We show that multiyear NAO and associated climate variations can be simulated reasonably accurately if results from a large number of experiments are averaged. We also show that the ambiguous results of previous NAO modeling studies were strongly influenced by the ensemble size, which was much smaller than that in the present study. The implications of these results for understanding and predictability of the NAO are discussed.

82. Barnett, T P., and Thomas L Delworth, et al., 1999: Detection and attribution of recent climate change: A status report. Bulletin of the American Meteorological Society, (80), 12, 2631-2660.
    [ Abstract PDF ]

    This paper addresses the question of where we now stand with respect to detection and attribution of an anthropogenic climate signal. Our ability to estimate natural climate variability, against which claims of anthropogenic signal detection must be made, is reviewed. The current situation suggests control runs of global climate models may give the best estimates of natural variability on a global basis, estimates that appear to be accurate to within a factor of 2 or 3 at multidecadal timescales used in detection work. Present uncertainties in both observations and model-simulated anthropogenic signals in near-surface air temperature are estimated. The uncertainty in model simulated signals is, in places, as large as the signal to be detected. Two different, but complementary, approaches to detection and attribution are discussed in the context of these uncertainties. Applying one of the detection strategies, it is found that the change in near-surface, June through August air temperature field over the last 50 years is generally different at a significance level of 5% from that expected from model-based estimates of natural variability. Greenhouse gases alone cannot explain the observed change. Two of four climate models forced by greenhouse gases and direct sulfate aerosols produce results consistent with the current climate change observations, while the consistency of the other two depends on which model's anthropogenic fingerprints are used. A recent integration with additional anthropogenic forcings (the indirect effects of sulfate aerosols and tropospheric ozone) and more complete tropospheric chemistry produced results whose signal amplitude and pattern were consistent with current observations, provided the model's fingerprint is used and detection carried out over only the last 30 years of annually averaged data. This single integration currently cannot be corroborated and provides no opportunity to estimate the uncertainties inherent in the results, uncertainties that are thought to be large and poorly known. These results illustrate the current large uncertainty in the magnitude and spatial pattern of the direct and indirect sulfate forcing and climate response. They also show detection statements depend on model-specific fingerprints, time period, and seasonal character of the signal, dependencies that have not been well explored. Most, but not all, results suggest that recent changes in global climate inferred from surface air temperature are likely not due solely to natural causes. At present it is not possible to make a very confident statement about the relative contributions of specific natural and anthropogenic forcings to observed climate change. One of the main reasons is that fully realistic simulations of climate change due to the combined effects of all anthropogenic and natural forcings mechanisms have yet to be computed. A list of recommendations for reducing some of the uncertainties that currently hamper detection and attribution studies is presented.

83. Delworth, Thomas L., Anthony J Broccoli, Keith W Dixon, Isaac M Held, Thomas R Knutson, P J Kushner, Michael J Spelman, Ronald J Stouffer, K Y Vinnikov, and Richard T Wetherald, 1999: Coupled climate modelling at GFDL: Recent accomplishments and future plans. Clivar Exchanges, 4(4), 15-20.
    [ PDF ]
84. Delworth, Thomas L., Jerry D Mahlman, and Thomas R Knutson, 1999: Changes in heat index associated with CO₂ -induced global warming. Climatic Change, 43(2), 369-386.
    [ Abstract PDF ]

    Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO₂), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765-2065 is used to force the model, and a third (fourth) integration in which atmospheric CO₂ increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.

85. Dixon, Keith W., Thomas L Delworth, Michael J Spelman, and Ronald J Stouffer, 1999: The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM climate change experiment. Geophysical Research Letters, 26(17), 2749-2752.
    [ Abstract PDF ]

    The mechanism by which the model-simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi-century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.

86. Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, and Ronald J Stouffer, 1999: Model assessment of regional surface temperature trends (1949-1997). Journal of Geophysical Research, 104(D24), 30,981-30,996.
    [ Abstract PDF ]

    Analyses are conducted to assess whether simulated trends in SST and land surface air temperature from two versions of a coupled ocean-atmosphere model are consistent with the geographical distribution of observed trends over the period 1949-1997. The simulated trends are derived from model experiments with both constant and time-varying radiative forcing. The models analyed are low-resolution (R15, ~4º) and medium-resolution (R30, ~2º) versions of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled climate model. Internal climate variability is estimated from long control integrations of the models with no change of external forcing. The radiatively forced trends are based on ensembles of integrations using estimated past concentrations of greenhouse gases and direct effects of anthropogenic sulfate aerosols (G+S). For the regional assessment, the observed trends at each grid point with adequate temporal coverage during 1949-1997 are first compared with the R15 and R30 model unforced internal variability. Nearly 50% of the analyzed areas have observed warming trends exceeding the 95th percentile of trends from the control simulations. These results suggest that regional warming trends over much of the globe during 1949-1997 are very unlikely to have occurred due to internal climate variability alone and suggest a role for a sustained positive thermal forcing such as increasing greenhouse gases. The observed trends are then compared with the trend distributions obtained by combining the ensemble mean G+S forced trends with the internal variability "trend" distributions from the control runs. Better agreement is found between the ensemble mean G+S trends and the observed trends than between the model internal variability alone and the observed trends. However, the G+S trends are still significantly different from the observed trends over about 30% of the areas analyzed. Reasons for these regional inconsistencies between the simulated and the observed trends include possible deficiencies in (1) specified radiative forcings, (2) simulated responses to specified radiative forcings, (3) simulation of internal climate variability, or (4) observed temperature records.

87. Delworth, Thomas L., and V M Mehta, 1998: Simulated interannual to decadal variability in the tropical and sub-tropical North Atlantic. Geophysical Research Letters, 25(15), 2825-2828.
    [ Abstract PDF ]

    The dominant pattern of tropical and sub-tropical North Atlantic sea surface temperature (SST) anomalies simulated in the GFDL coupled ocean-atmosphere model is identified and compared to observations. The spatial pattern and temporal variability of that pattern resemble observational results. On interannual time scales it is shown that anomalous surface heat fluxes, consistent with variations in the intensity of the sub-tropical high pressure system in the atmosphere and the associated Northeasterly Trade winds, appear to be the most important process for generating this SST pattern. This relationship is also true on decadal time scales, although the relative role of oceanic heat advection is somewhat larger than on the interannual time scales.

88. Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1997: Multidecadal climate variability in the Greenland Sea and surrounding regions: a coupled model simulation. Geophysical Research Letters, 24(3), 257-260.
    [ Abstract PDF ]

    Pronounced oscillations of ocean temperature and salinity occur in the Greenland Sea in a 2000 year integration of a coupled ocean-atmosphere model. The oscillations, involving both the surface and subsurface ocean layers, have a timescale of approximately 40-80 years, and are associated with fluctuations in the intensity of the East Greenland Current. The Greenland Sea temperature and salinity variations are preceded by large-scale changes in near-surface salinity in the Arctic, which appear to propagate out of the Arctic through the East Greenland Current. These anomalies then propagate around the subpolar gyre into the Labrador Sea and the central North Atlantic. These oscillations are coherent with previously identified multi-decadal fluctuations in the intensity of the North Atlantic thermohaline circulation. The oscillations in the Greenland Sea are related to atmospheric variability. Negative (cold) anomalies of surface air temperature are associated with negative (cold) sea surface temperature (SST) anomalies in the Greenland Sea, with amplitudes up to 2°C near Greenland declining to several tenths of a degree C over northwestern Europe. The cold SST anomalies and intensified East Greenland Current are also associated with enhanced northerly winds over the Greenland Sea.

89. Delworth, Thomas L., 1996: North Atlantic interannual variability in a coupled ocean-atmosphere model. Journal of Climate, 9(10), 2356-2375.
    [ Abstract PDF ]

    The primary mode of sea surface temperature variability in the North Atlantic on interannual timescales during winter is examined in a coupled ocean-atmosphere model. The model, developed at the Geophysical Fluid Dynamics Laboratory, is global in domain with realistic geography and a seasonal cycle of insolation. Analyses performed on a 1000-year integration of this model show that this mode is characterized by zonal bands of SST anomalies in the North Atlantic and bears a distinct resemblance to observational results. The largest anomalies in the model are to the southeast of Newfoundland. The model SST variations appear to be related to a north-south dipole in the atmospheric 500-mb geopotential height field, which resembles the North Atlantic oscillation and the Western Atlantic pattern. Analyses are presented that show that this mode of SST variability is primarily driven by perturbations to the surface heat fluxes, which are largely governed by atmospheric variability. Changes in model ocean circulation also contribute to this mode of variability but appear to be of secondary importance. Additional integrations are analyzed to examine the above conclusion. The same atmospheric model used in the above integration was coupled to a 50-m slab ocean and integrated for 500 years. The primary mode of SST variability in this model, in which there were no effects of ocean dynamics, resembles the primary mode from the coupled model, strengthening the conclusion that the surface fluxes are the primary mechanism generating this oceanic variability. One notable difference between the two models is related to the presence of deep vertical mixing at high latitudes in the model with a fully dynamic ocean. An additional 500-year integration of the atmospheric model with a prescribed seasonal cycle of SSTs lends further support to this conclusion, as do additional diagnostic calculations in which a 50-m slab ocean was forced by the time series of surface fluxes from both the prescribed SST and fully coupled model.

90. Delworth, Thomas L., and Syukuro Manabe, 1996: Climate variability and land surface processes In From Atmospheric Circulation to Global Change - Celebration of the 80th Birthday of Prof. YE Duzheng, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing: China, China Meteorological Press, 477-502.
    [ Abstract ]

    The coupled ocean-atmosphere-land climate system is characterized by substantial amounts of variability at a wide range of spatial and temporal scales. This natural variability of climate increases the difficulty of detecting climate change attributable to increasing greenhouse gas concentrations. A key issue in climate research is obtaining a better description of this variability and the physical mechanisms responsible for it. One of the important physical processes contributing to this variability is the interaction between the land surface and the atmosphere. Through its effect on the surface energy flux components, the land surface can exert a pronounced effect on the variability of the atmosphere. The potential importance of such interactions for climate variability is examined through the use of numerical modeling studies. The physical mechanisms governing the time scales of soil moisture variability in the model are outlined, and observational evidence is presented supporting this analysis. In addition, it is shown that interactions between soil wetness and the atmosphere can both increase the total variability of the atmosphere and lengthen the time scales of near-surface atmospheric fluctuations.

91. Delworth, Thomas L., 1995: Commentary on the paper of Parker et al In Natural Climate Variability on Decade-to-Century Time Scales, Washington, DC, National Academy Press, 251.
92. Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1995: North Atlantic Interdecadal variability in a coupled model In Natural Climate Variability on Decade-to-Century Time Scales, Washington, DC, National Academy Press, 432-439; 440-441.
    [ Abstract ]

    A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 40 to 50 years. The fluctuations appear to be driven by density anomalies in the sinking region of the thermohaline circulation combined with much smaller density anomalies of opposite sign in the broad, rising region. Anomalies of sea surface temperature associated with this oscillation induce surface air temperature anomalies over the northern North Atlantic, the Arctic, and northwestern Europe. The spatial pattern of sea surface temperature anomalies bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic.

93. Mehta, V M., and Thomas L Delworth, 1995: Decadal variability of the tropical Atlantic Ocean surface temperature in shipboard measurements and in a global ocean-atmosphere model. Journal of Climate, 8(2), 172-190.
    [ Abstract PDF ]

    Numerous analyses of relatively short (25-30 years in length) time series of the observed surface temperature of the tropical Atlantic Ocean have indicated the possible existence of decadal timescale variability. It was decided to search for such variability in 100-yr time series of sea surface temperature (SST) measured aboard ships and available in the recently published Global Ocean Surface Temperature Atlas (GOSTA). Fourier and singular spectrum analyses of the GOSTA SST time series averaged over 11 subregions, each approximately 1 x 106 km2 in area, show that pronounced quasi-oscillatory decadal (~8-20 yr) and multidecadal (~30-40 yr) timescale variability exists in the GOSTA dataset over the tropical Atlantic. Motivated by the above results, SST variability was investigated in a 200-yr integration of a global model of the coupled oceanic and atmospheric general circulations developed at the Geophysical Fluid Dynamics Laboratory (GFDL). The second 100 yr of SST in the coupled model's tropical Atlantic region were analyzed with a variety of techniques. Analyses of SST time series, averaged over approximately the same subregions as the GOSTA time series, showed that the GFDL SST anomalies also undergo pronounced quasi-osciallatory decadal and multidecadal variability but at somewhat shorter timescales than the GOSTA SST anomalies. Further analyses of the horizontal sturctures of the decadal timescale variability in the GFDL coupled model showed the existence of two types of variability in general agreement with results of the GOSTA SST time series analyses. One type, characterized by timescales between 8 and 11 yr, has high spatial coherence within each hemisphere but not between the two hemispheres of the tropical Atlantic. A second type, characterized by timescales between 12 and 20 yr, has high spatial coherence between the two hemispheres. The second type of variability is considerably weaker than the first. As in the GOSTA time series, the multidecadal variability in the GFDL SST time series has approximately opposite phases between the tropical North and South Atlantic Oceans. Empirical orthogonal function analyses of the tropical Atlantic SST anomalies revealed a north-south bipolar pattern as the dominant pattern of decadal variability. It is suggested that the bipolar pattern can be interpreted as decadal variability of the interhemispheric gradient of SST anomalies. The decadal and multidecadal timescale variability of the tropical Atlantic SST, both in the actual and in the GFDL model, stands out significantly above the background "red noise" and is coherent within each of the time series, suggesting that specific sets of proceses may be responsible for the choice of the decadal and multidecadal timescales. Finally, it must be emphasized that the GFDL coupled ocean-atmosphere model generates the decadal and multidecadal timescale variability without any externally applied force, solar or lunar, at those timescales.

94. Delworth, Thomas L., Syukuro Manabe, and Ronald J Stouffer, 1993: Interdecadal variations of the thermohaline circulation in a coupled ocean-atmosphere model. Journal of Climate, 6(11), 1993-2011.
    [ Abstract PDF ]

    A fully coupled ocean-atmosphere model is shown to have irregular oscillations of the thermohaline circulation in the North Atlantic Ocean with a time scale of approximately 50 years. The irregular oscillation appears to be driven by density anomalies in the sinking region of the thermohaline circulation (approximately 52°N to 72°N) combined with much smaller density anomalies of opposite sign in the broad, rising region. The spatial pattern of sea surface temperature anomalies associated with this irregular oscillation bears an encouraging resemblance to a pattern of observed interdecadal variability in the North Atlantic. The anomalies of sea surface temperature induce model surface air temperature anomalies over the northern North Atlantic, Arctic, and northwestern Europe.

95. Delworth, Thomas L., and Syukuro Manabe, 1993: Climate variability and land-surface processes. Advances in Water Resources, 16, 3-20.
    [ Abstract PDF ]

    The coupled ocean-atmosphere-land climate system is characterized by substantial amounts of variability on a wide range of spatial and temporal scales. This natural variability of climate increases the difficulty of detecting climate change attributable to increasing greenhouse gas concentrations. A key issue in climate research is obtaining a better description of this variability and the physical mechanisms responsible for it. One of the important physical processes contributing to this variability is the interaction between the land surface and the atmosphere. Through its effect on the surface energy flux components, the land surface can exert a pronounced effect on the variability of the atmosphere. The potential importance of such interactions for climate variability is examined through the use of numerical modeling studies. The physical mechanisms governing the time scales of soil moisture variability in the model are outlined, and observational evidence is presented supporting this analysis. In addition, it is shown that interactions between soil wetness and the atmosphere can both increase the total variability of the atmosphere and lengthen the time scales of near-surface atmospheric fluctuations.

96. Manabe, Syukuro, and Thomas L Delworth, 1990: The temporal variability of soil wetness and its impact on climate. Climatic Change, 16, 185-192.
    [ Abstract PDF ]

    The temporal variability of soil wetness and its interactions with the atmosphere were studied using a general circulation model of the atmosphere. It was found that time series of soil wetness computed by the model contain substantial amounts of variance at low frequencies. Long time-scale anomalies of soil moisture resemble the red noise response of the soil layer to white noise rainfall forcing. The dependence of the temporal variability of soil moisture on potential evaporation and precipitation is discussed.

97. Delworth, Thomas L., and Syukuro Manabe, 1989: The influence of soil wetness on near-surface atmospheric variability. Journal of Climate, 2(12), 1447-1462.
    [ Abstract PDF ]

    The influence of land surface processes on near-surface atmospheric variability on seasonal and interannual time scales is studied using output from two integrations of a general circulation model. In the first experiment, of 50 year s duration, soil moisture is predicted, thereby taking into consideration interactions between the surface moisture budget and the atmosphere. In the second experiment, of 25 years duration, the seasonal cycle of soil moisture is prescribed at each grid point based upon the results of the first integration, thereby suppressing these interactions. The same seasonal cycle of soil moisture is prescribed for each year of the second integration. Differences in atmospheric variability between the two integrations are due to interactions between the surface moisture budget and the atmosphere. Analyses of monthly data indicate that the surface moisture budget interacts with the atmosphere in such a way as to lengthen the time scales of fluctuation of near-surface relative humidity and temperature, as well as to increase the total variability of the atmosphere. During summer months at middle latitudes, the persistence of near-surface relative humidity, as measured by correlations of monthly mean relative humidity between successive months, increases from near zero in the experiment with prescribed soil moisture to as large as 0.6 in the experiment with interactive soil moisture, which corresponds to an e-folding time of approximately two months. The standard deviation of monthly mean relative humidity during summer is substantially larger in the experiment with interactive soil moisture than in the experiment with prescribed soil moisture. Surface air temperature exhibits similar changes, but of smaller magnitude. Soil wetness influences the atmosphere by altering the partitioning of the outgoing energy flux at the surface into latent and sensible heat compon ents. Fluctuations of soil moisture result in large variations in these fluxes, and thus significant variations in near surface relative humidity and temperature. Because anomalies of monthly mean soil moisture are characterized by seasonal and interannual time scales, they create persistent anomalous fluxes of latent and sensible heat, thereby increasing the persistence of near-surface atmospheric relative humidity and temperature.

98. Delworth, Thomas L., and Syukuro Manabe, 1988: Influence of potential evaporation on the variabilities of simulated soil wetness and climate. Journal of Climate, 1(5), 523-547.
    [ Abstract PDF ]

    An atmospheric general circulation model with prescribed sea surface temperature and cloudiness was integrated for 50 years to study atmosphere-land surface interactions. The temporal variability of model soil moisture and precipitation has been studied in an effort to understand the interactions of these variables with other components of the climate system. Temporal variability analysis has shown that the spectra of monthly mean precipitation over land are close to white at all latitudes, with total variance decreasing poleward. In contrast, the spectra of soil moisture are red and become more red with increasing latitude. As a measure of this redness, half of the total variance of a composite tropical soil moisture spectrum occurs at periods longer than nine months, while at high latitudes, half of the total variance of a composite soil moisture spectrum occurs at periods longer than 22 months. The spectra of soil moisture also exhibit marked longitudinal variations. These spectral results may be viewed in light of stochastic theory. The formulation of the GFDL soil moisture parameterization is mathematically similar to a stochastic process. According to this model, forcing of a system by an input white noise variable (precipitation) will yield an output variable (soil moisture) with a red spectrum, the redness of which is controlled by a damping term (potential evaporation). Thus, the increasingly red nature of the soil moisture spectra at higher latitudes is a result of declining potential evaporation values at higher latitudes. Physically, soil moisture excesses are dissipated more slowly at high latitudes, where the energy available for evaporation is small. Some of the longitudinal variations in soil moisture spectra result from longitudinal variations in potential evaporation, while others are explicable in terms of the value of the ratio of potential evaporation to precipitation. Regions where this value is less than one are characterized by frequent runoff and short time scales of soil moisture variability. By preventing excessive positive anomalies of soil moisture, the runoff process hastens the return of soil moisture values to their mean state, thereby shortening soil moisture time scales. Through the use of a second GCM integration with prescribed soil moisture, it was shown that interactive soil moisture may substantially increase summer surface air temperature variability. Soil moisture interacts with the atmosphere primarily through the surface energy balance. The degree of soil saturation strongly influences the partitioning of outgoing energy from the surface between the latent and sensible heat fluxes. Interactive soil moisture allows larger variations of these fluxes, thereby increasing the variance of surface air temperature. Because the flux of latent heat is directly proportional to potential evaporation under conditions of sufficient moisture, the influence of soil moisture on the atmosphere is greatest when the potential evaporation value is large. This occurs most frequently in the tropics and summer hemisphere extratropics.

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