Evaluating the performance of CMIP6 models in the tropical Atlantic: mean state, variability, and remote impacts

Tropical Pacific ◽

Boreal Summer ◽

Tropical Atlantic ◽

Westerly Wind ◽

Equatorial Atlantic ◽

Wide Range ◽

Mean State ◽

General circulation models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) are examined with respect to their ability to simulate the mean state and variability of the tropical Atlantic, as well as its linkage to the tropical Pacific. While, on average, mean state biases have improved little relative to the previous intercomparison (CMIP5), there are now a few models with very small biases. In particular the equatorial Atlantic warm SST and westerly wind biases are mostly eliminated in these models. Furthermore, interannual variability in the equatorial and subtropical Atlantic is quite realistic in a number of CMIP6 models, which suggests that they should be useful tools for understanding and predicting variability patterns. The evolution of equatorial Atlantic biases follows the same pattern as in previous model generations, with westerly wind biases during boreal spring preceding warm sea-surface temperature (SST) biases in the east during boreal summer. A substantial portion of the westerly wind bias exists already in atmosphere-only simulations forced with observed SST, suggesting an atmospheric origin. While variability is relatively realistic in many models, SSTs seem less responsive to wind forcing than observed, both on the equator and in the subtropics, possibly due to an excessively deep mixed layer originating in the oceanic component. Thus models with realistic SST amplitude tend to have excessive wind amplitude. The models with the smallest mean state biases all have relatively high resolution but there are also a few low-resolution models that perform similarly well, indicating that resolution is not the only way toward reducing tropical Atlantic biases. The results also show a relatively weak link between mean state biases and the quality of the simulated variability. The linkage to the tropical Pacific shows a wide range of behaviors across models, indicating the need for further model improvement.

Successive Modulation of ENSO to the Future Greenhouse Warming

Journal of Climate ◽

10.1175/2007jcli1500.1 ◽

2008 ◽

Vol 21 (1) ◽

pp. 3-21 ◽

Cited By ~ 55

Author(s):

Soon-Il An ◽

Jong-Seong Kug ◽

Yoo-Geun Ham ◽

In-Sik Kang

Keyword(s):

General Circulation ◽

Southern Oscillation ◽

Tropical Pacific ◽

Greenhouse Warming ◽

Delayed Response ◽

Subsurface Temperature ◽

Climate System Model ◽

The Mean ◽

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period of ENSO fluctuates in a multidecadal time scale. The climate system model outputs suggest that the multidecadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigenanalysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs in terms of the multidecadal modulations of the ENSO amplitude and period. In particular, the change in the vertical contrast, that is, change in difference between the subsurface temperature and SST, turns out to be more influential on the ENSO modulation than changes in the mean SST itself.

A CGCM Study on the Interaction between IOD and ENSO

Journal of Climate ◽

10.1175/jcli3797.1 ◽

2006 ◽

Vol 19 (9) ◽

pp. 1688-1705 ◽

Cited By ~ 174

Author(s):

Swadhin K. Behera ◽

Jing Jia Luo ◽

Sebastien Masson ◽

Suryachandra A. Rao ◽

Hirofumi Sakuma ◽

...

Keyword(s):

Indian Ocean ◽

General Circulation ◽

Southern Oscillation ◽

Walker Circulation ◽

Tropical Pacific ◽

Boreal Summer ◽

Enso Variability ◽

The Indian Ocean ◽

Sst Anomalies ◽

Abstract An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Niño–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian Ocean is reproduced. As in the observation, IOD peaks in boreal fall for both model experiments. In the absence of ENSO variability the interannual IOD variability is dominantly biennial. The ENSO variability is found to affect the periodicity, strength, and formation processes of the IOD in years of co-occurrences. The amplitudes of SST anomalies in the western pole of co-occurring IODs are aided by dynamical and thermodynamical modifications related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the alteration of western anomalies. It is found that 42% of IOD events affected by changes in the Walker circulation are related to the tropical Pacific variabilities including ENSO. The formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the noENSO experiment.

A Pacific Centennial Oscillation Predicted by Coupled GCMs*

Journal of Climate ◽

10.1175/jcli-d-11-00421.1 ◽

2012 ◽

Vol 25 (17) ◽

pp. 5943-5961 ◽

Cited By ~ 29

Author(s):

Kristopher B. Karnauskas ◽

Jason E. Smerdon ◽

Richard Seager ◽

Jesús Fidel González-Rouco

Keyword(s):

Time Scale ◽

General Circulation ◽

Southern Oscillation ◽

Heat Content ◽

Wave Pattern ◽

Tropical Pacific ◽

Equatorial Pacific ◽

Centennial Time Scale ◽

Abstract Internal climate variability at the centennial time scale is investigated using long control integrations from three state-of-the-art global coupled general circulation models. In the absence of external forcing, all three models produce centennial variability in the mean zonal sea surface temperature (SST) and sea level pressure (SLP) gradients in the equatorial Pacific with counterparts in the extratropics. The centennial pattern in the tropical Pacific is dissimilar to that of the interannual El Niño–Southern Oscillation (ENSO), in that the most prominent expression in temperature is found beneath the surface of the western Pacific warm pool. Some global repercussions nevertheless are analogous, such as a hemispherically symmetric atmospheric wave pattern of alternating highs and lows. Centennial variability in western equatorial Pacific SST is a result of the strong asymmetry of interannual ocean heat content anomalies, while the eastern equatorial Pacific exhibits a lagged, Bjerknes-like response to temperature and convection in the west. The extratropical counterpart is shown to be a flux-driven response to the hemispherically symmetric circulation anomalies emanating from the tropical Pacific. Significant centennial-length trends in the zonal SST and SLP gradients rivaling those estimated from observations and model simulations forced with increasing CO2 appear to be inherent features of the internal climate dynamics simulated by all three models. Unforced variability and trends on the centennial time scale therefore need to be addressed in estimated uncertainties, beyond more traditional signal-to-noise estimates that do not account for natural variability on the centennial time scale.

The Predictability of Tropical Pacific Decadal Variability: Insights from Attractor Reconstruction

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0114.1 ◽

2019 ◽

Vol 76 (3) ◽

pp. 801-819 ◽

Cited By ~ 1

Author(s):

Nandini Ramesh ◽

Mark A. Cane

Keyword(s):

General Circulation ◽

Decadal Variability ◽

Tropical Pacific ◽

The Other ◽

Intermediate Complexity ◽

Pacific Decadal Variability ◽

Attractor Reconstruction ◽

Climatic Impacts ◽

Abstract Tropical Pacific decadal variability (TPDV), though not the totality of Pacific decadal variability, has wide-ranging climatic impacts. It is currently unclear whether this phenomenon is predictable. In this study, we reconstruct the attractor of the tropical Pacific system in long, unforced simulations from an intermediate-complexity model, two general circulation models (GCMs), and the observations with the aim of assessing the predictability of TPDV in these systems. We find that in the intermediate-complexity model, positive (high variance, El Niño–like) and negative (low variance, La Niña–like) phases of TPDV emerge as a pair of regime-like states. The observed system bears resemblance to this behavior, as does one GCM, while the other GCM does not display this structure. However, these last three time series are too short to confidently characterize the full distribution of interdecadal variability. The intermediate-complexity model is shown to lie in highly predictable parts of its attractor 37% of the time, during which most transitions between TPDV regimes occur. The similarities between the observations and this system suggest that the tropical Pacific may be somewhat predictable on interdecadal time scales.

Interactive Feedback between the Tropical Pacific Decadal Oscillation and ENSO in a Coupled General Circulation Model

Journal of Climate ◽

10.1175/2009jcli2782.1 ◽

2009 ◽

Vol 22 (24) ◽

pp. 6597-6611 ◽

Cited By ~ 47

Author(s):

Jung Choi ◽

Soon-Il An ◽

Boris Dewitte ◽

William W. Hsieh

Keyword(s):

General Circulation ◽

Circulation Model ◽

Tropical Pacific ◽

Thermocline Depth ◽

Coupled General Circulation Model ◽

The Mean ◽

The Stability ◽

Interactive Feedback ◽

Mean State ◽

Abstract The output from a coupled general circulation model (CGCM) is used to develop evidence showing that the tropical Pacific decadal oscillation can be driven by an interaction between the El Niño–Southern Oscillation (ENSO) and the slowly varying mean background climate state. The analysis verifies that the decadal changes in the mean states are attributed largely to decadal changes in ENSO statistics through nonlinear rectification. This is seen because the time evolutions of the first principal component analysis (PCA) mode of the decadal-varying tropical Pacific SST and the thermocline depth anomalies are significantly correlated to the decadal variations of the ENSO amplitude (also skewness). Its spatial pattern resembles the residuals of the SST and thermocline depth anomalies after there is uneven compensation from El Niño and La Niña events. In addition, the stability analysis of a linearized intermediate ocean–atmosphere coupled system, for which the background mean states are specified, provides qualitatively consistent results compared to the CGCM in terms of the relationship between changes in the background mean states and the characteristics of ENSO. It is also shown from the stability analysis as well as the time integration of a nonlinear version of the intermediate coupled model that the mean SST for the high-variability ENSO decades acts to intensify the ENSO variability, while the mean thermocline depth for the same decades acts to suppress the ENSO activity. Thus, there may be an interactive feedback consisting of a positive feedback between the ENSO activity and the mean state of the SST and a negative feedback between the ENSO activity and the mean state of the thermocline depth. This feedback may lead to the tropical decadal oscillation, without the need to invoke any external mechanisms.

The Tropical Pacific ENSO–Mean State Relationship in Climate Models over the Last Millennium

Journal of Climate ◽

10.1175/jcli-d-19-0673.1 ◽

2020 ◽

Vol 33 (17) ◽

pp. 7539-7551

Author(s):

D. Allie Wyman ◽

Jessica. L. Conroy ◽

Christina Karamperidou

Keyword(s):

General Circulation ◽

Climate Models ◽

Global Climate ◽

Tropical Pacific ◽

Last Millennium ◽

Climate Simulations ◽

Intercomparison Project ◽

The Relationship ◽

AbstractENSO and the mean zonal sea surface temperature gradient (dSST) of the tropical Pacific are important drivers of global climate and vary on decadal to centennial time scales. However, the relationship between dSST and ENSO cannot be assessed with the short instrumental record, and is uncertain in proxy data, with intervals of both stronger and weaker ENSO postulated to occur with overall strong dSST in the past. Here we assess the ENSO–dSST relationship during the last millennium using general circulation models (GCMs) participating in phase 3 of the Paleoclimate Modeling Intercomparison Project. Last millennium GCM simulations show diversity in the strength and direction of the ENSO–dSST relationship. Yet, the models that best simulate modern tropical Pacific climate frequently have a more negative ENSO–dSST correlation. Thus, last millennium tropical Pacific climate simulations support the likelihood of enhanced ENSO during decadal to centennial periods of reduced tropical Pacific dSST. However, the alternating directional ENSO–dSST relationship in all model simulations suggests that this relationship is not constant through time and is likely controlled by multiple mechanisms.

Comparison and Sensitivity of ODASI Ocean Analyses in the Tropical Pacific

Monthly Weather Review ◽

10.1175/mwr3405.1 ◽

2007 ◽

Vol 135 (6) ◽

pp. 2242-2264 ◽

Cited By ~ 18

Author(s):

Chaojiao Sun ◽

Michele M. Rienecker ◽

Anthony Rosati ◽

Matthew Harrison ◽

Andrew Wittenberg ◽

...

Keyword(s):

General Circulation ◽

Tropical Pacific ◽

Global Ocean ◽

Geophysical Fluid Dynamics Laboratory ◽

Salinity Treatment ◽

Salinity Field ◽

Significant Difference ◽

The Impact ◽

Abstract Two global ocean analyses from 1993 to 2001 have been generated by the Global Modeling and Assimilation Office (GMAO) and Geophysical Fluid Dynamics Laboratory (GFDL), as part of the Ocean Data Assimilation for Seasonal-to-Interannual Prediction (ODASI) consortium efforts. The ocean general circulation models (OGCM) and assimilation methods in the analyses are different, but the forcing and observations are the same as designed for ODASI experiments. Global expendable bathythermograph and Tropical Atmosphere Ocean (TAO) temperature profile observations are assimilated. The GMAO analysis also assimilates synthetic salinity profiles based on climatological T–S relationships from observations (denoted “TS scheme”). The quality of the two ocean analyses in the tropical Pacific is examined here. Questions such as the following are addressed: How do different assimilation methods impact the analyses, including ancillary fields such as salinity and currents? Is there a significant difference in interpretation of the variability from different analyses? How does the treatment of salinity impact the analyses? Both GMAO and GFDL analyses reproduce the time mean and variability of the temperature field compared with assimilated TAO temperature data, taking into account the natural variability and representation errors of the assimilated temperature observations. Surface zonal currents at 15 m from the two analyses generally agree with observed climatology. Zonal current profiles from the analyses capture the intensity and variability of the Equatorial Undercurrent (EUC) displayed in the independent acoustic Doppler current profiler data at three TAO moorings across the equatorial Pacific basin. Compared with independent data from TAO servicing cruises, the results show that 1) temperature errors are reduced below the thermocline in both analyses; 2) salinity errors are considerably reduced below the thermocline in the GMAO analysis; and 3) errors in zonal currents from both analyses are comparable. To discern the impact of the forcing and salinity treatment, a sensitivity study is undertaken with the GMAO assimilation system. Additional analyses are produced with a different forcing dataset, and another scheme to modify the salinity field is tested. This second scheme updates salinity at the time of temperature assimilation based on model T–S relationships (denoted “T scheme”). The results show that both assimilated field (i.e., temperature) and fields that are not directly observed (i.e., salinity and currents) are impacted. Forcing appears to have more impact near the surface (above the core of the EUC), while the salinity treatment is more important below the surface that is directly influenced by forcing. Overall, the TS scheme is more effective than the T scheme in correcting model bias in salinity and improving the current structure. Zonal currents from the GMAO control run where no data are assimilated are as good as the best analysis.

A statistical-dynamical approach to improve subseasonal precipitation forecasts: application to the southwest tropical Pacific

10.5194/egusphere-egu21-1292 ◽

2021 ◽

Author(s):

Damien Specq ◽

Lauriane Batté

Keyword(s):

General Circulation ◽

Southern Oscillation ◽

Numerical Models ◽

Tropical Pacific ◽

Forecast Skill ◽

Post Processing ◽

Processing Scheme ◽

Probabilistic Forecasts ◽

Wide Range

Although there is an increasing interest in precipitation information at the subseasonal timescales in a wide range of sectors, the use of subseasonal precipitation forecasts from general circulation models is often impaired by poor reliability and low forecast skill. One crucial step to improve forecast quality is statistical correction and post-processing, which is particularly important for a parameterized variable like rainfall. This study introduces and evaluates a statistical-dynamical post-processing scheme, based on a Bayesian framework, that aims at providing more skillful and more reliable subseasonal forecasts of weekly precipitation. On the one hand, this method relies on the statistical relationship between observed and dynamically-forecast precipitation, that is determined in a set of reforecasts and depends on the lead time. On the other hand, it also takes advantage of the climatological impacts of large-scale drivers affecting rainfall, that are generally better represented by numerical models than rainfall itself. These two aspects of the method are respectively called calibration and bridging.This statistical-dynamical prediction scheme is illustrated with an application to the austral summer precipitation in the southwest tropical Pacific, using the M&#233;t&#233;o-France and ECMWF reforecasts in the Subseasonal-to-seasonal (S2S) database. Indices representing El Ni&#241;o Southern Oscillation and the Madden-Julian Oscillation &#8211; the major sources of predictability in the area &#8211; are used for bridging. Probabilistic forecasts of heavy rainfall spells are evaluated in terms of discrimination (ROC skill score) and reliability, which are both improved by the Bayesian method at all lead times (from week 1 to week 4). Additional results show that the calibration part of the method, using forecast precipitation as a predictor, is necessary to enhance forecast skill. The bridging part also provides additional discrimination skill, that is mostly due to the ENSO-related information.

The Seasonal Cycle over the Tropical Pacific in Coupled Ocean–Atmosphere General Circulation Models

Monthly Weather Review ◽

10.1175/1520-0493(1995)123<2825:tscott>2.0.co;2 ◽

1995 ◽

Vol 123 (9) ◽

pp. 2825-2838 ◽

Cited By ~ 326

Author(s):

C.R. Mechoso ◽

A.W. Robertson ◽

N. Barth ◽

M.K. Davey ◽

P. Delecluse ◽

...

Keyword(s):

Seasonal Cycle ◽

General Circulation ◽

Tropical Pacific ◽

Ocean Atmosphere ◽

Weakened impact of the Atlantic Niño on the future equatorial Atlantic and Guinean Coast rainfall

10.5194/esd-2021-46 ◽

2021 ◽

Author(s):

Koffi Worou ◽

Hugues Goosse ◽

Thierry Fichefet ◽

Fred Kucharski

Keyword(s):

General Circulation ◽

Rainfall Variability ◽

Summer Rainfall ◽

Boreal Summer ◽

Equatorial Atlantic ◽

Guinea Coast ◽

Guinean Coast ◽

The Future ◽

Atlantic Niño

Abstract. The Guinea Coast is the southern part of the West African continent. Its summer rainfall variability mostly occurs on interannual timescales and is highly influenced by the sea surface temperature (SST) variability in the eastern equatorial Atlantic, which is known as the Atlantic Niño (ATL3). Using historical simulations from 31 General Circulation Models (GCMs) participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we first show that these models are able to simulate reasonably well the rainfall annual cycle in the Guinea Coast, with, however, a wet bias during boreal summer. This bias is associated with too high mean summer SSTs in the eastern equatorial and south Atlantic regions. Next, we analyze the near-term, mid-term and long-term changes of the Atlantic Niño mode relative to the present-day situation, in a climate with a high anthropogenic emission of greenhouse gases. We find a gradual decrease of the equatorial Atlantic SST anomalies associated with the Atlantic Niño in the three periods of the future. This result reflects a possible reduction of the Atlantic Niño variability in the future due to a weakening of the Bjerkness feedback over the equatorial Atlantic. In a warmer climate, an oceanic extension of the Saharan Heat Low over the North Atlantic and an anomalous higher sea level pressure in the western equatorial Atlantic relative to the eastern equatorial Atlantic weaken the climatological trade winds over the equatorial Atlantic. As a result, the eastern equatorial Atlantic thermocline is deeper and responds less to Atlantic Niño events. Among the models that simulate a realistic rainfall pattern associated with ATL3 in the present-day climate, there are 15 GCMs which project a decrease of the Guinean Coast rainfall response related to ATL3, and 9 GCMs which show no substantial change in the patterns associated with ATL3. In these 15 models, the zonal wind response to the ATL3 over the equatorial Atlantic is strongly attenuated in the future climate. Similar results are found when the analysis is focused on the rainfall response to ATL3 over the equatorial Atlantic. There is a higher confidence in the reduction of the rainfall associated with ATL3 over the Atlantic Ocean than over the Guinea Coast. We also found a decrease of the convection associated with ATL3 in the majority of the models.