Tropical Atlantic SST Prediction with Coupled Ocean–Atmosphere GCMs

Abstract Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST.

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Reexamining the tropical Atlantic influence on ENSO using perfect model predictability experiments

10.5194/egusphere-egu21-12290 ◽

2021 ◽

Author(s):

Ingo Richter ◽

Yu Kosaka ◽

Hiroki Tokinaga ◽

Shoichiro Kido

Keyword(s):

Initial Conditions ◽

Boreal Summer ◽

Similar Amount ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Boreal Spring ◽

Modes Of Variability ◽

Control Simulation ◽

Starting Point ◽

Perfect Model

The potential influence of the tropical Atlantic on the development of ENSO has received increased attention over recent years. In particular equatorial Atlantic variability (also known as the Atlantic zonal mode or AZM) has been shown to be anticorrelated with ENSO, i.e. cold AZM events in boreal summer (JJA) tend to be followed by El Ni&#241;o in winter (DJF), and vice versa for warm AZM events. One problem with disentangling the two-way interaction between the equatorial Atlantic and Pacific is that both ENSO and the AZM tend to develop in boreal spring (MAM).Here we use a set of GCM sensitivity experiments to quantify the strength of the Atlantic-Pacific link. The starting point is a 1000-year free-running control simulation with the GFDL CM 2.1 model. From this control simulation, we pick years in which a cold AZM event in JJA is followed by an El Ni&#241;o in DJF. These years serve as initial conditions for &#8220;perfect model&#8221; prediction experiments with 10 ensemble members each. In the control experiments, the predictions evolve freely for 12 months from January 1 of each selected year. In the second set of predictions, SSTs are gradually relaxed to climatology in the tropical Atlantic, so that the cold AZM event is suppressed. In the third set of predictions, we restore the tropical Pacific SSTs to climatology, so that the El Ni&#241;o event is suppressed.The results suggest that, on average, the tropical Atlantic SST anomalies increase the strength of El Ni&#241;o in the following winter by about 10-20%. If, on the other hand, El Ni&#241;o development is suppressed, the amplitude of the cold AZM event also reduces by a similar amount. The results suggest that, in the context of this GCM, the influence of AZM events on ENSO development is relatively weak but not negligible. The fact that ENSO also influences the AZM in boreal spring highlights the complex two-way interaction between these two modes of variability.

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Weakened impact of the Atlantic Niño on the future Guinean coast rainfall

10.5194/egusphere-egu21-1168 ◽

2021 ◽

Author(s):

Koffi Worou ◽

Hugues Goosse ◽

Thierry Fichefet

Keyword(s):

Climate Models ◽

Climate Model ◽

Rainfall Variability ◽

Coupled Model ◽

Sea Surface ◽

Boreal Summer ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Sea Level Pressure ◽

Guinean Coast

Much of the rainfall variability in the Guinean coast area during the boreal summer is driven by the sea surface temperature (SST) variations in the eastern equatorial Atlantic, amplified by land-atmosphere interactions. This oceanic region corresponds to the center of action of the Atlantic Equatorial mode, also termed Atlantic Ni&#241;o (ATL3), which is the leading SST mode of variability in the tropical Atlantic basin. In years of positive ATL3, above normal SST conditions in the ATL3 area weaken the sea level pressure gradient between the West African lands and the ocean, which in turn reduces the monsoon flow penetration into Sahel. Subsequently, the rainfall increases over the Guinean coast area. According to observations and climate models, the relation between the Atlantic Ni&#241;o and the rainfall in coastal Guinea is stationary over the 20th century. While this relation remains unchanged over the 21st century in climate model projections, the strength of the teleconnection is reduced in a warmer climate. The weakened ATL3 effect on the rainfall over the tropical Atlantic (in years of positive ATL3) has been attributed to the stabilization of the atmosphere column above the tropical Atlantic. Analysis of historical and high anthropogenic emission scenario (the Shared Socioeconomic Pathways 5-8.5) simulations from 31 models participating in the sixth phase of the Coupled Model Intercomparison Project suggests an additional role of the Bjerkness feedback. A weakened SST amplitude related to ATL3 positive phases reduces the anomalous westerlies, which in turn increases the upwelling cooling effect on the sea surface. Both the Guinean coast region and the equatorial Atlantic experiment the projected rainfall reduction associated with ATL3, with a higher confidence over the ocean than over the coastal lands.

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Atlantic Zonal Mode-Monsoon Teleconnection in a Warming Scenario

10.21203/rs.3.rs-169135/v1 ◽

2021 ◽

Author(s):

Sabeerali C. T ◽

Ajaya Mohan Ravindran ◽

Praveen V

Keyword(s):

Indian Subcontinent ◽

Coupled Model ◽

Summer Monsoon Rainfall ◽

Convincing Evidence ◽

External Boundary ◽

Boreal Summer ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Warming Scenario ◽

Sst Variability

Abstract The dominant interannual SST variability in the eastern equatorial Atlantic referred to as the Atlantic Zonal Mode (AZM), which peaks in boreal summer impacts global weather patterns. The cold (warm) phase of this ocean-atmospheric coupled phenomenon enhances (weakens) the intensity of the Indian summer monsoon rainfall (ISMR). Observational studies show a strengthening relationship between AZM and ISMR in recent decades, providing a predictive signal for the ISMR. However, a suite of Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations in the highest emission scenario (SSP58.5) show a weakening relationship between ISMR and AZM in the future (2050-2099). The strengthening of atmospheric thermal stability over the tropical Atlantic in the warming scenario weakens the associated convection over the eastern equatorial Atlantic in response to the warm phase of AZM. This leads to weakening velocity potential response over the Indian subcontinent, resulting in a weak AZM-ISMR relationship. There is no convincing evidence to indicate that either the tropical Atlantic SST bias or the AZM-ISMR teleconnection bias plays a crucial role in the potential weakening of this relationship. These results imply that ISMR prediction will become more challenging in a warming scenario as one of the major external boundary forces that influence monsoon weakens.

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Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

Journal of Climate ◽

10.1175/jcli3922.1 ◽

2006 ◽

Vol 19 (20) ◽

pp. 5227-5252 ◽

Cited By ~ 14

Author(s):

Serena Illig ◽

Boris Dewitte

Keyword(s):

Mixed Layer ◽

El Niño ◽

Time Scale ◽

El Nino ◽

Coupled Model ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Layer Model ◽

Intermediate Coupled Model ◽

Mixed Layer Model

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

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Assessing ENSO Simulations and Predictions Using Adjoint Ocean State Estimation

Journal of Climate ◽

10.1175/3211.1 ◽

2004 ◽

Vol 17 (22) ◽

pp. 4301-4315 ◽

Cited By ~ 12

Author(s):

Dietmar Dommenget ◽

Detlef Stammer

Keyword(s):

State Estimation ◽

Positive Impact ◽

Coupled Model ◽

Estimation Procedure ◽

Time Interval ◽

Seasonal Forecasts ◽

Predictive Skill ◽

Sst Anomaly ◽

Ocean State Estimation

Abstract Simulations and seasonal forecasts of tropical Pacific SST and subsurface fields that are based on the global Consortium for Estimating the Circulation and Climate of the Ocean (ECCO) ocean-state estimation procedure are investigated. As compared to similar results from a traditional ENSO simulation and forecast procedure, the hindcast of the constrained ocean state is significantly closer to observed surface and subsurface conditions. The skill of the 12-month lead SST forecast in the equatorial Pacific is comparable in both approaches. The optimization appears to have better skill in the SST anomaly correlations, suggesting that the initial ocean conditions and forcing corrections calculated by the ocean-state estimation do have a positive impact on the predictive skill. However, the optimized forecast skill is currently limited by the low quality of the statistical atmosphere. Progress is expected from optimizing a coupled model over a longer time interval with the coupling statistics being part of the control vector.

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Forced modulations of Sahel rainfall at decadal timescale over the 20th Century.

10.5194/egusphere-egu21-4105 ◽

2021 ◽

Author(s):

Cassien Diabe Ndiaye ◽

Juliette Mignot ◽

Elsa Mohino

Keyword(s):

North Atlantic Ocean ◽

Coupled Model ◽

Internal Variability ◽

Boreal Summer ◽

Semiarid Region ◽

Rainfall Regime ◽

Anthropogenic Aerosols ◽

Coupled Models ◽

External Forcings ◽

Sahel Precipitation

The semiarid region of the Sahel was marked during the 20th Century by significant modulations of its rainfall regime. Part of these modulations has been associated with the internal variability of the climate system, mediated by changes in oceanic sea surface temperature (SST). We show here that the external forcings, and in particular anthropogenic aerosols, might have played a role more important than previously thought in setting these variations. The study is based on the recent simulations performed for CMIP6 with the IPSL-CM6A-LR coupled model. As in most coupled models, the maximum precipitation is limited to the southern Sahel during boreal summer in the IPSL-CM6A-LR model. A novel definition of the Sahel precipitation region is proposed in order to take this bias into account. Our results show that external forcings induce decadal modulations of Sahel precipitation that correlate significantly at 0.6 with the observed precipitations and that the anthropogenic aerosols explain more than 70% of these modulations. These results confirm recent results of CMIP6 highlighting an important role of aerosol forcing for the decadal climate in and around the North Atlantic ocean.

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A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-11383-2019 ◽

2019 ◽

Vol 19 (17) ◽

pp. 11383-11399

Author(s):

Jonathan K. P. Shonk ◽

Teferi D. Demissie ◽

Thomas Toniazzo

Keyword(s):

Boundary Layer ◽

General Circulation ◽

Marine Boundary Layer ◽

General Circulation Models ◽

Equatorial Atlantic ◽

Seasonal Forecasts ◽

Coupled Models ◽

Trade Winds ◽

Double Itcz ◽

Rapid Transition

Abstract. Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based on System 4, but with prescribed sea-surface temperatures. The development of the equatorial westerly bias in early April is marked by a rapid transition from a wintertime easterly, cold tongue bias to a springtime westerly bias regime that displays a marked double intertropical convergence zone (ITCZ). The transition is a seasonal feature of the model climatology (independent of initialisation date) and is associated with a seasonal increase in rainfall where a second branch of the ITCZ is produced south of the Equator. Excess off-equatorial convergence redirects the trade winds away from the Equator. Based on arguments of temporal coincidence, the results of our analysis contrast with those from previous work, and alleged causes hereto identified as the likely cause of the equatorial westerly bias in other models must be discarded. Quite in general, we find no evidence of remote influences on the development of the springtime equatorial bias in the Atlantic in the IFS-based models. Limited evidence however is presented that supports the hypothesis of an incorrect representation of the meridional equatorward flow in the marine boundary layer of the southern Atlantic as a contributing factor. Erroneous dynamical constraints on the flow upstream of the Equator may generate convergence and associated rainfall south of the Equator. This directs attention to the representation of the properties of the subtropical boundary layer as a potential source for the double ITCZ bias.

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

10.5194/egusphere-egu2020-12722 ◽

2020 ◽

Author(s):

Ingo Richter ◽

Hiroki Tokinaga

Keyword(s):

General Circulation ◽

General Circulation Models ◽

Tropical Pacific ◽

Boreal Summer ◽

Tropical Atlantic ◽

Westerly Wind ◽

Equatorial Atlantic ◽

Wide Range ◽

Mean State ◽

The Tropical Pacific

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.

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Weakening Satellite Era and Future Tropical Atlantic Sea Surface Temperature Variability

10.5194/egusphere-egu21-7067 ◽

2021 ◽

Author(s):

Arthur Prigent ◽

Rodrigue Anicet Imbol Koungue ◽

Joke Lübbecke ◽

Peter Brandt ◽

Jan Harlaß ◽

...

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Zonal Wind ◽

Coupled Model ◽

Sea Surface ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Sst Variability ◽

Remote Forcing ◽

Local Forcing

Since 2000, a substantial weakening in the equatorial and southeastern tropical Atlantic sea surface temperature (SST) variability is observed. Observations and reanalysis products reveal, for example, that relative to 1982&#8211;1999, the March&#8208;April&#8208;May SST variability in the Angola&#8208;Benguela area (ABA) has decreased by more than 30%. Both equatorial remote forcing and local forcing are known to play an important role in driving SST variability in the ABA. Here we show that compared to 1982&#8211;1999, since 2000, equatorial remote forcing had less influence on ABA SSTs, whereas local forcing has become more important. In particular, the robust correlation between the equatorial zonal wind stress and the ABA SSTs has substantially weakened, suggesting less influence of Kelvin waves on ABA SSTs. Moreover, the strong correlation linking the South Atlantic Anticyclone and the ABA SSTs has reduced. Multidecadal surface warming of the ABA could also have played a role in weakening the interannual SST variability.To investigate future changes in tropical Atlantic SST variability, an ensemble of nested high-resolution coupled model simulations under the global warming scenario RCP8.5 is analyzed. SST variability in both the ABA and equatorial cold tongue is found to decrease along with reduced western equatorial Atlantic zonal wind variability. &#160;

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Predictability of extreme events in UFS at subseasonal time scale

10.5194/egusphere-egu21-3175 ◽

2021 ◽

Author(s):

Christiana Stan

Keyword(s):

Extreme Events ◽

Time Scale ◽

Initial Conditions ◽

Coupled Model ◽

Forecast Skill ◽

Boreal Summer ◽

Boreal Winter ◽

Forecast System ◽

Cold Events ◽

Second Category

The predictability of extreme events over the continental United States (CONUS) in the Unified Forecast System (UFS) Couple Model is studied at subseasonal time scale. The benchmark runs of UFS (GFSv15), a coupled model consisting of atmospheric component (FV3GFS) with 28 km resolution and ocean (MOM6) and sea ice (CICE5) components with global 0.25&#176; resolution, for the period April 2011&#8211;December 2017 have been assessed. The model&#8217;s month-long forecasts initiated on the first and fifteenth of each month are used to examine the predictability of extreme events in precipitation and 2m temperature. The atmospheric and ice initial conditions are from CFSR data, and the ocean initial conditions are from 3Dvar CPC. The errors in the week 1&#8211;4 predictions and the corresponding spatial correlation between model and observation over CONUS are presented. The differences in the predictability of the extreme events between the boreal summer and winter are discussed. Two categories of extreme events are evaluated: 95th and 99th percentile, respectively. The forecast skill of extreme events in the 95th percentile is higher than the forecast skill of events in the second category. The forecast skill of warm and cold events in the 95th percentile shows seasonal dependence and is higher during the boreal winter.

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