Atlantic Zonal Mode-Monsoon Teleconnection in a Warming Scenario

Mapping Intimacies ◽

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.

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;

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.

Tropical Atlantic SST Prediction with Coupled Ocean–Atmosphere GCMs

Journal of Climate ◽

10.1175/jcli3947.1 ◽

2006 ◽

Vol 19 (23) ◽

pp. 6047-6061 ◽

Cited By ~ 68

Author(s):

Timothy N. Stockdale ◽

Magdalena A. Balmaseda ◽

Arthur Vidard

Keyword(s):

Initial Conditions ◽

Coupled Model ◽

Boreal Summer ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Seasonal Forecasts ◽

Coupled Models ◽

Equatorial Ocean ◽

Sst Anomalies

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.

Is There Evidence of Changes in Tropical Atlantic Variability Modes under AMO Phases in the Observational Record?

Journal of Climate ◽

10.1175/jcli-d-16-0459.1 ◽

2018 ◽

Vol 31 (2) ◽

pp. 515-536 ◽

Cited By ~ 25

Author(s):

Marta Martín-Rey ◽

Irene Polo ◽

Belén Rodríguez-Fonseca ◽

Teresa Losada ◽

Alban Lazar

Keyword(s):

Southern Oscillation ◽

Walker Circulation ◽

Atlantic Multidecadal Oscillation ◽

Boreal Summer ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

The North ◽

Sst Variability ◽

Tropical Atlantic Variability ◽

Atlantic Niño

The Atlantic multidecadal oscillation (AMO) is the leading mode of Atlantic sea surface temperature (SST) variability at multidecadal time scales. Previous studies have shown that the AMO could modulate El Niño–Southern Oscillation (ENSO) variance. However, the role played by the AMO in the tropical Atlantic variability (TAV) is still uncertain. Here, it is demonstrated that during negative AMO phases, associated with a shallower thermocline, the eastern equatorial Atlantic SST variability is enhanced by more than 150% in boreal summer. Consequently, the interannual TAV modes are modified. During negative AMO, the Atlantic Niño displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring. In contrast, a meridional seesaw SLP pattern evolving into a zonal gradient leads the Atlantic Niño during positive AMO. The north tropical Atlantic (NTA) mode is related to a Scandinavian blocking pattern during winter and spring in negative AMO, while under positive AMO it is part of the SST tripole associated with the North Atlantic Oscillation. Interestingly, the emergence of an overlooked variability mode, here called the horseshoe (HS) pattern on account of its shape, is favored during negative AMO. This anomalous warm (cool) HS surrounding an eastern equatorial cooling (warming) is remotely forced by an ENSO phenomenon. During negative AMO, the tropical–extratropical teleconnections are enhanced and the Walker circulation is altered. This, together with the increased equatorial SST variability, could promote the ENSO impacts on TAV. The results herein give a step forward in the better understanding of TAV, which is essential to improving its modeling, impacts, and predictability.

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.

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.

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.

Weakened Interannual Variability in the Tropical Atlantic

10.21203/rs.3.rs-1108528/v1 ◽

2021 ◽

Author(s):

Laura Sobral Verona ◽

Paulo Silva ◽

Ilana Wainer ◽

Myriam Khodri

Keyword(s):

Interannual Variability ◽

Warming Trend ◽

Precipitation Trend ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Bjerknes Feedback ◽

Trade Winds ◽

Modes Of Variability ◽

Sst Variability ◽

Wes Feedback

Abstract Climate variability in the Tropical Atlantic is complex with strong ocean-atmosphere coupling, where the sea surface temperature (SST) variability impacts the hydroclimate of the surrounding continents. We observe a decrease in the variability of the Tropical Atlantic after 1970 in both CMIP6 models and observations. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. The observed wind relaxation after 1970 in both the equatorial and Tropical North Atlantic (TNA) plays a role in the decreased variability. Concerning the AZM, a widespread warming trend is observed in the equatorial Atlantic accompanied by a weakening trend of the trade winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic and increasing the thermal damping. Even though individually the TNA and Tropical South Atlantic (TSA) show increased variability, the observed asymmetric warming in the Tropical Atlantic and relaxed northeast trade winds after the 70s play a role in decreasing the AMM variability. This configuration leads to positive Wind-Evaporation-SST (WES) feedback, increasing further the TNA SST, preventing AMM from changing phases as before 1970. Associated with it, the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which acts on maintaining the increased precipitation.

The impact of tropical Atlantic SST variability on the tropical atmosphere during boreal summer

Journal of Climate ◽

10.1175/jcli-d-20-0259.1 ◽

2021 ◽

pp. 1-57

Author(s):

Hong-Chang Ren ◽

Jinqing Zuo ◽

Weijing Li

Keyword(s):

Atmospheric Circulation ◽

Tropical Pacific ◽

Boreal Summer ◽

Tropical Atlantic ◽

Low Level ◽

Western Tropical Pacific ◽

Sst Variability ◽

Eastern Equatorial Pacific ◽

Community Earth System Model ◽

The Impact

AbstractThe interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anti-cyclones over the western tropical Pacific, and the NTA-related anti-cyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model Version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and though the response becomes stronger in the partially coupled experiments, obvious difference still exists between the simulations and observation. The ESTA-induced atmospheric circulation response is featured by an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña-like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavourable to enhancing the ESTA–Pacific teleconnection during boreal summer.

Weakening of the equatorial Atlantic SST variability under global warming

10.5194/egusphere-egu21-13003 ◽

2021 ◽

Author(s):

Lander R. Crespo ◽

Arthur Prigent ◽

Noel Keenlyside ◽

Ingo Richter ◽

Emilia Sánchez-Gómez ◽

...

Keyword(s):

Global Warming ◽

Atlantic Ocean ◽

Boreal Summer ◽

Historical Period ◽

Thermocline Depth ◽

Equatorial Atlantic ◽

Future Scenario ◽

Sst Variability ◽

Thermocline Feedback ◽

The Future

The eastern equatorial Atlantic is the region with the largest seasonal and interannual sea surface temperature (SST) variability in the entire tropical Atlantic Ocean. It is characterized by a rapid cooling during the boreal summer season, between June and September, that has large impacts in the regional climate. In this study we explore climate changes related to global warming in the cold tongue region using the CMIP5 and CMIP6 datasets as benchmarks. The historical simulations of both CMIP generations reproduce fairly well the spatial pattern of the observed warming &#8211; although weaker &#8211; in the Angola-Benguela region and most of the equatorial Atlantic band. The largest disagreements between model and observations are localized in the eastern equatorial Atlantic. The future business-as-usual scenario shows an intense and zonally homogeneous warming along the equatorial Atlantic band in CMIP5 and CMIP6. We also find a significant reduction of the June-July-August SST variability of 12% (17%) in the ensemble mean of the CMIP5 (CMIP6), in the future scenario (2050-2099) with respect to the historical period (1950-1999). The thermocline feedback, i.e., the local response of the SST anomalies to the thermocline depth anomalies, is weaker in the future scenario and appears to be the main driver of the change in interannual SST variability. The strong warming of the upper equatorial Atlantic Ocean in the future leads to a higher stratification which could explain the weaker thermocline feedback.