Tropical Atlantic climate: Ocean processes, air-sea interactions, remote impacts, and climate change

Observations and model simulations illustrate significant ocean variability and associated air-sea interactions in the tropical Atlantic basin from daily-to-decadal time scales. This session is devoted to the understanding of ocean dynamics in the tropical and subtropical Atlantic Ocean, its interaction with the overlying atmosphere from the equator to the mid-latitudes and its climate impacts on adjacent to remote areas. Relevant processes in the ocean include upper and deep ocean circulation, eddies, tropical instability waves, warm pools, cold tongues and eastern boundary upwellings. We are interested in air-sea interactions related to both the seasonal cycle and the development of modes of variability from local to basin scale (e.g. the Meridional Mode, the Atlantic Niño, and the Benguela Niño). We welcome studies on wind variations related to the development of these modes, as well as studies on high-frequency events, such as marine heat waves, the Madden-Julian Oscillation, tropical cyclones and convective systems. Furthermore, we seek studies on climate change in the region, and also of the climatic impacts of change and variability on marine ecosystems. Finally, we are also interested in contributions examining the causes and impacts of systematic model errors in simulating the local to regional Atlantic climate. Studies based on direct observations, reanalysis, reconstructions as well as model simulations are welcome.

Co-organized by AS2/CL2
Convener: Marta Martín-Rey | Co-conveners: Peter Brandt, Noel Keenlyside, Belen Rodríguez de Fonseca
| Thu, 26 May, 15:10–16:40 (CEST)
Room 1.15/16

Presentations: Thu, 26 May | Room 1.15/16

Chairpersons: Belen Rodríguez de Fonseca, Marta Martín-Rey
Characterization of present and future Tropical Atlantic variability
Virtual presentation
Laura Sobral Verona, Paulo Silva, Ilana Wainer, and Myriam Khodri

Climate variability in the Tropical Atlantic is complex and significantly different than that in the Pacific. A strong ocean-atmosphere coupling is present, sea surface temperature (SST) variability in this region impacts the hydroclimate of the surrounding continents and influences the meridional displacement of the Intertropical Convergence Zone (ITCZ).  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 equatorial and meridional modes. The Atlantic Zonal Mode (AZM) characterizes an equatorial cold tongue. The Atlantic Meridional Mode (AMM) represents an interhemispheric SST anomaly gradient.  Both modes respond to positive ocean-atmosphere feedback: the Bjerkens Feedback controls most of the dynamics underlying the AZM; and a thermodynamic feedback amplifies the AMM, the WES (wind-evaporation-SST) feedback .

            The observed winds relaxation after 1970 in both the equatorial Atlantic region and in the Tropical Northern Atlantic (TNA) plays a role in the decrease of Tropical Atlantic variability, for each mode predominant season. With respect to 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 Bjerkens 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 WES feedback, increasing further the TNA SST, preventing AMM from changing phases as before 1970.

            Associated with SST, trade wind trends and decreased Tropical Atlantic variability, the African Sahel shows a positive precipitation trend. The southwest wind anomaly (trade wind relaxation) over the Tropical North Atlantic carries more humidity into the Sahel region, therefore increasing precipitation. As a consequence of the observed trends and decreased variability especially in the AMM, the ITCZ tends to shift northward, which acts on maintaining the increased precipitation over the Sahel.

How to cite: Sobral Verona, L., Silva, P., Wainer, I., and Khodri, M.: Variability Changes in the Tropical Atlantic in CMIP6, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13162, https://doi.org/10.5194/egusphere-egu22-13162, 2022.

On-site presentation
Ingo Richter, Hiroki Tokinaga, Yuko Okumura, and Noel Keenlyside

The equatorial Atlantic is subject to interannual variability that is centered in the eastern cold tongue region and is known as the Atlantic Zonal Mode (AZM). Previous studies have indicated that AZM variability has declined over the recent decades and this tendency is projected to continue based on climate change simulations. The period 2000 to mid-2019 was arguably most conspicuous in this regard, as it did not contain any major AZM event. In late 2019, however, the strongest event in more than 40 years developed. This was followed, in 2021, by an equally warm event. In the present work we examine the mechanisms behind these recent events. We show that while the accompanying wind stress forcing was strong, it cannot account for the exceptional strength of the two events. Analysis suggests that Ekman pumping north of the equator contributed to the strength of the events by generating downwelling Rossby waves that were reflected into downwelling Kelvin waves at the equator. In addition, an examination of observed sea-surface height and ocean temperature from reanalysis and PIRATA buoys suggests that there was a steady buildup of heat in the eastern equatorial region (20W-10E, 10S-5N) since about 2015. This excessive heat content was discharged during the 2019 and 2021 events and may have contributed to their exceptional strength. Our results highlight the need for a close monitoring of oceanic conditions in the region. This will not only have implications for seasonal prediction but also for the long-term development of AZM variability.

How to cite: Richter, I., Tokinaga, H., Okumura, Y., and Keenlyside, N.: Is equatorial Atlantic variability resurging?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11006, https://doi.org/10.5194/egusphere-egu22-11006, 2022.

Noel Keenlyside

In 2021 there was an exceptionally strong Atlantic Niño—stronger than the last major event in 1996. Positive SST anomalies developed in May and peaked in June-August. There was a build up of heat content in the spring in the western north Atlantic that could be related to local wind stress curl anomalies.  The event appears to have been triggered by zonal wind anomalies in April and May in the western equatorial Atlantic, when strong rainfall anomalies were also observed along the equator. The event terminated with rainfall anomalies shifting northward in late summer. Interestingly, there was also a strong Benguela Niño that developed already in April and has persisted into boreal summer. Furthermore, the event may have contributed to the La Niña event that developed later in the year in the Pacific.

How to cite: Keenlyside, N.: The Super Atlantic Niño of 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13546, https://doi.org/10.5194/egusphere-egu22-13546, 2022.

Paola Castellanos, Estrella Olmedo, Edmo Campos, Wlademir Santis, and Joaquim Dias

The spatiotemporal evolutions of sea surface salinity measurements from the SMOS satellite reveal presence of a local salinity maximum in the northwestern tropical Atlantic beginning in September increasing with a Maximum in October and disappearing in January. Its structure and variability are analyzed through SMOS SSS daily products derived with advanced techniques developed at the Barcelona Expert Centre during 9 years. The results are compared with in situ data along the North Brazil Current (NBC) from the Prediction and Research moored Array in the Tropical Atlantic - PIRATA program. This seasonal tropical SSS maximum, produces the salty signature Northward of the NBC, which is seen as a localized salinity maximum on satellite imagery, in contrast to the fresh signature present in summer-early fall. These changes suggest a change in the composition of water masses that enter in the South Atlantic contributing to an alteration in the dynamics of global circulation.

How to cite: Castellanos, P., Olmedo, E., Campos, E., Santis, W., and Dias, J.: Surface salinity maximum in the western boundary of the Tropical Atlantic as observed from SMOS salinity maps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13538, https://doi.org/10.5194/egusphere-egu22-13538, 2022.

Virtual presentation
Arthur Prigent, Rodrigue Anicet Imbol Koungue, Joke F. Lübbecke, Peter Brandt, Tobias Bayr, Jan Harlaß, and Mojib Latif

Tropical Atlantic interannual sea surface temperature (SST) variability has significantly weakened since 2000. Here, we use a coupled ocean-atmosphere model with an embedded high-resolution nest in the tropical Atlantic Ocean to investigate future changes in the southeastern tropical Atlantic SST variability in response to anthropogenic global warming. In the model, the Angola-Benguela Area (ABA) is among the regions in the tropical Atlantic that exhibit the largest surface warming. Relative to 1970-1999, the SST variability in the ABA during the peak season, May-June-July (MJJ), decreases by about 24% during 2070-2099 under the worst-case scenario of the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5). The MJJ interannual temperature variability weakens along the Angolan and Namibian coasts in the top 40 m of the ocean. This reduction appears to be due to a smaller temperature response to thermocline-depth variations, i.e. a weaker thermocline feedback. The weaker thermocline feedback is found where the thermocline deepens the most. Our model results suggest that the trend towards a weakening of the interannual SST variability in the ABA observed during the recent decades could persist in the future under a worst-case global warming scenario.

How to cite: Prigent, A., Imbol Koungue, R. A., Lübbecke, J. F., Brandt, P., Bayr, T., Harlaß, J., and Latif, M.: Future weakening of southeastern Tropical Atlantic Ocean interannual SST variability in a nested coupled model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5292, https://doi.org/10.5194/egusphere-egu22-5292, 2022.

Virtual presentation
Marie Pontoppidan, Chiara De Falco, Priscilla A. Mooney, and Jerry Tjiputra

Marine ecosystems are largely impacted by marine heat waves (MHWs). That includes coral reefs which are experiencing coral bleaching and subsequently loss of marine biodiversity because of MHWs. Such reefs are crucial habitat of fish stocks feeding the world’s population. As ocean temperatures increase, the occurrences of MHWs become more frequent. A further solid mechanistic understanding is therefore urgently required for adaptation and mitigation of future changes in MHWs. Importantly, this knowledge is needed on a local-scale.

Here we use a coupled ocean-atmosphere regional modelling system (COAWST), consisting of the atmospheric model WRF and the ocean model ROMS, to dynamically downscale an area over the Caribbean Sea and the Gulf of Mexico. Compared to a global model with coarser horizontal resolution, our 12 km grid spacing resolves smaller scale phenomena and ensures a skilled representation of the air-sea interactions which are important for a correct representation of MHWs. We show the results of a 20-year regional climate simulation and compare the output with two global climate model simulations (NorESM2-MM and NorESM2-MH) to address the added value of the regional simulation. Our high-resolution simulation represents the temporal distribution (frequency and duration) of MHWs well compared to the coarser global models which produce too few, but too long heatwaves in the area.

How to cite: Pontoppidan, M., De Falco, C., Mooney, P. A., and Tjiputra, J.: Marine heat waves: The added value of a high resolution, coupled atmosphere-ocean regional climate model , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4629, https://doi.org/10.5194/egusphere-egu22-4629, 2022.

Drivers of Tropical Atlantic Variability: air-sea interactions and ocean dynamics
On-site presentation
Marta Martín-Rey, Ignasi Vallès-Casanova, and Josep Lluis Pelegri

The scarcity of in-situ measurements and the variability among individual events has limited our understanding of the drivers and impacts of the tropical Atlantic Ocean circulation. Here we investigate the response of the surface and subsurface ocean circulation to the two main modes of tropical Atlantic Variability (TAV): the Meridional Mode (MM) and Equatorial Mode (EM). For this purpose, we use three oceanic reanalyses and an interannual forced-ocean simulation covering the period 1982-2018. The developing phase of the MM is associated with a spring intensification of the North Equatorial Countercurrent (NECC), the Equatorial Undercurrent (EUC) and the north South Equatorial Current (nSEC) in the eastern equatorial margin. It also triggers Rossby waves that reach the western boundary and are reflected as equatorial Kelvin waves that weaken the ocean surface and subsurface transports and cause anomalous warm equatorial conditions in boreal summer. During the developing spring-summer phase of the EM, the westward surface zonal transport is considerably reduced with no clear impact at subsurface levels. During the fall EM decaying phase, the reflected Kelvin wave reverses the zonal pressure-gradients at the equator and the westward equatorial nSEC is reinforced. This is accompanied by a weakening of the EUC that suggests an additional off-equatorial forcing. Our results reveal that the ocean circulation responds to both MM and EM, endorsing the key role played by the propagating zonal waves in connecting the tropical and equatorial ocean transports.

How to cite: Martín-Rey, M., Vallès-Casanova, I., and Pelegri, J. L.: Response of the upper ocean circulation to tropical Atlantic interannual modes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6900, https://doi.org/10.5194/egusphere-egu22-6900, 2022.

On-site presentation
Mia Sophie Specht, Johann Jungclaus, and Jürgen Bader

Tropical Instability Waves (TIWs) at the equatorial Atlantic Ocean lead to SST cooling due to enhanced mixing and heat fluxes above the EUC core. This phenomenon has been studied predominantly at the equator and to the north, where TIWs are most pronounced. However, a recent study has shown the presence of subsurface TIWs in the Atlantic Ocean, which frequently occur to the south of the equator. As TIW induced subsurface mixing leads to SST cooling at the equator, we suspect a similar cooling may occur in the Southern Hemisphere due to the presence of subsurface TIWs. Using one decade of high-resolution ICON ocean simulations, we investigate such effect of subsurface TIWs in the southern hemisphere on SST in the tropical Atlantic Ocean. The analysis of all terms of the mixed layer heat budget allows for the investigation and quantification of the processes involved in subsurface TIW induced SST changes.

How to cite: Specht, M. S., Jungclaus, J., and Bader, J.: Influence of subsurface tropical instability waves on sea surface temperature in the tropical Atlantic , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12325, https://doi.org/10.5194/egusphere-egu22-12325, 2022.

Virtual presentation
Mareike Körner, Peter Brandt, and Marcus Dengler

The Angolan shelf system represents a highly productive ecosystem that exhibits pronounced seasonal variability. Productivity peaks in austral winter when seasonally prevailing upwelling favorable winds are weakest. Thus, other processes than local wind-driven upwelling contribute to the near-coastal cooling and nutrient supply during this season. Possible processes that lead to changes of the mixed-layer heat content does not only include local mechanism but also the passage of remotely forced coastally trapped waves. Understanding the driving mechanisms of changes in the mixed-layer heat content that may be locally or remotely forced is also vital for understanding of upward nutrient supply and biological productivity off Angola. Here, we investigate the seasonal mixed layer heat budget by analyzing atmospheric and oceanic causes for heat content variability. By using different satellite and in-situ data, we derive monthly estimates of surface heat fluxes, horizontal advection, diapycnal heat fluxes and local heat storage. The results show that the contribution of horizontal heat advection is small. When considering surface heat fluxes and horizontal heat advection only, the local mixed layer heat budget cannot be closed and the resulting residuum increases closer to the coast. Diapycnal heat fluxes at the base of the mixed layer and uncertainties of surface heat fluxes are suggested to explain the residuum. Our data suggests that the magnitude of diapycnal heat fluxes is controlled by stratification with stronger stratification reducing diapycnal heat fluxes. We conclude that local and remote impacts on stratification need to be examined in order to understand the mixed layer heat budget variability off Angola.

How to cite: Körner, M., Brandt, P., and Dengler, M.: Seasonal mixed layer heat budget in coastal waters off Angola, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5559, https://doi.org/10.5194/egusphere-egu22-5559, 2022.

Virtual presentation
Folly Serge Tomety, Mathieu Rouault, Founi Mesmin Awo, and Noel Sebastian Keenlyside

The Angola Benguela Front (ABF), is a very dynamic area, characterized by a high-temperature gradient of up to 4°C per degree latitude. It fluctuates in position and intensity seasonally which strongly affects the local marine ecosystem. A lot of research, in the past decades, has focused on the SST variability at the interannual timescale in the ABF and the Angolan and Northern Namibian coast to the north and south of it in the contest of Benguela Niños and Niñas. A warming trend since the 1980’s in that region has been reported in the literature and was attributed to a decreasing trend in wind speed. In this study, we look at the processes responsible for the warming in the ABF region. The OGCM NEMO model is used for that matter. The results suggest that the warming is due to various processes acting during different seasons. In autumn, the modelled SST warming trend occurs along the Angolan sector and it is associated with a positive trend in net surface heat flux (Qnet) and with the weakening of the vertical flow associated with the upwelling of cooler water to the surface. In early summer (November-January), the modelled SST warming trend occurs along the Angolan and Namibian sector and it is primarily associated with the intensification of a coastal poleward flow bringing more warm water from the tropics into the ABF region and with the weakening of vertical flow, while locally, Qnet trend generates a cooling trend. The modelled SST cooling trend that occurred south of the ABF, especially in winter and early spring, is primarily associated with a northwards trend in the horizontal subsurface current that advects cooler water from the south and an intensification of the upwelling of cold water to the surface.


How to cite: Tomety, F. S., Rouault, M., Awo, F. M., and Keenlyside, N. S.: Origin of the recent warming along the Angola Namibia coast, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11894, https://doi.org/10.5194/egusphere-egu22-11894, 2022.

Virtual presentation
Badara Sané, Alban Lazar, Malick Wade, and Amadou Thierno Gaye

The Azores High shows a strong intra-seasonal variability that, transmitted to the whole North Atlantic by the Trade Wind, generates a multi-factor variability of the Canary Islands eastern edge upwelling system. In this work, we study the cold season (March to April), using satellite observations and numerical simulation, and how the variability of the wind at the equator, the Kelvin and coastal waves, and the local wind along the North-West African coast combine
to force upwelling variability. Composite analyses show how, in 80% of the cases, the pulsations of the anticyclone at 40 d excite equatorial waves that arrive in the Senegalese upwelling 15 d later, precisely at the time of the phase change of coastal wind anomalies.  These waves trapped at the coast, from upwelling or downwelling, reinforce the local wind anomaly. The intra-seasonal variability of the SST is thus the result of a double local and remote effect whose respective contributions we quantify 

How to cite: Sané, B., Lazar, A., Wade, M., and Gaye, A. T.: Pulsations of the Azores anticyclone at intra-seasonal scale: how oceanic waves and coastal wind anomalies combine constructively to force the variability of the north-eastern boundary upwelling system in winter-spring., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12733, https://doi.org/10.5194/egusphere-egu22-12733, 2022.

Tropical and Extratropical teleconnections of Tropical Atlantic Variability
Long-term trend of equatorial Atlantic zonal SST gradient linked to the tropical Pacific cold tongue mode under global warming
Yang Li, Quanliang Chen, Nan Xing, Zhigang Cheng, Yulei Qi, Fan Feng, and Minggang Li
On-site presentation
Fraser Goldsworth, David Marshall, and Helen Johnson

Models, theory and observations suggest that symmetric instability is excited in the North Brazil Current after it crosses the equator. The instability is fuelled by the advection of waters with anomalous potential vorticity from the Southern to the Northern Hemisphere. There also exists a deep western boundary current which sits below the North Brazil Current. This current advects anomalous potential vorticity across the equator too, and so also becomes symmetrically unstable upon crossing it. Numerical models and scaling arguments will be used to predict the similarities and differences between the action of symmetric instability in the surface and deep currents. We will then explore how the excitement of the instability affects the structure of the deep western boundary current, and how this impacts the development of mesoscale features further down-stream.

How to cite: Goldsworth, F., Marshall, D., and Johnson, H.: Symmetric instability in the surface and deep components of the Atlantic Meridional Overturning Circulation close to the equator, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5802, https://doi.org/10.5194/egusphere-egu22-5802, 2022.

Teresa Losada, Belén Rodríguez-Fonseca, C. Roberto Mechoso, Elsa Mohino, and Antonio Castaño-Tierno

Tropical interbasin teleconnections at inter-annual time scales are receiving much attention in the last years. However, their controlling factors and long-term changes are still under debate. In this work, we investigate whether selected features in the climatology, the position of the ITCZ and strong tropical convection, can influence the teleconnections between the tropical Atlantic and Pacific basins at inter-annual timescales.

For investigation, we contrast a CGCM control simulation with an experiment in which the climatological position of the ITCZ is shifted in latitude by artificially reducing the shortwave radiation incident in a region of the south Atlantic sector. The perturbation magnitude and sign are such that the local model’s biases in Atlantic SST are reduced. The experiment shows stronger interannual variability over the tropical Atlantic and Pacific oceans, a westward extension of the Atlantic Niño pattern, and enhanced interannual teleconnections between equatorial Atlantic and Pacific.

We examine the mechanisms at work for these changes. We find several factors as major contributors to enhance the tropical interbasin teleconnections. One is the modified Walker circulation resulting from the westward extension of SST anomalies during the Atlantic Niño and concurrent westward displacement of convection. The other factors are the enhancement of the precipitation at the equator and the shallowing of thermocline in the Pacific, which make the latter basin more sensitive to both local and remote perturbations.

On the contrary, the North Tropical Atlantic – equatorial Pacific teleconnection is weakened in the experiment, despite the strongest impact of the NTA anomalies in the north tropical Pacific winds. due to the opposite effect on divergence exerted by the off equatorial winds related to NTA and the equatorial winds related to the concomitant warming in the eastern and central equatorial Pacific.

How to cite: Losada, T., Rodríguez-Fonseca, B., Mechoso, C. R., Mohino, E., and Castaño-Tierno, A.: Interhemispheric asymmetries, ITCZ location and interannual tropical Atlantic-Pacific interactions produced by South Atlantic cooling., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12308, https://doi.org/10.5194/egusphere-egu22-12308, 2022.

Belen Rodriguez-Fonseca, Irene Polo Sanchez, Elsa Mohino Harris, Teresa Losada Doval, Marta Martin del Rey, Noel Keenlyside, and Carlos Roberto Mechoso

Observational studies have reported that tropical Atlantic interannual variability impacts
on ENSO in different seasons and periods: Atlantic Ni ̃nos (AN) in boreal summer during
negative phases of the Atlantic Multidecadal Variability (AMV); and tropical north Atlantic
(TNA) in boreal spring during positive AMV. Nevertheless, this relation is not clear for the
whole observational record. This paper is an step forward towards understanding of tropical
Atlantic impacts on ENSO: how and when do they occur? Using observations and a pool of
preindustrial control simulations from the CMIP5 initiative we investigate the background
ocean and atmospheric conditions promoting these tropical interbasin connections.Periods
with a negative AN-ENSO connection appear characterized by a shallower thermocline
over the western Pacific and deeper in the east, together with an increase in interannual
SST variability over the tropics. Periods with a negative TNA-ENSO connection appear
characterized by a steeper thermocline in the Pacific and positive interhemispheric SST
gradient in the the Atlantic. A decrease in tropical Pacific atmospheric and ocean variability
characterizes these periods.

How to cite: Rodriguez-Fonseca, B., Polo Sanchez, I., Mohino Harris, E., Losada Doval, T., Martin del Rey, M., Keenlyside, N., and Mechoso, C. R.: Multidecadal Modulations of Tropical Atlantic impact on ENSO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12602, https://doi.org/10.5194/egusphere-egu22-12602, 2022.