OS1.4

In this study, we investigate the mesoscale flow field and how it enables energy to propagate vertically in form of near-inertial waves. As part of the EURAC4A-OA campaign the research vessels RV Maria S. Merian and NO L’Atalante simultaneously surveyed mesoscale eddy fronts in the western tropical North Atlantic. From velocity profile data, measured by a shipboard Acoustic Doppler Current Profiler (sADCP), we reconstruct eddies in the upper 1000m of the surveyed area, by fitting a Rankine Vortex model. The model derives an idealized velocity structure of the eddy as well as the location of its centre. Multiple occurrences of stacked eddies are identified and often surrounded by current shear structures associated with near-inertial waves. Using data from ship sections, where both research vessels operated less than 1nm apart, the vertical component of the relative vorticity (zeta) is calculated using different methods (single ship, two ships)[Shcherbina et al. 2013]. It is found that in particular zeta outside of the eddy cores is sensitive to the way the vorticity is calculated and may even change sign. Furthermore, the resulting zeta sections and its impact on the ability of near-inertial waves propagating vertically below the mixed layer is discussed.

How to cite: Rudloff, D., Karstensen, J., Fischer, T., Schütte, F., Bendinger, A., Speich, S., L'Hegaret, P., Carton, X., Reverdin, G., Laxenaire, R., and Gula, J.: Observations of vertical propagation of near-inertial Waves in a complex vorticity field during the EURAC4A-OA campaign in the tropical western North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15402, https://doi.org/10.5194/egusphere-egu21-15402, 2021.

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic Ocean with the overall objective of understanding the role of fine scale processes in the internal ocean dynamics and air-sea interaction. Four oceanographic vessels, the French Atalante, German Maria S Merian and Meteor, and the American Ron Brown, closely coordinated with air-borne observations and autonomous ocean platforms (gliders, saildrones, and drifters) to simultaneously measure the ocean and atmosphere east of the island of Barbados and the coast of Guyana in the western Tropical Atlantic. A whole battery of instruments measuring the thermohaline and dynamic characteristics of the region was launched. The fixed CTD stations, reaching great depths while measuring salinity, temperature, and oxygen concentrations, serve as a reference to calibrate and validate other devices, in particular, shallower uCTD, TSG, and MVP, acquired during ship transits, and autonomous gliders and saidrones. Combined, these datasets increase the horizontal resolution and thus the description of structures ranging from mesoscale to fine scale.

The Northwest Tropical Atlantic Ocean is a dynamical region filled with mesoscale eddies of different origins and transporting various water masses across the region. These eddies have rich and diverse characteristics ranging from shallow cyclonic and anticyclonic eddies to the deep reaching North Brazil Current (NBC) Rings. On the surface, down to 200 m depth, the signatures of shallow cyclones and anticyclones (NBC rings) were measured. The shallow mesoscale eddies, with core centered around a density of 25.5 kg m-3, advect highly saline and warm waters, with low oxygen concentrations compared to the surrounding water masses. Below, evolving at density around 26.7 kg m-3, thick anticyclones were observed, characterized by low temperature and salinity but with high values of oxygen, indicative of a South Atlantic origin. One was observed drifting slowly northward and another one at the NBC retroflection. Similarly, mesoscale cyclonic eddies were also observed both at the surface and at depth. Surface and subsurface eddies are not aligned vertically and they seem to evolve independently. 

The large number and diversity (ship-mounted or autonomous) of observing platforms implemented in the project made made it possible to innovatively sample the upper-ocean frontal scales and stratification. It has been found that the interaction between the particularly fresh waters from the Amazon River, flowing northward along the shelf-break, and NBC rings create a rich variety of submesoscale fronts and a strong barrier layer, leading to interleaving. With the high vertical and horizontal resolutions, we quantify the layering and mixing processes at play.

How to cite: L'Hégaret, P., Speich, S., Chen, Y., Manta, G., Olivier, L., Reverdin, G., Poupon, M., Schütte, F., Karstensen, J., Carton, X., Laxenaire, R., Zhang, D., and Foltz, G.: EUREC4A-OA/ATOMIC experiment  : Themohaline and dynamical descriptions of mesoscale and submesoscale structures of the Northwest Tropical Atlantic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10991, https://doi.org/10.5194/egusphere-egu21-10991, 2021.

During the EUREC4A field campaign in 2020, three ocean gliders were deployed to the tropical North Atlantic, upwind of Barbados. We present preliminary results from this three week deployment, focusing on the finescale temperature and salinity variability below the pycnocline.

The three gliders completed a total of 580 dive cycles to 750 m in virtual mooring and bowtie patterns around a 10 km square. A research vessel occupied a 250 km meridional transect 2 km east of the glider square. The gliders and research vessel observed staircases in temperature and salinity from 300 m to 500 m depth, with a typical vertical scale of 50 m and temperature steps of 0.5 to 1.0 C. The staircase structure was observed by all three gliders’ temperature/salinity sensors and the research vessel's main CTD. The finescale (O 10 cm) vertical structure of the steps, was clearly resolved by a FP07 fast thermistor mounted on one of the gliders. The finescale layers of uniform temperature appear also to be uniform in salinity. These large stairsteps persisted for an average of two days before eroding, and were observed to be spatially coherent over at least 10 km. Smaller stairstep structures at the base of the pycnocline (O 10 m, 0.2 C) persisted throughout the observational period.

Halfway through the deployment, a density-compensated front moving through the region increased temperature at 400 m by 2 C. Simultaneous observations from the three gliders and research vessel enabled analysis of the evolution of this structure. The temperature change was greatest at 400 m, tapering to the limit of detectability at 200 m and 600 m. Along the edge of the front on the warm side, staircase structures were observed. These structures persisted for over a week before eroding.

How to cite: Rollo, C., Heywood, K. J., and Hall, R. A.: Coherent finescale temperature structures characterised at high resolution by a fast thermistor on an ocean glider, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-863, https://doi.org/10.5194/egusphere-egu21-863, 2021.

During the collaborative project "Role of Eddies in the Carbon Pump of Eastern Boundary 

Upwelling Systems" (REEBUS), that took place in the eastern tropical North Atlantic in 2019, 

three cyclonic mesoscale eddies were intensely surveyed by ship and autonomous systems. The 

three eddies were located at different distances to the coast, the most intense of them 

(vorticity about 0.5 times f) was found in lee of the Cabo Verdian island of Fogo. Here we 

present the reconstruction of the 3-D structure for the three eddies from ship ADCP and 

hydrographic sections. Divergence estimates suggest the existence of a downwelling cell in the 

center of all three eddies. This cell extends from below the thermocline down to some hundred 

meters, at a diameter of about 10 nautical miles. Surface signatures of the eddies indicate 

elliptic and oscillating behavior which is further investigated using the interior ocean data. 

In this work we also explore the limitations of section-based ocean mesoscale eddy 

reconstructions.

How to cite: Fischer, T., Karstensen, J., Dengler, M., and Bendinger, A.: Multiplatform observation of cyclonic eddies during the REEBUS experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6537, https://doi.org/10.5194/egusphere-egu21-6537, 2021.

The upper limb of the Atlantic Meridional Overturning Circulation draws waters with negative potential vorticity from the southern hemisphere into the northern hemisphere. The North Brazil Current is one of the cross-equatorial pathways in which this occurs. It is known that upon crossing the equator fluid parcels within this current must modify their potential vorticity, to render them stable to symmetric (inertial) instability and to merge smoothly with the ocean interior.

A hierarchy of models predict the excitement of inertial instability in cross-equatorial flows dynamically similar to the North Brazil Current. A linear stability analysis of a barotropic flow is able to predict the structure and growth rate of the instability. A two-dimensional numerical model verifies these predictions and shows how the instability is able to stabilise unstable potential vorticity configurations. A simplified three-dimensional model demonstrates how large anti-cyclonic rings spun up at the equator entrain waters with negative PV, before the rings themselves become inertially unstable. The high-resolution, observationally constrained, MITgcm LLC4320 model is probed for signs of this instability process.

How to cite: Goldsworth, F., Marshall, D., and Johnson, H.: Symmetric (inertial) instability in cross-equatorial western boundary currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1159, https://doi.org/10.5194/egusphere-egu21-1159, 2021.

In austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind-driven upwelling contribute to near-coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three-dimensional ocean model to simulate the generation, propagation and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200- and 500-m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross-shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially-averaged generation, onshore flux and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

How to cite: Zeng, Z., Brandt, P., Lamb, K., Greatbatch, R., Dengler, M., Claus, M., and Chen, X.: Three Dimensional Numerical Simulations of Internal Tides in the Angolan Upwelling Region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1445, https://doi.org/10.5194/egusphere-egu21-1445, 2021.

Potential vorticity (PV) is a key parameter to analyze the generation and dynamics of mesoscale eddies. Numerical studies have shown how adiabatic (displacement of particles within a background gradient of PV) and diabatic (diapycnal mixing and friction) processes can be involved in the generation of localized PV anomalies and vortices. Such processes are however difficult to evaluate in the ocean because PV is difficult to evaluate at mesoscale. In this study, we argue that qualitative analysis can be done, based on the link between PV anomalies and isopycnal temperature/salinity anomalies (Ɵ’/S’). Indeed, in the ocean, eddies created by diapycnal mixing or isopycnal advection of water-masses, are associated with PV anomalies and significant isopycnal Ɵ’/S’. In contrast, eddies created by friction are associated with PV anomalies but without isopycnal Ɵ’/S’. In this study, based on 18 years of satellite altimetry data and vertical Ɵ/S profiles acquired by Argo floats, we analyze the isopycnal Ɵ’/S’ within new-born eddies in the tropical Atlantic Ocean (TAO) and discuss the possible mechanisms involved in their generation. Our results show that on density-coordinates system, both anticyclonic (AEs) and cyclonic (CEs) eddies can exhibit positive, negative, or non-significant Ɵ’/S’. Almost half of the sampled eddies do not have significant Ɵ’/S’ at their generation site, indicating that frictional effects probably play a significant role in the generation of their PV anomalies. The other half of eddies, likely generated by diapycnal mixing or isopycnal advection, exhibits significant positive or negative anomalies with typical Ɵ’ of ±0.5°C. More than 70% of these significant eddies are subsurface-intensified, having their cores below the seasonal pycnocline. Refined analyses of the vertical structure of new-born eddies in three selected subregions of the TAO where the strongest anomalies were observed, show the dominance of cold (warm, respectively) subsurface AEs (CEs) likely due to isopycnal advection of large scale PV and temperature.

How to cite: Aguedjou, H. M., Chaigneau, A., Dadou, I., Morel, Y., Pegliasco, C., Da-Allada, C. Y., and Baloïtcha, E.: What can we learn from observed temperature and salinity isopycnal anomalies at eddy generation sites?  Application in the Tropical Atlantic Ocean., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9472, https://doi.org/10.5194/egusphere-egu21-9472, 2021.

We analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20^◦W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcel swith PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20^◦W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5^◦ and 5^◦S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.

How to cite: Assene, F., Morel, Y., Delpech, A., Aguedjou, M., Jouanno, J., Cravatte, S., Marin, F., Ménesguen, C., Chaigneau, A., Dadou, I., Alory, G., Holmes, R., Bourlès, B., and Koch-Larrouy, A.: From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5371, https://doi.org/10.5194/egusphere-egu21-5371, 2021.

The shallow meridional overturning cells of the Atlantic Ocean, the subtropical cells (STCs), consist of poleward Ekman transport at the surface, subduction in the subtropics, equatorward flow at thermocline level and upwelling along the equator and at the eastern boundary. In this study, we provide the first observational estimate of transport variability associated with the horizontal branches of the Atlantic STCs in both hemispheres based on Argo float data and supplemented by reanalysis products.

Thermocline layer transport convergence and surface layer transport divergence between 10°N and 10°S are dominated by seasonal variability. Meridional thermocline layer transport anomalies at the western boundary and in the interior basin are anti-correlated and partially compensate each other at all resolved time scales. It is suggested that the seesaw-like relation is forced by the large-scale off-equatorial wind stress changes through low-baroclinic-mode Rossby wave adjustment. We further show that anomalies of the thermocline layer interior transport convergence modulate sea surface temperature (SST) variability in the upwelling regions along the equator and at the eastern boundary at time scales longer than 5 years. Phases of weaker (stronger) interior transport are associated with phases of higher (lower) equatorial SST. At these time scales, STC transport variability is forced by off-equatorial wind stress changes, especially by those in the southern hemisphere. At shorter time scales, equatorial SST anomalies are, instead, mainly forced by local changes of zonal wind stress.

How to cite: Tuchen, F. P., Lübbecke, J. F., Brandt, P., and Fu, Y.: Observed Transport Variability of the Atlantic Subtropical Cells and Their Connection to Tropical Sea Surface Temperature Variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1267, https://doi.org/10.5194/egusphere-egu21-1267, 2021.

Equatorial deep jets (EDJ) are strong zonal currents in the deep tropical oceans that alternate in direction with depth and
time. In the Atlantic below the thermocline, they are the dominant variability on interannual timescales. They propagate
energy upwards and are suggested to impact surface climate variables on interannual timescales. They are also
important for the distribution of tracer in the mid-depth tropical ocean, for example by enhanced oxygen ventilation of
the eastern deep oxygen minimum zones, both through advection by the EDJ themselves and because the EDJ
nonlinearly drive time mean flow. Observations of equatorial deep jets are available but scarce, given the EDJs’ location
at depth and their long periodicity of several years. In the last few years, Argo floats have added a significant amount of
measurements at intermediate depth. We therefore perfomed a new EDJ scale analysis based on Argo float
measurements, the results of which we show here. At 1000 m depth, very weak or no EDJ signals can be detected in the
Indian and Pacific Oceans. In the Atlantic, however, the EDJ signal is strong at 1000 m depth, allowing us to obtain
good estimates of their frequency, amplitude, phase, zonal wavelength, and meridional structure.

How to cite: Bastin, S., Claus, M., Brandt, P., and Greatbatch, R. J.: Atlantic equatorial deep jets in Argo float data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2285, https://doi.org/10.5194/egusphere-egu21-2285, 2021.

The northward flow in the western tropical Atlantic Ocean is carried mainly by North Brazil Current (NBC), hence playing a major role in the cross-equatorial exchange of properties. As thermocline waters reach the equator, they largely retroflect to feed the Equatorial Undercurrent (EUC), a quasi-permanent zonal current that brings salty and highly-oxygenated waters to the eastern side of the basin. This retroflection system is governed by the zonal pressure gradient, which is driven by the trade winds. Hence, the wind fluctuations represent the major source of variability at seasonal and interannual scales. However, at shorter time scales, the variability of the retroflection system may be associated with both interior and coastal waves. In the present study we describe the water mass balance at the NBC-EUC retroflection area using a combination of shipboard observations and numerical reanalysis. The observations, from an oceanographic campaign in April 2010, provide a synoptic view of the retroflection region and allow assessing the goodness of the numerical data. We then use the ocean reanalysis GLORYS2v4 to analyse the temporal variability of this region, from intra-seasonal to seasonal scales, and use Lagrangian simulations to identify the principal water mass pathways feeding the retroflection. We find a substantial seasonal cycle in the boundary and interior (southern and northern) origins of those waters that feed the EUC. Our results also show the propagation of high-frequency waves (16-30 days) along the coast from the south, probably as coastal trapped waves, while waves with 30-60 days period come from the northern hemisphere, probably as westward Rossby waves reach the coast of America and follow south as Kelvin waves. These short-term fluctuations have a high impact on the water mass pathways that feed the EUC and the retroflection structure itself.

How to cite: Vallès Casanova, I., Pelegrí, J. L., Martín Rey, M., van Sebille, E., and Olivé Abelló, A.: From weekly to seasonal variability of the North Brazil - Equatorial Undercurrent retroflection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13045, https://doi.org/10.5194/egusphere-egu21-13045, 2021.

How to cite: Hernandez, F., Dimoune, D. M., Birol, F., Leger, F., and Araujo, M.: New insight of the West Tropical Atlantic Circulation based on 25 years of satellite altimetry, PIRATA data and GLORYS ocean reanalysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12222, https://doi.org/10.5194/egusphere-egu21-12222, 2021.

The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 Wm⁻² over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.

How to cite: Hummels, R., Dengler, M., Rath, W., Foltz, G. R., Schütte, F., Fischer, T., and Brandt, P.: Surface cooling caused by rare but intense near-inertial wave induced mixing in the tropical Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9777, https://doi.org/10.5194/egusphere-egu21-9777, 2021.

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface atmospheric flow with different, and sometimes contrasting, impacts on coastal and open-ocean upwelling systems. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e. filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonal dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine-ecosystem.

How to cite: Bordbar, M. H., Mohrholz, V., and Schmidt, M.: On the connection between coastal Ekman upwelling and wind-stress-curl-driven upwelling off the southwest African coasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12206, https://doi.org/10.5194/egusphere-egu21-12206, 2021.

The impact of intra-seasonal coastally trapped waves on SST in the Canary upwelling system is studied in satellite estimates of sea surface height, wind, and temperature, using a composite analysis of propagating upwelling and downwelling events. We focus on Spring, the season of strongest SST variability at this frequency. The results obtained show that the average wave reaches an amplitude at sea level of +/- 2 cm and is associated with an SST signal of +/-0.4 °C in the vicinity of the upwelling front, located off Senegal. Strikingly, this composite wave is reinforced by a constructive meridional wind anomaly when it reaches the upwelling front, the wind signal is likely as important as the wave in terms of SST impacts. We discuss the possible cause of this synchronicity in terms of basin-scale atmosphere and ocean waves.
Keywords:
- Impact
- Coastal Kelvin waves
- Intra-seasonal
- Boundary upwelling systems
- Composite analysis of spring
- Tropical Atlantic

How to cite: Sané, B., Lazar, A., and Wade, M.: Impact of intra-seasonal coastal Kelvin waves on SST in the Canary upwelling system: composite analysis in Spring, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14117, https://doi.org/10.5194/egusphere-egu21-14117, 2021.

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 mechanism of changes in the mixed-layer heat content that may be locally or remotely forced are 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. We calculate monthly estimates of surface heat fluxes, horizontal advection from near-surface velocities, horizontal eddy advection, and vertical entrainment. Additionally, diapycnal heat fluxes at the mixed-layer base are determined from shipboard and glider microstructure data. The results are discussed in reference to the variability of the eastern boundary circulation, surface heat fluxes and wind forcing.

How to cite: Körner, M., Brandt, P., and Dengler, M.: Seasonal mixed layer heat budget in coastal waters off Angola, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6122, https://doi.org/10.5194/egusphere-egu21-6122, 2021.

El Niño–Southern Oscillation (ENSO) is a key mode of climate variability with worldwide climate impacts. Recent studies have highlighted the impact of other tropical oceans on its variability. In particular, observations have demonstrated that summer Atlantic Niños (Niñas) favor the development of Pacific Niñas (Niños) the following winter, but it is unclear how well climate models capture this teleconnection and its role in defining the seasonal predictive skill of ENSO. Here we use an ensemble of seasonal forecast systems to demonstrate that a better representation of equatorial Atlantic variability in summer and its lagged teleconnection mechanism with the Pacific relates to enhanced predictive capacity of autumn/winter ENSO. An additional sensitivity study further shows that correcting SST variability in equatorial Atlantic improves different aspects of forecast skill in the Tropical Pacific, boosting ENSO skill. This study thus emphasizes that new efforts to improve the representation of equatorial Atlantic variability, a region with long standing systematic model biases, can foster predictive skill in the region, the Tropical Pacific and beyond, through the global impacts of ENSO.

How to cite: Exarchou, E., Ortega, P., Rodrıguez de Fonseca, M. B., Losada Doval, T., Polo Sanchez, I., and Prodhomme, C.: Impact of Equatorial Atlantic Variability on ENSO Predictive Skill, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9436, https://doi.org/10.5194/egusphere-egu21-9436, 2021.

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 Niño 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 Niño in DJF. These years serve as initial conditions for “perfect model” 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 Niño event is suppressed.

The results suggest that, on average, the tropical Atlantic SST anomalies increase the strength of El Niño in the following winter by about 10-20%. If, on the other hand, El Niño 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.

How to cite: Richter, I., Kosaka, Y., Tokinaga, H., and Kido, S.: Reexamining the tropical Atlantic influence on ENSO using perfect model predictability experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12290, https://doi.org/10.5194/egusphere-egu21-12290, 2021.

Interaction between the tropical Pacific, Atlantic, and Indian Ocean basins is increasingly recognized as a key factor in understanding climate variability on interannual to decadal timescales. Most of the studies deal with the connection between pair of basins and less attention has been paid to analyze the degree of collective interaction among the three tropical oceans and its variability along time.In this study, we consider a complex network perspective to analyze the collective connectivity among the three tropical basins. To do so, we first construct a climate network considering as network’ nodes the indices that represent the variability of the SST over the tropical Pacific, the tropical north Atlantic, the equatorial Atlantic and the tropical Indian Ocean. Then, we focus on detecting periods of maximum degree of collective connectivity (synchronization periods) using the mean network distance definition.Results show that the degree of collective connectivity among the three tropical oceans present a large muti-decadal variability and that during the observed period there were two synchronization periods: one developed over the period (1900-1935) and the other from 1975 to present. A period center in the 1950’s is characterized by being the three basins uncoupled .Using this information, an analysis of background conditions in the ocean and the atmosphere has been conducted in order to elucidate causes for this change in connectivity.

How to cite: Rodríguez de Fonseca, B., Martín-Gómez, V., and Aliganga, J. M.: Complex networks approach for detecting tropical basin interactions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16230, https://doi.org/10.5194/egusphere-egu21-16230, 2021.

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 1982–1999, the March‐April‐May SST variability in the Angola‐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 1982–1999, 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.  

How to cite: Prigent, A., Imbol Koungue, R. A., Lübbecke, J., Brandt, P., Harlaß, J., and Latif, M.: Weakening Satellite Era and Future Tropical Atlantic Sea Surface Temperature Variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7067, https://doi.org/10.5194/egusphere-egu21-7067, 2021.

The objective of this work is to understand how the seasonal tendances of the tropical Atlantic SST influence the migration of the Intertropical Convergence Zone (ITCZ) and the West African precipitation associated with it. For this we carried out different sensitivity tests to the SST, climatological, with the regional atmospheric model WRF-ARW. Our results, based on the July-August period, show a strong influence of SST anomalies in the Dakar Nino (DN) and Atlantic cold tongue (ACT) regions on the marine ITCZ and West African precipitation. Above the ocean, the cooling of the tropical northeast Atlantic induces a strong reduction in precipitation north of 10°N, associated with the southward displacement of the ITCZ which is located between 5°-10°N with a slight increase in rains. On the other hand, the warming of the SST of the tropical south-eastern Atlantic induces an increase in marine precipitations, with a maximum centered on 5°N, explained by the location of the ITCZ further south than that associated with the cooling in the region of DN. On the continent, the influence of these SST tendances is characterized by the presence of a zonal dipole of rainfall anomalies over the Sahelian regions. The SST cooling effect in the DN region is more marked in the western Sahel, particularly in Senegal, with a sharp drop in rainfall in this region. While that of warming in the LEF region is more marked in the Sahel, which also induces a strong reduction in the intensity of the rains in this region. However, the combined experience of these two type anomalies shows a dipole of rainfall anomalies over the ocean and over the continent. This dipole is characterized by a decrease (increase) in Sahelian (Guinean) rainfall. Our results also show that, for all simulations, the increase (reduction) in precipitation is more explained by the convective (non-convective) part of the rain. The influence of the SST of DN contributes 40% to 100% on the decrease in rainfall in the West Sahel, while the SST of the ACT reduces rainfall in the eastern Sahel by 40% to 100%. Thus, this work underlines the importance of taking into account the effect of the seasonal anomaly of the SST of DN on Sahelian precipitations in forecasting models.

How to cite: Wane, D., de Coëtlogon, G., Alban, L., Wade, M., and Gaye, A. T.: Atmospheric response to SST tendancy in the Eastern topical Atlantic in July-August, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16204, https://doi.org/10.5194/egusphere-egu21-16204, 2021.

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 Niño (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 Niño and the rainfall in coastal Guinea is stationary over the 20^th century. While this relation remains unchanged over the 21^st 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.

How to cite: Worou, K., Goosse, H., and Fichefet, T.: Weakened impact of the Atlantic Niño on the future Guinean coast rainfall, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1168, https://doi.org/10.5194/egusphere-egu21-1168, 2021.

We aim to assess the recent change of upper-ocean stratification towards tropical conditions at the sea surface in the SETA region and explore its driving mechanisms as well as possible consequences for the primary productivity and fisheries off Angola and Namibia, in order to improve our understanding of what is happening as a result of intensified upper-ocean stratification in upwelling regions.

How to cite: Roch, M., Brandt, P., and Schmidtko, S.: Southeast tropical Atlantic changing from subtropical to tropical conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-833, https://doi.org/10.5194/egusphere-egu21-833, 2021.

The upper wind-driven circulation in the tropical Atlantic plays a key role in the basin wide distribution of water mass properties and affects the transport of heat, freshwater, and biogeochemical components like oxygen or nutrients. It is an important component of the Atlantic climate system and the marine ecosystems. Hence, it is crucial to improve our understanding of its long-term variability which largely relies on model simulations due to sparse observational data coverage in earlier periods. In this study the impact of two different wind forcing products on the tropical Atlantic zonal current field is studied in a high-resolution ocean general circulation model. The first forcing product is the Coordinated Ocean-Ice Reference Experiments (CORE) v2 dataset covering the period 1948 to 2009 (Griffies et al., 2009). It has a horizontal resolution of 2°x2° and temporal resolution of 6-hours. The second forcing product is the new JRA55-do surface dataset (Tsujino et al., 2018). This dataset stands out due to its high horizontal (~55 km) and temporal resolution (3 h) which now covers the entire observational period (1958 to present).

While CORE simulations had difficulties to realistically simulate off-equatorial zonal currents in the tropical Atlantic, in model simulations forced with JRA55-do preliminary results show a clearly improved structure of the equatorial current system. In this study, the used CORE simulation tends to overestimate the strength and vertical extend of the zonal currents especially north of the equator compared to the here used JRA55-do simulation and observations. This might be due to the low resolution of the CORE forcing which cannot resolve smaller scale wind stress and wind stress curl structures.

Furthermore, the CORE wind forcing exhibit suspicious multidecadal wind variability (He et al., 2016) which presumable impacts the multidecadal variability of the simulated wind-driven circulation in the tropical Atlantic. Here, largest differences of zonal wind stress anomalies (up to ~0.03 N m^-2) between both forcing products occur north of the equator between 30°-10°W before 1990. CORE shows stronger eastward wind stress anomalies between 1958 and 1970 and stronger westward wind stress anomalies between 1970 and 1990. How this impacts the variability of the equatorial current system is investigated in this study.

How to cite: Burmeister, K., Schwarzkopf, F. U., Biastoch, A., Brandt, P., Lübbecke, J. F., and Inall, M.: How do different wind forcing products impact the zonal current variability in the tropical Atlantic?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2165, https://doi.org/10.5194/egusphere-egu21-2165, 2021.

Multiple lines of new evidence suggest that the Atlantic Ocean plays an active role in the modulation of global climate. Special attention deserves tropical Atlantic extreme events that have increased from 2000s causing severe winter conditions in the Euro-Atlantic region and originating the most devastating hurricane seasons on record (Foltz and McPhaden 2006; Bucham et al. 2014; Lim et al. 2018; Klotzbach et al. 2018). In 2017, the north Tropical Atlantic (NTA) experienced a profound warming, resembling the Atlantic Meridional Mode (AMM) pattern, that originated a destructive hurricane season with catastrophic social and economic damages (Klotzbach et al. 2018). Previous studies focused their attention on the description of the precursors and predictability of the 2017 hurricane season. Nevertheless, the impact of the 2017 NTA warming on equatorial SST variability has not been explored so far. Recent findings put forward the key role of the AMM-associated cross-equatorial wind to trigger oceanic waves that impact on equatorial SSTs (Martín-Rey and Lazar 2019; Foltz and McPhaden 2010).

Thus, in the present study, we investigate the connection between NTA and equatorial variability during 2017, as well as the importance of an accurate ocean forcing to correctly simulate this event. For such purpose, a suite of three initialized climate predictions, performed with the climate model EC-Earth (version3.3), are analyzed. Two sets of predictions apply a wind stress correction over the Tropical Atlantic (35S-35N) using two distinct wind stress products: ERA-Interim (ERAI) reanalysis and a new ERAI-corrected (ERA*) wind product, which are compared to a control prediction with model-generated wind stress (MOD). ERA* has been developed based on means of a geolocated scatterometer-based correction applied to the ERA-interim reanalysis (Trindade et al. 2019). The high-quality of the scatterometer stress-equivalent winds (Portabella and Stoffelen 2009; De Kloe et al., 2017) allows ERA* to contain some of the physical processes missing or misrepresented (i.e., small-scale ocean processes, such as wind-current interaction) in ERAI.

Using more realistic surface wind stress (ERAI or ERA* with respect to MOD) considerably improves the simulation of eastern NTA and equatorial warming. The novel wind stress product (ERA*) respect its precursor ERAI, better represents the off-shore warm SSTs in the NTA and along eastern equatorial Atlantic and south African coast. It is worth mentioning that oceanic wave activity proves highly sensitivity when forced by realistic ERAI and ERA* wind stress products. In the wind-corrected experiments, an anomalous wind stress curl north of the equator during March-April excites a downwelling Rossby wave that propagates to the west and is boundary reflected in June-July, becoming an equatorial downwelling Kelvin wave (dKW). This dKW displaces eastward favouring the development of an equatorial warming in late-summer and fall. ERA* does not show significant changes in the RW generation, but in the amplitude of equatorial KW during summer season.

Our results highlight the importance of using improved wind stress products to achieve a correct simulation of ocean wave activity and in turn equatorial Atlantic SST variability. This information is of great value for improving current seasonal forecast systems.

How to cite: Trindade, A., Martín-Rey, M., Portabella, M., Exarchou, E., Ortega, P., and Gómara, I.: Dominant role of North tropical Atlantic 2017 warm event on equatorial variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-828, https://doi.org/10.5194/egusphere-egu21-828, 2021.

Here we investigate tropical cyclone (TC) activity and intensity within a 100km radius of Bermuda between 1955 and 2019. Our results show a more easterly genesis over time and significant increasing trends in tropical cyclone intensity (maximum wind speed (Vmax)) with a decadal Vmax median value increase of 30kts from 33 to 63kts, together with significant increasing August, September, October (ASO) sea surface temperature (SST) of 1.1°C (0.17 °C per decade)  and ocean temperature between 0.5–0.7°C (0.08-0.1°C per decade)  in the depth range 0-300m. The strongest correlation is found between TC intensity and ocean temperature averaged through the top 50m ocean layer (T_50m) r=0.37 (p<0.01). 

We show how tropical cyclone potential intensity estimates are closer to actual intensity by using T_50m opposed to SST using the Bermuda Atlantic Timeseries Hydrostation S dataset. We modify the widely used sea surface temperature potential intensity index by using T_50m to provide a closer estimate of the observed minimum sea level pressure (MSLP), and associated Vmax than by using SST, creating a T_50m potential intensity (T_50m_PI) index. The average MSLP difference is reduced by 12mb and proportional to the SST/ T_50m temperature difference. We also suggest the index could be used over a wider area of the subtropical/tropical Atlantic where there is a shallow mixed layer depth. Finally, we outline the TC wind-pressure relationship observed for the subtropical Atlantic around Bermuda, explaining 77% of the variance, which may prove useful for future prediction.

(Environmental Research Letters, 2020, in revision)

How to cite: Hallam, S., Guishard, M., Josey, S., Hyder, P., and Hirschi, J.: Increasing tropical cyclone intensity and potential intensity in the subtropical Atlantic around Bermuda from an ocean heat content perspective 1955- 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7803, https://doi.org/10.5194/egusphere-egu21-7803, 2021.

Most state-of-the-art earth system model still exhibit large biases in the tropical Atlantic. This study aims to investigate how the physical bias influences the marine biogeochemical processes in the tropical Atlantic using Norwegian Earth System Model (NorESM). We assess four different configurations of NorESM: NorESM version 1is taken as benchmark (NorESM-CTL), a version of this model with a physical bias correction using anomaly coupling (NorESM-AC), and NorESM version 2 with low and medium atmospheric resolution (NorESM-LM/NorESM-MM) is also utilized.

With respect to NorESM-CTL, the annual-mean sea surface temperature (SST) bias is improved largely in NorESM-AC and NorESM-MM in the equatorial Atlantic and southeast Atlantic. On the other hand, the improvement of seasonal cycle of SST can be seen in NorESM-AC and the two versions of NorESM2; development of Atlantic Cold Tongue (ACT) is realistic in terms of location and timing. Corresponding to the ACT seasonal cycle, the primary production in the equatorial Atlantic is also improved and in particular, the Atlantic summer bloom is well represented in NorESM-AC and NorESM-MM even though the amount of production is still much smaller than satellite observations. This realistic summer bloom can be related to the well-represented shallow thermocline and associated nitrate supply from the subsurface ocean at the equator.

How to cite: Koseki, S., Rodriguez Crespo, L., and Keenlyside, N.: An assessment of marine biogeochemical processes in the tropical Atlantic in NorESMs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9592, https://doi.org/10.5194/egusphere-egu21-9592, 2021.

Since 2011, Sargassum seaweed has proliferated across the tropical North Atlantic, evident in Floating Algae Index (FAI) images for the Central Atlantic region (38-63°W, 0-22°N) over 2000-2020. To investigate the role of physical drivers in post-2011 Sargassum blooms, conditions are examined across the wider tropical Atlantic. Of particular consequence for the growth and drift of Sargassum are patterns and seasonality of winds and currents. In years when the FAI index is high (2015, 2018), the Intertropical Convergence Zone (where Sargassum accumulates) was displaced southward, towards nutrient-rich waters of the Amazon river plume and the equatorial upwelling zone. Strong enhancement of the North Brazil Current retroflection and North Equatorial Counter Current circulation system in 2015 and 2018 may have increased nutrient availability/uptake for Sargassum in the North Equatorial Recirculation Region. To first order, these changes are associated with modes of natural variability in the tropical Atlantic, notably a negative phase of the Atlantic Meridional Mode in 2015 and 2018, and a positive phase of the Atlantic Niño in 2018. The influence of anomalous winds and currents on Sargassum drift during years of high and low FAI are explored with virtual particle tracking, using surface currents from an eddy-resolving ocean model hindcast and optional % windage, to quantify the variable partitioning between Sargassum that is westward-bound to the Caribbean and eastward-bound to west Africa.

How to cite: Marsh, R., Skliris, N., Oxenford, H., and Appeaning Addo, K.: Post-2011 variability of the great Atlantic Sargassum belt attributed to changing winds and currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12381, https://doi.org/10.5194/egusphere-egu21-12381, 2021.

How to cite: Coelho, P., Tchipalanga, P., Macuéria, M., van der Plas, A., N’Guessan, B., Desiré, K., Makaoui, A., Bessa, I., Idrissi, M., Ettahiri, O., Hilmi, K., Halo, I., Schmidtko, S., Dengler, M., Brandt, P., Cervantes, D., Lødemol, H., Chierici, M., Ostrowski, M., and Bâ, M. L. and the WG TOMSWA - Ad-Hoc Working Group on Transboundary Oxygen Monitoring Status in West Africa: An inventory of dissolved oxygen conditions along the eastern boundary of tropical and subtropical Atlantic: building oxygen monitoring capacity in West African countries, 2013-2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15806, https://doi.org/10.5194/egusphere-egu21-15806, 2021.