The tropical oceans are home to some of the strongest subsurface current systems of the world. Among these are the Equatorial Undercurrent (EUC) which flows eastward below the thermocline, and the Equatorial Intermediate Current System (EICS) consisting of latitudinally alternating zonal jets between 15S and 15N. Ocean and climate models consistently struggle in correctly representing these subsurface currents, but especially the EUC is quite important for tropical Atlantic climate, e.g. the evolution of the Atlantic cold tongue and the associated Atlantic Niño. We use the ocean component of the ICON model to test how the representation of the Atlantic subsurface current systems reacts to different model parameter choices. In general, the EUC is too weak in our ICON configuration. We can show that the EUC is stronger, i.e. better represented with the TKE vertical mixing scheme than with the KPP scheme. Using the TKE scheme, different parameters are tested and it can be shown that the EUC reacts sensitively to the value of the important tuning parameter c_k, being stronger when c_k is smaller. We also test the sensitivity of the EUC strength in the model to the formulation of the Prandtl number in the TKE mixing scheme, which also includes a changeable constant. Apart from changes in the vertical mixing scheme, we also look at the effect of the vertical resolution of the near-surface ocean. We compare a vertical level thickness of 2m in the upper 20m to 10m in the upper 20m, with the same level distribution below 20m depth for both configurations. We can show that for the thinner surface levels, the EUC is much weaker than for the thicker levels. Intriguingly, also the EICS react to the change in near-surface vertical resolution despite being located at much larger depths. The EICS is, like the EUC, generally too weak in ICON, but becomes stronger when the near-surface levels are thinner.

How to cite: Bastin, S., Putrasahan, D., and Jungclaus, J.: Sensitivity of tropical Atlantic subsurface currents to different model parameter choices in ICON-O, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14070, https://doi.org/10.5194/egusphere-egu23-14070, 2023.

In this study, we address the role of the ocean fine scales in north-west tropical Atlantic Ocean air-sea interactions. With this purpose, we use satellite observations of the ocean and the atmosphere, the ERA5 atmospheric reanalysis and a set of regional numerical simulations of the lower atmosphere. In particular, we focus on the coupling between the sea-surface temperature (SST) and the marine atmospheric boundary layer (MABL). We also evaluate the latent heat flux (LHF) sensitivity to SST. The results suggest that the SST-MABL coupling depends on the spatial scale of interest. At scales larger than the ocean mesoscale (larger than 150 km), negative correlations are observed between near-surface wind speed (U_10m) and SST and positive correlations between near-surface specific humidity (q_2m) and SST. However, when smaller scales (1 – 150km, i.e., encompassing the ocean mesoscale and a portion of the submesoscale) are considered, the U_10m-SST and q_2m-SST correlate inversely. This is interpreted in terms of an active ocean modifying the near-surface atmospheric state, driving convection, mixing and entrainment of air from the free troposphere into the MABL.

The estimated values of the ocean-atmosphere coupling at the ocean small-scale are then used to develop a linear and SST-based downscaling method aiming to include and further investigate the impact of these fine-scale SST features into an available low-resolution latent heat flux (LHF) data set. The results show that they induce a significant increase of LHF (30% - 40% per °C of SST). We identify two mechanisms causing such a large increase of LHF: (1) the thermodynamic contribution that only includes the increase in LHF with larger SSTs associated with the Clausius-Clapeyron dependence of saturating water vapor pressure on SST and (2) the dynamical contribution related to the change in vertical stratification of the MABL as a consequence of SST anomalies. Using different downscaling setups, we conclude that largest contribution comes from the dynamic mode (28% against 5% for the thermodynamic mode). To validate our approach and results, we have implemented a set of high-resolution WRF numerical simulations forced by high-resolution satellite SST that we have analyzed in terms of LHF using the same algorithm.

To provide further validation to our results we use the high spatio-temporal resolution of in-situ data collected during the EUREC⁴A-OA/ATOMIC campaigns that go beyond the coarse spatial grid of available satellite observations and include additional variables to the SST such as the impact of ocean currents and the local vertical stratification of the upper ocean.

How to cite: Fernández, P., Speich, S., Borgnino, M., Meroni, A., Desbiolles, F., and Pasquero, C.: On the importance of the atmospheric coupling to the small-scale ocean in the modulation of latent heat flux, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-311, https://doi.org/10.5194/egusphere-egu23-311, 2023.

Chlorophyll-a surface concentration is partially determined by environmental conditions and its variability, as the highest concentrations are generally found in wind-driven oceanic upwelling regions. These wind regimes that affect upwelling strength can be determined by local and remote drivers, such as sea surface temperature (SST) anomaly patterns (e.g., Pacific and Atlantic Niños/Niñas) that trigger tropical basin interactions.

By performing a Maximum Covariance Analysis (MCA) between chlorophyll-a concentration from Copernicus Satellite data and SST anomalies from OISST (January 1998-December 2019), we here identify the individual SST patterns and the associated atmospheric responses that lead to an increase in chlorophyll concentration in two regions of the tropical Atlantic: the Senegalese coast and the equator during their seasonal maxima (February to May and June to September, respectively). The present study shows how an Atlantic El Niño is capable of promoting a Pacific La Niña, whose atmospheric response affects either the tropical north Atlantic and the equatorial Atlantic, producing an SST cooling in early spring in the former and in summer in the latter, both related to an increase of chlorophyll concentration.

A cross-validated hindcast based on Maximum Covariance Analysis (MCA) is used to assess chlorophyll predictability through these individual SST variability modes.

Key words: chlorophyll-a concentration, SSTs, atmospheric responses, statistical prediction.

How to cite: Calvo Miguélez, E., Rodríguez-Fonseca, B., and Gómara, I.: Variability and predictability of surface chlorophyll in the Atlantic upwelling systems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-854, https://doi.org/10.5194/egusphere-egu23-854, 2023.

Coastal countries in West Africa heavily rely on the ocean, which is a major source of food and employment. This is mainly due to the presence of coastal upwelling, upward motion of sea water bringing the nutrient-rich deeper waters into the illuminated surface layers in the coastal zone, where they become available for photosynthesis. The resulting phytoplankton production, the base of the food chain, render coastal upwelling the most productive of large marine ecosystems in the world’s oceans. Recently, decadal variability and predictability of coastal upwelling systems has received a lot of attention, since near-term changes of upwelling systems could have a strong impact of living marine resources and hence surrounding countries economy. On this aspect, recent progress has been made in generating near-term (“decadal”) predictions of physical using Earth system models (ESMs). Initialized forecasts have shown significant predictability from 1 to 10 years in advance for climate events showing substantial decadal variability.

Our objective here is two-fold: first we investigate the decadal variability of the Senegalo-mauritanian upwelling system (SMUS) in the reanalysis and historical simulations from eleven climate models using indices based on the SST and wind stress and also identify the processes controlling this variability. Second, we exploit the decadal prediction experiments of CMIP6 (DCPP-A), to investigate this upwelling predictability. Our results show that the SMUS is characterized by a strong decadal variability, in part linked to the Atlantic Multidecadal Variability Consequently, the DCPP- A experiment shows strong and generally significant correlation prediction scores at various lead times for the dynamical indices (Ekman transport and Ekman pumping). However, even though coastal SST are also significantly predictable, non-significant ACC scores are found for the thermal upwelling indices. The analysis concludes on trying to qualify and quantify the predictable components of the SMUS and possible applications.

How to cite: Sylla, A. and Mignot, J.: Decadal variability and predictability of Senegalo-mauritanian upwelling system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12692, https://doi.org/10.5194/egusphere-egu23-12692, 2023.

Like other Eastern Boundary Upwelling Systems, the upwelling near the southwest African coasts is primarily alongshore-wind-driven, whereas it is controlled mainly by the wind stress curl farther offshore. The surface wind regime across the Benguela Upwelling System (BUS) is strongly related to the South Atlantic Anticyclone (SAA), which is believed to migrate poleward in response to anthropogenic global warming. Here, we investigate multi-decadal changes of the SAA and its impacts on the coastal Ekman transport as a primary driver of coastal upwelling and the wind-stress-curl-driven upwelling across the BUS by using the ERA-5 data sets. Our findings indicate that the SAA plays a significant role in the regional wind-driven upwelling with different impacts on the coastal Ekman transport and the offshore wind-stress-curl-driven upwelling. Further, the upwelling in the equatorward sector is significantly affected by the anticyclone intensity. In contrast, the poleward portion is also influenced by the meridional position of the anticyclone. The multi-decadal trend in the sea level pressure across the South Atlantic renders a considerable heterogeneity in space. However, the trend is broadly associated with a small signal-to-noise ratio, which can be attributed to internal climate variability. This view is further supported by the multi-decadal trend in coastal offshore transport and the wind-stress-curl-driven upwelling in multiple upwelling cells, which hardly depict any significant systematic changes.

How to cite: Bordbar, M. H., Mohrholz, V., and Schmidt, M.: The importance of internal climate variability in the multi-decadal trend of the wind-driven upwelling on the west African coasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6033, https://doi.org/10.5194/egusphere-egu23-6033, 2023.

The Atlantic meridional mode (AMM) is characterized by north-south bands of alternate anomalies in surface-ocean temperatures, and winds from colder bands to the warmer, at a periodicity of 10-15 years. The AMM has been linked to variations in Atlantic hurricanes, global surface-air temperature, and climate variability over the Sahel, South American, North American, and European. Despite these far-reaching impacts, the role of ocean circulation remains uncertain, and the prevailing AMM theories are based on thermodynamic air-sea interactions. Here we we uncover ocean-circulation variability that is linked to the AMM using twentieth century observations. Specifically, sea level-derived index of ocean circulation variabilityleads the AMM pattern by several years, through the interactions of overturning and gyre circulations with Kelvin wave anomalies that propagate from the North Atlantic to the low latitudes and by the thermocline feedback in the Atlantic cold tongue region. The peak of the sea surface temperature variability in the tropical Atlantic in turn drives inter-hemispheric atmospheric teleconnections represented by negative NAO phase over the North Atlantic. These findings imply that, rather than a passive role postulated by the prevailing thermodynamic paradigm, ocean circulation plays an active role in AMM variability.

How to cite: Nnamchi, H., Farneti, R., Keenlyside, N., Kucharski, F., Latif, M., Reintges, A., and Martin, T.: Ocean circulation underlies the Atlantic meridional mode, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7587, https://doi.org/10.5194/egusphere-egu23-7587, 2023.

During the boreal summer of 2021, the central and eastern equatorial Atlantic experienced persistent sea surface temperature (SST) anomalies of more than +1°C for several months. These episodic extreme events are referred to as Atlantic Niños, with 2021 marking the strongest event since 1984. Atlantic Niños are known to have far-reaching impacts on, for instance, rainfall over the surrounding continents, and on ocean dynamics through changes in thermohaline gradients and circulation. A pronounced event like the 2021 Atlantic Niño in combination with the steadily expanding coverage by satellite and in-situ observations provides the rare opportunity to study an Atlantic Niño event in unprecedented detail. Here we focus on the influence of the 2021 Atlantic Niño on tropical instability wave (TIW) activity and surface chlorophyll concentration.

The 2021 Atlantic Niño was initiated by a strong downwelling Kelvin wave excited by westerly wind bursts in the western and central equatorial Atlantic. The eastward propagating Kelvin wave induced strong eastward flow anomalies on the equator causing a reduction of the meridional shear of the near-surface zonal flow in the central equatorial Atlantic. At the same time, the Kelvin wave-induced deepening of the thermocline weakened the seasonal development of the equatorial Atlantic cold tongue. The reduction of both the meridional SST gradient and the meridional shear of zonal velocity largely suppressed barotropic and baroclinic instability, which is required for the generation of TIWs. Consistent with these changes, we find that 2021 was one of the least active years in terms of TIW-related temperature, salinity, sea level, and current variability. The overall reduction in TIW activity is characterized by weak TIW activity before and enhanced TIW activity after the climatological TIW peak resulting in a delay of the TIW season. This delayed onset of TIW activity is expected to have considerable consequences for the local heat and freshwater budgets. Low TIW activity and positive SST anomalies also impacted the concentration and distribution of surface chlorophyll as observed by daily gap-free satellite observations. After the initial Kelvin wave, surface chlorophyll concentration dropped to extraordinarily low values and was anti-correlated with the evolution of SST anomalies. The absence of TIWs is also apparent in weaker than normal surface chlorophyll concentration variability on intraseasonal time scales, highlighting the interplay of TIWs and chlorophyll. Our results demonstrate how the 2021 Atlantic Niño impacted oceanic variability, but further analysis is needed to better understand the consequences of such events for regional heat and freshwater budgets as well as for nutrients, productivity, and marine ecosystems.

How to cite: Tuchen, F. P., Perez, R. C., Foltz, G. R., and Brandt, P.: Modulation of Tropical Instability Waves and chlorophyll concentration by equatorial waves during the 2021 Atlantic Niño, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10576, https://doi.org/10.5194/egusphere-egu23-10576, 2023.

Northern tropical and subtropical North Atlantic Ocean is exposed to large plumes of atmospheric dust mostly from Saharan dust outbreaks. The scattering and absorbing radiation in the dust air layer modifies the air column temperature. These temperature changes strongly impact the atmospheric wind forcing and deep atmospheric convection resulting in changes in the latitudinal position of the Intertropical Convergence Zone and the distribution of sea surface temperature anomalies (SSTA). In this study, we analyze the interaction between the interannual variability of Aerosol Optical Depth (AOD) in the North Tropical Atlantic (NTA) and SSTA in the equatorial Atlantic in the period 1996-2020. Observational results show that AOD-induced SSTA in the north tropical Atlantic may impact the equatorial Atlantic variability by different mechanisms such as an intensification of cross-equatorial winds and the excitation of oceanic waves. In particular, the AOD in NTA in boreal winter/spring seems to impact on the onset, intensity and spatial configuration of the Atlantic Zonal Mode, also known as Atlantic Niño. Our findings suggest that atmospheric dust can be a potential precursor for equatorial Atlantic Variability. 

How to cite: Vallès-Casanova, I., Adam, O., and Martín-Rey, M.: Influence of atmospheric dust on the equatorial Atlantic Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16606, https://doi.org/10.5194/egusphere-egu23-16606, 2023.

The South American continental slope hosts a variety of topographic waves. We use a 27-year-long global ocean reanalysis (1/12° Spatial resolution) to examine trapped waves (TWs) around South America at periods ranging from 40 to 130 days. The waves propagate from the Equatorial Pacific to the Tropical Atlantic (22°S) with phase velocities between 1.8 and 7 m/s according to the local background characteristics, such as stratification, slope steepness, latitude, mean flow and shelf width. The Madden-Julian Oscillation (MJO) plays a key role in forcing the TWs in two ways (a) through an oceanic connection implying equatorial Kelvin waves reaching the western American Coast and (b) through an atmospheric teleconnection enhancing southerly winds in the south-east Pacific. Furthermore, local winds, not necessarily linked with the MJO, modulate and trigger waves in specific locations, such as the Brazil-Malvinas Confluence. Trapped waves impact the along-shore currents: during the positive phase of the waves the near-surface flow is enhanced by about 0.1 m/s.

How to cite: Poli, L., Artana, C., and Provost, C.: Topographically Trapped Waves Around South America: Oceanic Teleconnections between Equatorial Pacific and Tropical Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3178, https://doi.org/10.5194/egusphere-egu23-3178, 2023.

The El Niño Southern Oscillation (ENSO) underwent a major shift in in the 1970’s, becoming stronger and more predictable. This shift has been attributed to changes in the tropical Pacific mean state. However, around the 1970’s, tropical Atlantic – Pacific variability became coupled, with Atlantic SST leading opposite signed changes in the Pacific by around 6-months. Here we assess the role of the Atlantic in driving the ENSO regime shift using pacemaker experiments with two climate models: ECHAM5/MPIOM and SPEEDY/RGO. In these experiments, model SST is restored to observations in the tropical Atlantic, while elsewhere the models are fully coupled. Both models capture the observed changes in inter-basin interactions and strengthening on ENSO variability after the 1970’s. The warming of the equatorial and south Atlantic and southward shift of the inter-tropical convergence zone causes inter-basin interactions to become active after the 1970’s in the models. In ECHAM5/MPIOM, this leads to Atlantic Niño variability driving increased ENSO activity. In SPEEDY/RGO, the increase in ENSO activity appears more related to induced mean state changes in the Pacific. In addition, experiments with two different versions of the Norwegian Earth System Model and two nudging approaches (anomaly and full field SST) have been performed as part of the CLIVAR RF Tropical Basin Interactions. Initial analysis reveals are rather muted impact of the tropical Atlantic on ENSO. Further analysis is being performed and results will also be presented.

How to cite: Keenlyside, N., Ding, H., Martín del Rey, M., Polo, I., Rodriguez-Fonseca, B., Kucharski, F., and Chiu, P.-G.: Tropical Atlantic forcing of increased ENSO variability since the 1970’s, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9943, https://doi.org/10.5194/egusphere-egu23-9943, 2023.