Alti-ETAO - Characterization and analysis of circulation and mesoscale dynamics in the Eastern Tropical Atlantic Ocean using altimetry data -OSTST-AO2016 2017-2020

Isabelle Dadou (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France)

CoAuthors

Yves Morel (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Gaël Alory (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Alexis Chaigneau (ICMPA-UNESCO Chair, University of Abomey-Calavi, IRHOB, Cotonou, Rep. Benin); Julien Jouanno (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Micaël Aguedjou (ICMPA-UNESCO Chair, University of Abomey-Calavi, IRHOB, Cotonou, Rep. Benin); Habib Dieng (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Samuel Dédoni (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Mesmin Awo (ICMPA-UNESCO Chair, University of Abomey-Calavi, IRHOB, Cotonou, Rep. Benin); Fernand Assene (Department of Oceanography, Institute of Fisheries and Aquatic Sciences, University of Douala at Yabassi, Douala-Bassa, Cameroon); Fabien Leger (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Marie-Hélène Radenac (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Frederic Marin (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France); Bernard Bourles (US-IMAGO, Délégation Régionale Ouest, Plouzane, France); Florence Birol (LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France, France)

Event: 2020 Ocean Surface Topography Science Team Meeting (virtual)

Session: Salient results from the 2017-2020 OSTST PIs

Presentation type: Type Forum

During the Alti-ETAO OSTST project, we used long-term time series of altimetry measurements, both the single-mission along track data (T/P, ERS, JASON, etc.) and multi-mission gridded products, to study the dynamics of the Eastern Tropical Atlantic Ocean (ETAO), especially at mesoscale (10-500 km) from intraseasonal to interannual timescales. This mesoscale activity in the ETAO is not well known and interacts with a broad range of processes (large-scale circulation, equatorial and coastal waves, near-coastal dynamics). Combining these altimetry data with other remotely sensed data such as surface winds (ERS, QuikSCAT, ASCAT), sea surface temperature (AVHRR, TMI, ect.), ocean color (SeaWiFS, MODIS/Aqua, MERIS, Sentinel3, GlobColour multi-mission product), Sea Surface Salinity (SMOS, SMAP), in situ data (PIRATA, drifters, Argo floats, tide gauges) and high-resolution numerical models, the main following results in the ETAO region were obtained during this project:
General characteristics of the mesoscale dynamics and eddy detectability using satellite data (Aguedjou et al., 2019; Morel et al., 2019; Assene et al., 2020): 1) 60% more eddies are found in the tropics compared to the equatorial zone; 2) Eddies form in the eastern part of the basin and propagate to the west (faster speed at the equator) with vortex lifetimes of up to 140 days in tropical areas; 3) Waves represent 10% of the structures detected by applying a vortex/wave criterion; 4) Eddy seasonal cycle amplitude can reach up to ± 50% of the mean value for the eddy properties (radius, EKE) and are linked to the ocean circulation seasonal cycle; 5) Vortices are generated mainly by barotropic instability to the east of the North Brazil current retroflexion, along the North Equatorial Counter-Current, according to a newly defined criterion based on altimetry and validated by academic modeling; 6) Vortices present T/S anomalies at the surface (equatorial zone) and in subsurface (tropical zones); 7) In equatorial zone (beta plane), vortices are much easier to follow with PV rather than with SSH; 8) Based on PV, vortices were classified in 3 types in the Gulf of Guinea: surface vortices detectable by SSH and subsurface eddies with either short or long lifetime without detectable signature in satellite data.
Large scale current variability (equatorial, tropical), equatorial waves, coastal dynamics (currents, upwelling) and their influence on the mesoscale dynamics (Awo et al., 2018; Ohde and Dadou., 2018; Dieng et al., 2019; Alory et al., 2020): 1) We found an SSS signature of the tropical Atlantic interannual modes of variability in the equatorial band and around the plumes of large rivers, visible by SMOS, and identified driving processes; 2) A theoretical model of the dynamics of equatorial Kelvin waves was successfully applied to the Atlantic to explain the SLA and SST anomalies observed in July-October 2009, with strong consequence on the African Monsoon; 3) Coastal upwelling in the northern gulf of Guinea is weakened by about 50% due to geostrophic flow in its eastern part, with partial contribution of the Niger river resulting in a one-degree temperature increase in the upwelling tongue; 4) Coastal circulation off the Congo river is mainly due to the ageostrophic wind-driven processes, with also a strong influence of the geostrophic processes off the river plume and, inertial and secondary cross-shelf circulation close to the river mouth during the river discharge seasonal peak; 5) Small vortices exist in the surface layer close to the coast with small radius (~ 50 km) and short lifespan (~ 1 month) in relation with the coastal dynamics (currents, upwelling, ect.) in the northern part of the Gulf of Guinea, 6) Close to the Equator, the strong zonal surface currents and undercurrents stir and destroy vortical structures but there could exist long-life equatorial subsurface vortices; 7) The inter-annual variability of eddies is lower than the seasonal cycle and linked to large-scale ocean circulation.
We also validated the coastal altimetry data (XTRACK) with tide gauges data in the ETAO, except in the Benguela upwelling system where additional corrections and work are needed (Dieng et al., 2019). Furthermore, new diagnostics were proposed for the generation and structure of vortices (Aguedjou et al., 2019), in particular combining altimetry with other observations (both satellite and in situ), some of which having already been successfully applied to other regions or numerical simulations (Assassi et al., 2016). Numerical results from realistic high resolution models of the region and also academic modeling, were used as a virtual reality to test these diagnostics but also to identify their limitations.
Refs:Aguedjou et al., GRL, 2019; Alory et al., rev. Front. in Mar. Sc.; Assassi et al., JPO,2016; Assene et al., Fluids, 2020; Awo et al., JGR,2018; Dieng et al., Adv. in Sp. Res.; Morel et al., Ocean Mod., 2019; Ohde and Dadou, PLoS ONE, 2018.
 
Isabelle Dadou
LEGOS - University of Toulouse , CNES, CNRS, IRD, Toulouse, France
France
isabelle.dadou@legos.obs-mip.fr