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

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 Assene F. auteur aut IRD Morel Y. auteur aut IRD Delpech A. auteur aut IRD Aguedjou M. auteur aut IRD Jouanno Julien auteur aut IRD Cravatte Sophie auteur aut IRD Marin Frédéric auteur aut IRD Menesguen C. auteur aut IRD Chaigneau Alexis auteur aut IRD Dadou I. auteur aut IRD Alory G. auteur aut IRD Holmes R. auteur aut IRD Bourlès Bernard auteur aut IRD Koch Larrouy Ariane auteur aut IRD text journalArticle eng text/pdf reformatted digital access In this paper, 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 degrees 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 parcels with 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 degrees W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5 degrees and 5 degrees 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. specialized equatorial atlantic NEUC (North Equatorial Undercurrents) SEUC (South Equatorial Undercurrents) vortices mixing friction ATLANTIQUE GUINEE GOLFE ZONE EQUATORIALE 032 Fluids 5 3 147 [34 p.] 2020 https://www.documentation.ird.fr/hor/fdi:010079858 10.3390/fluids5030147 [F B010079858] https://www.documentation.ird.fr/hor/fdi:010079858 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/divers20-11/010079858.pdf IRD - Base Horizon / Pleins textes 2020-12-08 2025-02-24 fdi:010079858 fre