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

Type Article
Date 2020-09
Language English
Author(s) Assene Fernand1, 2, Morel Yves1, Delpech Audrey1, Aguedjou Micael1, 7, Jouanno Julien1, Cravatte Sophie1, Marin Frederic1, Ménesguen ClaireORCID3, Chaigneau Alexis1, 7, 8, Dadou Isabelle1, Alory Gael1, Holmes Ryan4, Bourlès Bernard5, Koch-Larrouy Ariane1, 6
Affiliation(s) 1 : LEGOS, Université de Toulouse, CNES, CNRS, IRD, 31400 Toulouse, France
2 : Department of Oceanography, Institute of Fisheries and Aquatic Sciences, University of Douala at Yabassi, BP 7236 Douala-Bassa, Cameroon
3 : LOPS, Université de Bretagne Occidentale, Ifremer, CNRS, IRD, 29280 Plouzané, France
4 : CCRC, ARC CLEX and the School of Mathematics and Statistics, University of New South Wales, NSW 2052 Sydney, Australia
5 : IMAGO, IRD, 29280 Plouzané, France
6 : Mercator Ocean, 31520 Ramonville-Saint-Agne, France
7 : International Chair in Mathematical Physics and Applications (ICMPA-UNESCO Chair), University of Abomey-Calavi, 072BP50 Cotonou, Rep. Benin
8 : Institut de Recherches Halieutiques et Océanologiques du Bénin (IRHOB), 03BP1665 Cotonou, Rep. Benin
Source Fluids (2311-5521) (MDPI AG), 2020-09 , Vol. 5 , N. 3 , P. 147 (36p.)
DOI 10.3390/fluids5030147
WOS© Times Cited 2
Note This article belongs to the Special Issue Submesoscale Processes in the Ocean
Keyword(s) equatorial atlantic, NEUC (North Equatorial Undercurrents), SEUC (South Equatorial Undercurrents), vortices, mixing, friction
Abstract

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∘ 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∘ 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.

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Assene Fernand, Morel Yves, Delpech Audrey, Aguedjou Micael, Jouanno Julien, Cravatte Sophie, Marin Frederic, Ménesguen Claire, Chaigneau Alexis, Dadou Isabelle, Alory Gael, Holmes Ryan, Bourlès Bernard, Koch-Larrouy Ariane (2020). 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. Fluids, 5(3), 147 (36p.). Publisher's official version : https://doi.org/10.3390/fluids5030147 , Open Access version : https://archimer.ifremer.fr/doc/00645/75697/