Zonal jets and recirculations in the upper tropical Atlantic Ocean

Abstract

We use in situ Argo float, AVISO and numerical ECCO2 data to examine the seasonal variability of the velocity fields in the upper tropical Atlantic Ocean, here defined as comprising surface and central waters (down to about 500 m) between 10ºS and 20ºN. This upper-ocean is characterized by the dominance of zonal jets, with substantial annual and semi-annual variability. Our results show that these jets are directly (through the surface down-wind stress) and indirectly (through latitudinal Ekman transports) driven by the seasonal latitudinal march of the Inter-Tropical Convergence Zone (ITCZ). One major element is the seasonal accumulation and release of surface waters, through latitudinal Ekman convergence-divergence, and the associated geostrophic meridional circulations at surface and subsurface levels, in what constitutes the upper and lower limbs of the tropical and subtropical cells. Their most remarkable surface manifestation is the connection between the South Equatorial Current and the North-Equatorial Counter Current, which builds up between April and September from East to West, starting as a westward extension of the Guinea Current and eventually connecting with the retroflection of the North Brazil Current (NBC). The reported recirculations are responsible for the long-residence times in the upper-tropical Atlantic, only broken by the inter-hemispheric transport of the NBC, and favor the existence of high-remineralization and low-oxygen zones. The long-term expansion of the low-oxygen tropical regions will rely on the coupled evolution of the ITCZ and the surface-ocean