New insight of the west tropical atlantic circulation based on 25 years of satellite altimetry, pirata data and glorys ocean reanalysis

Abstract

The western tropical Atlantic is one of the most dynamic regions of the whole ocean where most of the zonal currents flows transporting water masses with salt and heat. Its importance is mostly due to its interactions with atmosphere which influence the regional climate and the precipitations of the region. During this thesis, the main goal was to take advantage of the larger dataset of the altimetry-geostrophic currents and the GLORYS12V1 (G12V1) reanalysis available over the 1993-2017 period to revisit the western tropical Atlantic circulation and improve the knowledge about the spatiotemporal variability of the currents at the seasonal and interannual timescales. To do so, 9 sections crossing the currents were used to follow the evolution of the currents and the redistribution of the water masses. Data from Ship- mounted acoustic Doppler current profiler (SADCP) and near-surface drifter were used to prove the effectiveness of the G12V1. From both the altimetry-geostrophic currents data and the G12V1 reanalysis we found 2 different behaviours of the North Brazil Current (NBC) strength. In the southern hemisphere, the NBC depicts an annual cycle with a maximum (minimum) amplitude/transport during boreal spring (fall) while in the northern hemisphere, its cycle reverses. During boreal spring, the southward recirculation of the northern branch of the South Equatorial Current (nSEC) joins the weak NBC retroflected branch (rNBC) to feed a surface eastward flow along the equatorial region and to generate a cyclonic circulation between 0°- 5°N, 35°W-45°W. The volume transport of the NBC in the upper layer (0-100 m) shows an equatorial retroflection of 1.8 Sv joining the weak flow from the rNBC-nSEC (4.6 Sv) to maintain this cyclonic circulation. From September to December, the NBC shows another retroflection region in the upper layer between 4-6°N which is particularly intense in October. During boreal fall, the upper rNBC connects to the North Equatorial Countercurrent (NECC) which shows a two-core structure, and eventually 2 separated branches from November to the first half of the year. The seasonal variability of the transport/amplitude/core location of both branches and the currents in the northern hemisphere respond to the remote wind seasonal changes in the eastern basin. The higher contribution of the rNBC to the NECC occurs during boreal fall with an input of 15.2 Sv. In the thermocline layer (100-350 m), the higher contribution to the Equatorial Undercurrent (EUC) occurs during boreal spring and fall (22.3 Sv and 20 Sv, respectively) when the contribution of the northern hemisphere waters to the rNBC is also higher. The investigation of the year-to-year variations of the altimetry- geostrophic currents crossing the 9 sections (amplitude and core velocity/location) show that the interannual variability of the NECC amplitude/location and the nSEC amplitude at 42°W might be associated to the Atlantic meridional mode (AMM) phases whereas it is associated to the zonal mode (AZM) phases at 32°W. Unlike, the interannual variability of the NECC transport at 38°W and of the surface equatorial eastward flow amplitude at 32°W appears to be associated to both modes.