Revisiting the tropical Atlantic western boundary circulation from a 25-year time series of satellite altimetry data

Abstract. Geostrophic currents derived from altimetry are used to investigate the surface circulation in the Western Tropical Atlantic over the 1998–2017 period. Using six horizontal sections defined to capture the current branches of the study area, we investigate their respective variations at both seasonal and interannual time-scales as well as the spatial distribution of these variations. Our results show that the central branch of the South Equatorial Current, the North Brazil Current component located south of the equator, the Guyana Current and the northern branch of the South Equatorial Current at 42° W have similar annual cycles, with maxima/minima during boreal winter-spring/October–November. In contrast, the seasonal cycles of the North Brazil Current branch located between the equator and 7–8° N, the North Brazil Current retroflected branch and the North Equatorial Countercurrent show maxima/minima during boreal fall/May. West of 42° W, an eastward current is observed between 0°–2° N, identified as the equatorial extension of the retroflected branch of the North Brazil Current. It is part of a large cyclonic circulation observed between 0°–6° N and 35°–45° W during boreal spring. The North Equatorial Countercurrent shows a two-core structure during the second half of the year, when we also observe the two regions where the North Brazil Current retroflects. The latter can be related to the wind stress curl seasonal changes. At interannual scales, depending on which side of the equator, the North Brazil Current exhibits two opposite scenarios related to the tropical Atlantic Meridional Mode phases. The interannual variability of the North Equatorial Countercurrent and of the northern branch of the South Equatorial Current (in terms of both strength and/or latitudinal shift) at 42° W are also associated to the Atlantic Meridional Mode, while they are associated to the zonal mode phases at 32° W.