On How the Seasonal Variability of the ITCZ Drives Subtropical-Tropical Exchange in the Atlantic Ocean

The Inter-Tropical Convergence Zone (ITCZ) oscillates seasonally in the North Atlantic Ocean, its mean position moving between the Equator in February and 10°N in August-September. Further, during its annual march, the ITCZ remains oriented SW-NE, with 10° of latitude difference across the Atlantic. As a consequence, Ekman convergence-divergence changes zonally and seasonally, setting up the sea-surface absolute dynamic topography (ADT) that drives geostrophic currents in the tropical ocean and the adjacent subtropical gyres. The clearest example is the seasonal growth of the North Equatorial Counter Current (NECC) at 6-8°N, as an intense eastward jet in the tropical North Atlantic amid the westward South Equatorial Current (SEC). The NECC starts in the eastern Atlantic in May and progresses west, fed by a northern diversion of the SEC, until it reaches the western boundary in August, causing the retroflection of the North Brazil Current (NBC). The NECC results from the boreal-summer northward penetration of the ITCZ and the associated South Atlantic High: the surface winds cause zonal bands of meridional Ekman convergence (3-7°N) and divergence (7-13°N) that lead to a ridge-valley ADT pattern, with the NECC unfolding at those latitudes of large latitudinal ADT gradients (6-8°N). The NECC intensifies through input from the interior gyres – anticyclonic near the low-latitude ridge and cyclonic around the northern valley, the latter related to the northern subtropical cell – and only after August it fuels from the NBC retroflection. The northern position of the ITCZ prevents analogous jets and retroflections to occur in the southern hemisphere, yet the location where the SEC bifurcates (into the northward NBC and the southward Brazil Current) changes between January (8°S) and June (16°S). Further, between April and July, a ridge-valley ADT distribution appears in the western tropical South Atlantic, giving rise to cyclonic-anticyclonic motions near 12°S-6°S. Here we (1) integrate recent work on the tropical Atlantic, (2) examine several types of observational and numerical data, and (3) use an idealized wind-driven model for the surface ocean, in order to stress the relevance of the ITCZ motion on both the surface circulation patterns and the net latitudinal transport in the tropical and subtropical Atlantic.