Lagrangian observations in the Intermediate Western Boundary Current of the South Atlantic

Subsurface float measurements at 800 m depth carried out from 1994 to 2003 in the Brazil Basin are used to characterise the equatorward Intermediate Western Boundary Current (IWBC) and its connections to the ocean interior. Transversally, the boundary flow is less than 100 km wide, and most intense at 10–20 km from the 800 m isobath. Its average velocities range from ∼0.1 ms−1 to 0.3 ms−1 depending on latitude, with individual daily values as high as 0.7 ms−1. The flow meridional extent exhibits 3 contrasted domains: (i) from 27°S to the Vitoria–Trindade Ridge at 20°30′S, the boundary flow intensifies northward along a relatively smooth topography. A counter current adjacent to it on its seaward side feeds it with intermediate water from the northern limb of the subtropical gyre. (ii) At latitudes 20–15°S characterised by a very irregular topography, the IWBC becomes weaker with even no real proof of its presence at 18–15°S. An intense mesoscale variability prevails there, which apparently takes over from the boundary flow to ensure the northward transport of water to 15°S, where the IWBC re-forms. (iii) North of this latitude, the boundary flow increases again to ∼10°S along smooth isobaths, then decreases when encountering a rougher topography and the zonal jets of the equatorial current system. A counter current present from ∼5°S to 14°S, partly fed from the boundary flow, contributes to its drainage. The IWBC shows two main input locations, at 27–23°S and 15–12°S in the southern parts of the two latitudinal domains of smooth topography where the northward current increases. Output locations coincide with major capes in the continental slope geometry, at 20°S and 18°S (the southeastern and northeastern corners of the Abrolhos Bank), at 8°S near the Recife Plateau, and at 5°S near Cape São Roque.