Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

Abstract

Surface drifters and subsurface floats drifting at depths near 800 m were used to study the pathways of warm salty Indian Ocean water leaking into the South Atlantic that is a component of the upper limb of the Atlantic meridional overturning circulation. Four drifters and 5 floats drifted from the Agulhas Current directly into the Benguela Current. Others looped for various amounts of time in Agulhas rings and cyclones, which translated westward into the Atlantic contributing a large part of Indian Ocean leakage. Agulhas rings translated into the Benguela Current where they slowly decayed. Some large blob-like Agulhas rings with irregular shapes were found in the southeastern Cape Basin. Drifter trajectories suggest these rings become more circular with time eventually evolving into the circular rings observed west of the Walvis Ridge. Agulhas cyclones, which form on the north side of the Agulhas south of Africa, translated southwestward (to 6°E) and contributed water to the southern Cape Basin. A new discovery is a westward extension from the mean Agulhas retroflection measured by westward drifting floats near 41ºS out to at least 5ºW with some floats as far west as 25ºW. The Agulhas extension appears to split the South Atlantic Current into two branches and to transport Agulhas water westward where it is mixed and blended with eastward-flowing water from the western Atlantic. The blended mixture flows northeastward in the northern branch of the South Atlantic Current and into the Benguela Current. Agulhas leakage transport was estimated from drifters and floats to be at least 15 Sv in the upper 1,000 m, which is equivalent to the transport of the upper layer meridional overturning circulation. It is suggested that the major component of the upper layer overturning circulation in the Atlantic is Agulhas leakage.