The Exchange of Water Between the South Indian and South Atlantic Oceans

Abstract

The circulation at mid-latitudes of the South Atlantic Ocean accommodates a substantial, though intermittent, leakage of tropical and subtropical water from the South Indian Ocean past the southern tip of Africa. This transferral of water masses is for the most part, but not exclusively, due to large Agulhas rings that are shed at the Agulhas retroflection. Direct transfer in the form of filaments also occurs. These inter-basin exchanges of waters influence global climate, local ocean dynamics, biogeographic patterns and possibly even local fish recruitment.

Agulhas rings are major mesoscale features that average 250 km in diameter and extend to more than 1000 m depth. They drift into the South Atlantic at rates of about 7 km/day (8 cm/s). An estimated 6 to 9 are formed each year with a range of dimensions. Recent studies have estimated the total inter-basin heat flux due to Agulhas rings as being 0.05 PW per annum, and the salt flux as 78 x 10¹² kg per annum.

A secondary, and minor, exchange between the ocean basins is due to Agulhas filaments. These are detached from the landward edge of the southern Agulhas Current and are then advected into the South Atlantic by the Benguela drift. These features are only about 50 m deep on average, lose most of their heat to the atmosphere, but contribute a salt flux that is estimated to be 29 x 10¹² kg per annum, or about 12% of that due to Agulhas rings.

Agulhas rings are formed through complex processes. Perturbations in the flow paths of the Agulhas Return Current, and that of the Agulhas Current proper, cause the opposing flows to coalesce and thus to pinch off a ring. In this process quantities of cold, Subantarctic Surface Water are forced into the South Atlantic, further complicating the array of water types already present. The driving forces that bring about perturbations in the Agulhas system are poorly understood. Meanders in the southern Agulhas Current are similar to those observed in other western boundary currents, but are, by comparison, small. A large soliton meander on the current, the Natal Pulse, occurs at irregular intervals, about 8 times per year. Recent analyses suggest that the passage of each Natal Pulse is followed by the spawning of an Agulhas ring. The triggering of a Natal Pulse occurs far upstream in the Natal Bight and may be caused by the impingement of deep-sea eddies on the Agulhas Current, or by fluctuations in the velocity of the current leading to barotropic instabilities.