Interaction of Agulhas filaments with mesoscale turbulence: a case study

Introduction Heat and salt are distributed and circulated throughout the world’s oceans via the global thermohaline circulation. The oceans adjacent to southern Africa form an important component of this circulation. The Agulhas Current is the western boundary current of the South Indian Ocean and acts as a link between the Indian and Atlantic oceans in the following way. South of Africa the current retroflects abruptly and about 90% of its water mass transport is returned to the South Indian Ocean. Shedding of huge Agulhas rings at the retroflection and the advection of filaments—from the upper layers of the landward border of the current—account for the remaining 10% of Agulhas Current water mass transport. These Agulhas Current products leak into the South Atlantic Ocean; the annual contribution to inter-basin salt exchange of filaments being about 15% of that of Agulhas rings. The South Atlantic is unique as an ocean basin since it is the only one in which there is a net equatorward heat transport. This may be largely controlled by the process of inter-ocean exchange south of Africa. In fact, modelling studies indicate that the transfer of water between the Atlantic and Indian oceans controls the rate of thermohaline overturning of the whole Atlantic Ocean. The fate of Agulhas rings, as well as the mechanisms by which they distribute their heat and salt content into the Atlantic Ocean, have been subjected to intense scientific scrutiny, e.g. in the Mixing of Agulhas Rings Experiment (MARE) and in the Cape of Good Hope Experiments (KAPEX). Investigations by Schouten et al. have demonstrated, for instance, that rings may dissipate very rapidly in the Cape Basin, west of Cape Town. Persistent ring–eddy interactions between Agulhas rings and smaller cyclonic eddies suggest one possible mechanism by which the physical properties of an Agulhas ring may rapidly be eroded. The Cape Basin therefore plays an integral role as a region of mixing and stirring in the Indian–Atlantic inter-ocean exchange. The flux of salt by the other leakage component, Agulhas filaments, has been estimated as making a much smaller contribution to inter-ocean leakage than that of Agulhas rings. The mixing processes undergone by these features have therefore been considered of lesser importance and have not been studied as yet. Early work has suggested that they follow the coastal upwelling front along the southwestern coast of South Africa, thereby intensifying this front. Fortuitous observations at sea have now made it possible to make a first contribution to an understanding of the mixing processes of Agulhas filaments.