The Role of Mesoscale Turbulence in the Agulhas Current System

Abstract

The terminal region of the Agulhas Current has for some time been recognised for its particularly intense mesoscale turbulence field. This has been evident from statistical analyses of historic hydrographic data, satellite altimetric data as well as the geographic distribution of eddy kinetic energy as measured by free-drifting weather buoys. This area of higher than average mesoscale variability is geographically extensive. Investigations in the area, making use of satellite remote sensing as well as dedicated research cruises, have identified a number of regional mechanisms involved in the generation of mesoscale vortices. Amongst these are instabilities in the Agulhas retroflection which lead to the budding off of Agulhas rings. These rings have been observed to drift into the south-east Atlantic Ocean and may dominate the heat and momentum flux of a large part of this ocean. Perturbations in the Subtropical Convergence have been observed to cause eddy formation at this front. Fluctuations in the Rossby waves on the Agulhas Return Current have been identified as being responsible for the shedding of both warm and cold eddies. This eddy spawning across the Subtropical Convergence may be held responsible for substantial meridional heal flux into the Southern Ocean. It is further conjectured that the Subantarclic Front south of Africa may in fact be maintained by a balance in the meridional flux of such warm subtropical and cold Antarctic eddies. Recent observational results have suggested that circulation features in the sub-mesoscale range may act as triggering mechanisms for some of the larger fluctuations in the flow patterns. implying that modelling of the system will have to take cognizance of features on these finer scales.