Agulhas Eddies: A Synoptic View Using Geosat ERM Data

Abstract

Warm core rings formed in the Agulhas Retroflection transfer water from the Indian Ocean to the South Atlantic. In an attempt to measure the strength of this exchange, a combination of satellite altimeter and hydrographic data are used to examine Agulhas eddy paths and decay rates in the South Atlantic. Because the surface dynamic height of a warm core eddy is higher than surrounding waters, the rings are visible in satellite altimeter measurements. Over 20 Agulhas eddies have been tracked from maps of anomalous sea surface height (SSH) derived from the Gcosat Exact Repeat Mission (ERM) dataset. The correlation (r2) of dynamic height referenced to 2000 dbar and anomaly SSH for one coincidentally sampled area is 97% within an Agulhas eddy, dropping to a fraction of that outside of it, indicating that the SSH anomaly signal is a reliable measure for strong features like Agulhas eddies. The sizes and distribution of the Agulhas eddies in the ERM record compare favorably with those in recent hydrographic records from the area. Individual eddy tracks from the ERM show the influence of topography, with slowed trimslation over areas of steep relief. The eddies tracked take a generally WNW course across the South Atlantic, propelled by the mean flow and internal dynamics. While propagating westward, Agulhas eddies decay in amplitude with an e-folding distance of O( 1700-3000 km) alongtrack. As tbey approach the western boundary of the South Atlantic, at 40oW, the eddies have O( 10%) of their initial amplitude remaining. This study finds the residence time of an Agulhas eddy in the South Atlantic to be 3-4 years. On average, the authors find six eddies per year form by the retroflection that enter the South Atlantic. The 20 eddies tracked therefore represent 50%-60% of the population that would have been extant during the ERM. The Agulhas eddies appear to contribute a minimum of 5 X 106 m3 5-1 to the Indian-South Atlantic water mass transfer, with a corresponding energy flux on the order of 1011 J.