SST Observations of the Agulhas and East Madagascar Retroflections by the TRMM Microwave Imager

Abstract

The retroflections of the East Madagascar Current and Agulhas Current are complex rapidly-evolving systems, the latter controlling the passage of warm salty water from the Indian Ocean to the Atlantic. The TRMM Microwave Imager (TMI) provides frequent observations of sea surface temperature through clouds, allowing us to monitor the evolution of these systems. We develop a simple feature-tracking system that obviates the need for user intervention, and use its results to guide more focussed studies. In the period 1997-1999, we observe westward progradation of the Agulhas Retroflection (associated with ring shedding) about eight times per year, agreeing with previous estimates from infra-red data, and many rings moving westward or north-westward. However, this behaviour is seen to change in the 2000-2001 time period, with the Agulhas Retroflection occurring further to the east. A few Natal pulses are seen, but cannot be linked conclusively to the spawning of rings due to TMI's limited latitudinal coverage. The majority of features originating at the East Madagascar Retroflection appear to migrate southwestwards. A new observation from the data is that, although the first northward meander of the Agulhas Return Current is constrained by bathymetry, its position does vary intermittently, remaining fixed in a given location for up to six months at a time. Southward propagation of features is noted along two ridges: although eddies have been found before along the eastern slope of the Mozambique Ridge, the new results for the Madagascar Ridge indicate an extra pathway for the eddies. Eddy-like features are also found leading from the Agulhas Return Current back toward the Agulhas Current. The narrow 'corridor' of these features suggests that it is controlled by the gyre recirculation in the southwest Indian Ocean.

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