Following the fate of the East Madagascar Current G. D. QUARTLY and M.A. SROKOSZ Southampton Oceanography Centre, Empress Dock, Southampton, Hants, UK, e-mail: gdq / mas @soc.soton.ac.uk ABSTRACT The current regime to the south of Madagascar is a particularly interesting region. There have been various observations from buoys and satellites that suggest that the poleward-flowing East Madagascar Current undergoes a retroflection to the south of the island. The complex current pattern is believed to shed eddies, like the Agulhas Retroflection does further south. Here we examine SeaWiFS data for the region as the current patterns are often marked by sharp fronts between waters of different chlorophyll concentration (CC). Along the south coast of the island there is significant upwelling, which is marked by low temperatures and high chlorophyll concentrations. The CC is lowest during summer, but the upwelling region is clearly delimited throughout the year. The boundary of the upwelled region may remain fixed for more than a month, but at other times can change within the course of a week. 'Blobs' of high CC shed from this region appear to move in a variety of directions. Eddy-like features are also found further north in the Mozambique Channel, where they are highlighted by the swirls of high CC water drawn around them from coastal regions. KEYWORDS — East Madagascar Current, Agulhas, SeaWiFS, chlorophyll, retroflection, eddies, ocean colour fronts 1. INTRODUCTION The poleward-flowing East Madagascar Current (EMC) is ~100 km wide 1 , with a core speed of 0.9 m s -1 . Ship-drift data suggest that the flow is fastest in October (spring) and slowest in February (summer) 2 . Upon reaching the southern tip of Madagascar it proceeds in a southwesterly direction. Cooler nutrient-rich water is upwelled on the landward-side of the current, both at the southern end of the east coast and the whole region to the south of the island. Lutjeharms and Machu 3 have shown that the current itself can cause some upwelling in the absence of wind, but favourable winds can enhance the amount of upwelling. The waters brought to the surface are characterised by low temperatures and high nutrient content, allowing phytoplankton growth and leading to high chlorophyll concentrations (CC). These contrast markedly with the waters of the EMC which are warmer and contain less chlorophyll than the neighbouring water masses. This means that ocean colour data may be used, with caution, in the monitoring of changes in current flow. 2. SOURCE AND PROCESSING OF DATA For this study we use chlorophyll concentrations (in mg m -3 ) from the SeaWiFS sensor, which are provided by NASA/GSFC as daily files gridded at 18 km resolution. We composite the data in 6-day periods (hexads), and apply a spatial smoother to remove the finest scale features (which are not of interest in this study) and to overcome problems of persistent cloud, since frequent 'missing data' values would make subsequent processing more difficult. The subsequent analysis is in terms of log 10 (CC) since CC values tend to be log-normally distributed 4 and we wish to display changes over the entire dynamic range. Figure 1a is an example of the composites produced for the full region of interest. Typically there are higher values in the north than the south, and there are particularly high CC values in certain coastal regions e.g. Toolamaro (Fort Dauphin), Maputo and Durban. For more detailed studies we focus on the region just to the south of Madagascar (Fig. 1b). There are strong seasonal changes in this region, with peaks around July and February (Fig. 2). The former corresponds to an early spring bloom; the