VARIABILITY OF WATER MASSES IN THE WESTERN INDIAN-OCEAN

In 1987, conductivity-temperature-depth and current data were collecte d in two oceanographic surveys carried out around the Indian Ocean cor al atolls of Aldabra (45 degrees 20' E, 9 degrees 25' S) and Cosmoledo (47 degrees 30' E, 9 degrees 45' S) between April and July. During th e surveys, significant changes were observed in the temperature and sa linity of the upper layers. From temperature-salinity analysis, the do minant water masses in the upper layers (0-400 m) were categorized and their origin determined. During the first cruise, TS characteristics changed only slightly. Low salinity waters (S = 34.5-34.6 and T = 28 d egrees-30 degrees C), brought from the east by the South Equatorial Cu rrent, were dominant in the surface layers. During the second cruise, the surface waters were saltier and cooler (S = 35.1 and T = 25 degree s-26 degrees C) and more homogeneous in the upper 200 m as a result of a greater admixture of a southern water mass. Seasonal cooling at thi s time was responsible for a deeper surface mixed layer, Near the end of the cruise, TS conditions changed suddenly due to an invasion of so uthern subtropical water masses in the layers above 200 m. Current met er records were dominated by the regional 50-day oscillation and highe r-frequency variability and did not show the expected increase in stre ngth of the South Equatorial Current to indicate onset of the southwes t monsoon. The water mass changes were not correlated with changes in the local currents and were probably caused by advection of patches by the large-scale flow.