SEASONAL-VARIATIONS OF THERMOCLINE CIRCULATION AND VENTILATION IN THEINDIAN-OCEAN

Two seasonal hydrographic data sets, including temperature, salinity, dissolved oxygen, and nutrients, are used in a mixing model which comb ines cluster analysis with optimum multiparameter analysis to determin e the spreading and mixing of the thermocline waters in the Indian Oce an. The mixing model comprises a system of four major source water mas ses, which were identified in the thermocline through cluster analysis . They are Indian Central Water (ICW), North Indian Central Water (NIC W) interpreted as aged ICW, Australasian Mediterranean Water (AAMW), a nd Red Sea Water (RSW)/Persian Gulf Water (PGW). The mixing ratios of these water masses are quantified and mapped on four isopycnal surface s which span the thermocline from 150 to 600 m in the northern Indian Ocean, on two meridional sections along 60 degrees E and 90 degrees E, and on two zonal sections along 10 degrees S and 6 degrees N. The mix ing ratios and pathways of the thermocline water masses show large sea sonal variations, particularly in the upper 400-500 m of the thermocli ne. The most prominent signal of seasonal variation occurs in the Soma li Current, the western boundary current, which appears only during th e SW (summer) monsoon. The northward spreading of ICW into the equator ial and northern Indian Ocean is by way of the Somali Current centered at 300-400 m on the sigma(theta) = 26.7 isopycnal surface during the summer monsoon and of the Equatorial Countercurrent during the NE (win ter) monsoon. More ICW carried into the northern Indian Ocean during t he summer monsoon is seen clearly in the zonal section along 6 degrees N. NICW spreads southward through the western Indian Ocean and is str onger during the winter monsoon. AAMW appears in both seasons but is s lightly stronger during the summer in the upper thermocline. The westw ard flow of AAMW is by way of the South Equatorial Current and slightl y bends to the north on the sigma(theta) = 26.7 isopycnal surface duri ng the summer monsoon, indicative of its contribution to the western b oundary current. Outflow of RSW/PGW seems effectively blocked by the c ontinuation of strong northward jet of the Somali Current along the we stern Arabian Sea during the summer, giving a rather small contributio n of only up to 20% in the Arabian Sea. A schematic summer and winter thermocline circulation emerges from this study. Both hydrography and water - mass mixing ratios suggest that the contribution of the water from the South Indian Ocean and from the Indo-Pacific throughflow cont rols the circulation and ventilation in the western boundary region du ring the summer. However, during the winter the water is carried into the eastern boundary by the Equatorial Countercurrent and leaks into t he eastern Bay of Bengal, from where the water is advected into the no rthwestern Indian Ocean by the North Equatorial Current. The so-called East Madagascar Current as a southward flow occurs only during the su mmer, as is suggested by both hydrography and water-mass mixing patter ns from this paper. During the winter (austral summer) the current see ms reversal to a northward flow along east of Madagascar, somewhat sym metrical to the Somali Current in the north.