INTERMEDIATE WATER CIRCULATION AND VENTILATION OF THE INDIAN-OCEAN DERIVED FROM WATER-MASS CONTRIBUTIONS

The intermediate water circulation and ventilation of the Indian Ocean is somewhat unique among the world oceans (in terms of the source wat ers). This has been studied with historical and recently obtained hydr ographic data including potential temperature, salinity, dissolved oxy gen, phosphate and silicate in a mixing model of applying optimum mult iparameter analysis (OMP). The mixing model comprises three source wat er masses, Antarctic Intermediate Water (AAIW) (applied the transforme d AAIW north of the Antarctic frontal zone and central South Indian Oc ean), Indonesian Intermediate Water (IIW) and Red Sea Intermediate Wat er (RSIW) (including the influence of Persian Gulf Intermediate Water) . A possible source from south of Australia has also been considered a nd accommodated into the water type definition of AAIW. This study was performed on six closely spaced neutral density surfaces which encomp ass the intermediate layer of the Indian Ocean from 500 m (in the nort hern Indian Ocean) to 1500 m (in the subtropical latitudes) with a dis tance of about 100-150 m between a pair of surfaces. Water-mass mixing contributions were plotted on the neutral surfaces and in three cross sections, the western Indian Ocean along 60E, the eastern Indian Ocea n along 90E, and a zonal section along 10S. The intermediate water cir culation and ventilation of the Indian Ocean can thus be inferred from the spreading paths and mixing patterns of these source water masses. A schematic intermediate water circulation of the Indian Ocean theref ore emerges from the water-mass and dynamical information. The latter is derived from the acceleration potential (10 m(2) s(-2)) mapped on t he neutral surfaces. The equatorward AAIW enters the Indian Ocean from the mid-ocean of the southern Indian Ocean, is advected with the subt ropical gyre and transits to the north through the western boundary. I n the western equatorial Indian Ocean, AAIW flows northeastward to eas tward. At about 80E, AAIW bifurcates into northward and southward flow s. The former continues into the Bay of Bengal through the western bou ndary (east of Sri Lanka) with up to 10% of the contribution. It retur ns southward in the eastern Bay of Bengal and along the Sumatra and Ja va Islands, zonally westward with IIW. The latter recirculates southwa rd and then westward, forming a cyclonic gyre. The AAIW then turns sou thward into the Agulhas Current system through either side of Madagasc ar. AAIW contributes about 10-20% of its water into the equatorial Ind ian Ocean. Its northward flow in the western Indian Ocean is limited t o 5N. IIW flows zonally westward and bifurcates into a northward and a southward how in the western Indian Ocean. The direction of the latte r is southward into the Agulhas Current system through either side of Madagascar. The former flows northward by the way of AAIW. Although AA IW does not flow into the Arabian Sea, IIW is found flowing into the A rabian Sea via the west coast of India. The main how path of IIW into the Bay of Bengal is through the south of Sri Lanka. IIW largely contr ibutes about 50-60% of its water into the Bay of Bengal. The northward flow of IIW is interrupted at the central equatorial region by the ea stward AAIW. These circulations form two cyclonic gyres in the western and eastern equatorial Indian Ocean. The latter gyre straddles the eq uator in the eastern Indian Ocean. RSIW flows to the southern Indian O cean through both the western and eastern boundaries with much stronge r flow in the west. The western boundary flow directly feeds the Agulh as Current and the Agulhas Return Current.