Abstract

The intermediate water circulation and ventilation of the Indian Ocean is somewhat unique among the world oceans (in terms of the source waters). This has been studied with historical and recently obtained hydrographic data including potential temperature, salinity, dissolved oxygen, phosphate and silicate in a mixing model of applying optimum multiparameter analysis (OMP). The mixing model comprises three source water masses, Antarctic Intermediate Water (AAIW) (applied the transformed AAIW north of the Antarctic frontal zone and central South Indian Ocean), Indonesian Intermediate Water (IIW) and Red Sea Intermediate Water (RSIW) (including the influence of Persian Gulf Intermediate Water). A possible source from south of Australia has also been considered and accommodated into the water type definition of AAIW. This study was performed on six closely spaced neutral density surfaces which encompass the intermediate layer of the Indian Ocean from 500 m (in the northern Indian Ocean) to 1500 m (in the subtropical latitudes) with a distance 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 Ocean along 90E, and a zonal section along 10S. The intermediate water circulation 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 therefore emerges from the water-mass and dynamical information. The latter is derived from the acceleration potential (10 m2 s-2) mapped on the neutral surfaces. The equatorward AAIW enters the Indian Ocean from the mid-ocean of the southern Indian Ocean, is advected with the subtropical gyre and transits to the north through the western boundary. In the western equatorial Indian Ocean, AAIW flows northeastward to eastward. At about 80E, AAIW bifurcates into northward and southward flows. The former continues into the Bay of Bengal through the western boundary (east of Sri Lanka) with up to 10% of the contribution. It returns southward in the eastern Bay of Bengal and along the Sumatra and Java Islands, zonally westward with IIW. The latter recirculates southward and then westward, forming a cyclonic gyre. The AAIW then turns southward into the Agulhas Current system through either side of Madagascar. AAIW contributes about 10-20% of its water into the equatorial Indian Ocean. Its northward flow in the western Indian Ocean is limited to 5N. IIW flows zonally westward and bifurcates into a northward and a southward flow in the western Indian Ocean. The direction of the latter is southward into the Agulhas Current system through either side of Madagascar. The former flows northward by the way of AAIW. Although AAIW does not flow into the Arabian Sea, IIW is found flowing into the Arabian Sea via the west coast of India. The main flow path of IIW into the Bay of Bengal is through the south of Sri Lanka. IIW largely contributes 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 eastward AAIW. These circulations form two cyclonic gyres in the western and eastern equatorial Indian Ocean. The latter gyre straddles the equator in the eastern Indian Ocean. RSIW flows to the southern Indian Ocean through both the western and eastern boundaries with much stronger flow in the west. The western boundary flow directly feeds the Agulhas Current and the Agulhas Return Current.

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