ON THE ORIGINS OF DEEP AND BOTTOM WATERS OF THE INDIAN-OCEAN

The characteristics of the deep and bottom waters of the Indian Ocean, when illustrated on potential-density anomaly surfaces, indicate that the waters enter from both the Atlantic and Pacific Oceans. The paths of spreading are constrained by the complex topography, and characteristics are seen to be altered by exchange with the overlying and underlying water and with the sediments, especially in the northern Indian Ocean. The Weddell Sea contributes to the densest waters found in the western basins and the Ross Sea and Adelie coast to the densest waters found in the eastern basins. Both dense water varieties are altered by and incorporated in the less dense water above; initially, water carried by the circumpolar current, then water from the north Atlantic, and finally by deep water whose characteristics are derived in the northern Indian Ocean. Contact with the sediments increases the silica content of the bottom water in the Southern Ocean. In the northern Indian Ocean the sediments alter the silica of the water at the bottom and, together with enhanced salinity from diffusion of saline overflows from the marginal seas above, imprint unique markers to the deep water that flows back to the south. At middepths the series of ridges between Madagascar and Australia confine the flow to a series of gyres that carry characteristics from the circumpolar current equatorward and the northern Indian Ocean characteristics southward. Within the citcumpolar current, low-oxygen deep water from the Pacific is carried across the Atlantic and into the Indian Ocean south of Africa. Part flows around the cyclonic Weddell Sea Gyre, and part extends across the Southern Ocean. Water from another Pacific source can be seen near 2000 m extending westward from the Tasman Sea, south of Australia and across the Indian Ocean, and perhaps to the Aghulas Current region southeast of Africa.