Abstract
Chemical and hydrographic tracer data obtained during the GEOSECS and WOCE expeditions along with historical hydrographic data are used to examine the deep overturning circulation in the Indian Ocean. The mean upwelling transport of bottom water in the Indian Ocean north of 30°S is estimated using the GEOSECS Indian Ocean radiocarbon and carbonate chemistry data set. The study uses an "adjusted radiocarbon concentration" which is corrected for the effects of addition of particulate radiocarbon to the deep ocean. Vertical transfer processes are estimated using horizontally averaged concentration and fluxes. The resulting total upwelling flux, north of 30°S, is 8.2 +/- 1.5 x 106 m3s--1 . The estimated upwelling rate is roughly three times the upwelling rate diagnosed for the Pacific Ocean. The results from this analysis are combined with the results from a simple horizontal advection-diffusion model used to interpret the meridional salinity distribution in the deep basins of the Indian Ocean. It is found about 40% of the inflowing Circumpolar Deep Water is exported southward as North Indian Deep Water and the remaining 60% is converted into Intermediate waters. The southward export of North Indian Deep Water appears to be topographically controlled and takes place mainly in the Madagascar and Central Indian Basins.Tracer data from WOCE cruises I7N, I9N, I8 and I1 indicate (1) A deep boundary current along the northern flank of the Carlsberg Ridge supplying bottom water to the Arabian Basin during the Southwest monsoon period of 1995; (2) a cyclonic deep circulation in the Bay of Bengal; (3) input of nutrients and silica to the deep Arabian Sea and Bay of Bengal and (4) the presence of previously unidentified sources of mantle helium in the northeastern Indian Ocean. Based on these data, the helium degassing flux from the Indian Ocean is estimated as 0.6 x 1010 atms.m--2 s--1 and 5.8 x 104 atms.m--2s--1 for 4He and 3He, respectively.