The dissolved silica budget as a constraint on the meridional overturning circulation of the Indian Ocean

The geostrophic circulation at 32 degrees S in the Indian Ocean, based on hydrographic data collected in 1987 on the R.R.S. Charles Darwin, is re-examined with the inclusion of the advective flux of dissolved silica. Consideration of the physical mechanisms of silica transport at 32 degrees S in conjunction with the silica budget of the Indian Ocean basin requires modification of the geostrophic reference levels. The resulting meridional overturning circulation composed of northward flowing deep waters returning south at shallower levels becomes 11.9+/-2.7 x 10(9) kg s(-1) roughly half the magnitude of a previous estimate using the same data. The bulk of the northward flowing bottom and lower deep water is converted to intermediate and upper deep water, which exits the basin as southward flow across 32 degrees S. Less than one-third of the northward flowing deep waters upwells into the thermocline. Though the magnitude of the overturning circulation is reduced compared with previous estimates with this data set, the maximum basin-mean upwelling velocity required for mass continuity, 4.5 x 10(-5) cm s(-1), is consistent with other estimates for the Indian Ocean basin and remains large compared with estimates for the deep Pacific basin. Since the meridional circulation is a major conveyor of heat and salinity in the Indian Ocean basin, a constraint on the magnitude of the meridional circulation provides bounds on the net heat and freshwater budgets of the region. The divergence of heat across the Indian Ocean basin north of 32 degrees S is estimated to be 0.42 +/- 0.19 Petawatts and the convergence of freshwater to be 0.31 +/- 0.09 x 109 kg s(-1). Both of these values are consistent with independent estimates, but reduced compared with calculations that do not consider the dissolved silica budget. (C) 1997 Elsevier Science Ltd.