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 s ilica. Consideration of the physical mechanisms of silica transport at 32 degrees S in conjunction with the silica budget of the Indian Ocea n basin requires modification of the geostrophic reference levels. The resulting meridional overturning circulation composed of northward fl owing 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 lowe r deep water is converted to intermediate and upper deep water, which exits the basin as southward flow across 32 degrees S. Less than one-t hird of the northward flowing deep waters upwells into the thermocline . Though the magnitude of the overturning circulation is reduced compa red 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 r emains large compared with estimates for the deep Pacific basin. Since the meridional circulation is a major conveyor of heat and salinity i n the Indian Ocean basin, a constraint on the magnitude of the meridio nal circulation provides bounds on the net heat and freshwater budgets of the region. The divergence of heat across the Indian Ocean basin n orth of 32 degrees S is estimated to be 0.42 +/- 0.19 Petawatts and th e convergence of freshwater to be 0.31 +/- 0.09 x 109 kg s(-1). Both o f these values are consistent with independent estimates, but reduced compared with calculations that do not consider the dissolved silica b udget. (C) 1997 Elsevier Science Ltd.