Inferring meridional mass and heat transports of the Indian Ocean by fitting a general circulation model to climatological data

The meridional overturning and heat transport of the Indian Ocean are studied by fitting the steady state dynamics of a general circulation model (GCM) to climatological annual mean temperatures, salinities, and surface forcings using the GCM and its adjoint. By estimating target temperatures and salinities near the artificially closed side boundaries as part of the optimization procedure, a steady solution that is consistent with climatological data within limits of observational errors is found. The resultant meridional overturning is vigorous (14 Sv; 1 Sv = 10(6) m(3)/s) only in the upper 1000 m. The estimated deep inflow entering the Indian Ocean from the south is weak. Requiring a large net northward inflow at depth does not result in strong inferior upwelling but leads to unrealistically large baroclinic mass exchange and implied vertical mixing near the Indonesian throughflow region. The shallow overturning is the main carrier of the southward heat transport, which has a maximum of 0.8 PW (1 PW = 10(15) W) near 15 degrees S. Wind forcing plays a key role in driving the estimated overturning of the Indian Ocean. This result is in disagreement with previous interpretations about the central role of surface heat flux in driving a vigorous deep overturning of the Indian Ocean.