The Indian Ocean general circulation is estimated by fitting the MIT Ocean
General Circulation Model to the annual mean climatological hydrography and
surface forcing, using the model and its computer-generated adjoint. Open
boundary conditions are implemented to the west of the Indonesian Archipela
go and near 30S. The approach simultaneously optimizes the initial conditio
ns of the hydrographic fields, surface fluxes, and the open boundary condit
ions (temperature, salinity, and horizontal velocities).
Compared to previous results obtained in a closed domain, the estimated vel
ocity field shows a marked improvement near the southern boundary, with a r
easonably strong Agulhas Current leaving the model domain. The Indonesian t
hroughflow (ITF) is estimated as 2.7 Sv (1 Sv is 10(6) m(3)/s) westward, wh
ich is on the low end of the range of previous estimates. The model is able
to sharpen fronts in surface salinity, compared to climatology, and sugges
ts that the low surface salinity values in the eastern equatorial region ar
ise from advection out of the Bay of Bengal rather than from the ITF.
Consistent with the closed-domain results, the meridional overturning is do
minated by a shallow (above 500 m), wind-driven cell of 16 Sv maximum, whic
h carries the bulk of the southward heat transport. We have defined general
izations of meridional heat and freshwater transports appropriate in the pr
esence of a throughflow. The estimated meridional heat transport has a maxi
mum of 0.8 PW at 12S, maximum freshwater transport is 0.29 Sv southward at
9S. The meridional transport divergences are well balanced by the surface h
eat and freshwater fluxes, indicating near-steady state and small influence
of the ITF.