Impact of 4D-variational assimilation of WOCE hydrography on the meridional circulation of the Indian Ocean

World Ocean Circulation Experiment (WOCE) hydrographic sections and a sea-surface climatology are combined with a ocean general circulation model through a 4D-variational method to analyze the meridional overturning of the Indian Ocean. The regional model is run with realistic surface forcings over year 1995 for which most of WOCE Indian Ocean sections were made. The assimilation controls the initial temperature and salinity fields, surface forcings and open-boundary velocities, temperature and salinity. When no observations are assimilated, the model shows that the deep (below 1000 m) meridional overturning is weak compared to observation-based estimates. This is a common feature of general circulation models. In contrast, after the assimilation, the model develops a deep overturning of 17×106 m3 s−1 at 32°S when a 10×106 m3 s−1 Indonesian Throughflow (ITF) is prescribed. The mass flux of bottom waters that moves northward below 3200 m is balanced by a southward mass flux of deep waters between 1000 and 3200 m. The deep overturning carries 10% of the total southward energy flux of 1.2 PW at 32°S. The intensity of this deep overturning changes only by ±2×106 m3 s−1 when the annual mean ITF is zero or 30×106 m3 s−1. The upper circulation is less constrained by the assimilation because of the large temporal and spatial variability of this ocean and also because of limitations in the representation of the mixed layer physics during the assimilation process. Limitations in the physics of the model also are thought to be the source of the slow erosion of the deep overturning when the model is run for several years from its optimal state.