The shallow overturning circulation of the Indian Ocean

The Indian Ocean differs from the other two large oceans in not possessing an annual-mean equatorial upwelling regime. While the subtropical cells (STCs) of the Atlantic and Pacific Oceans connect subtropical subduction regimes with tropical upwelling via equatorward thermocline flows and coastal undercurrents, much of the upwelling in the Indian Ocean occurs in the coastal regimes of the northern hemisphere. Consequently, the counterpart of the STCs of the other oceans has to be a cross-equatorial cell connecting the southern subtropical subduction zone via the Somali Current with the upwelling areas off Somalia and Oman. The southward return flow is by interior Ekman transports. This annual-mean picture is accomplished by a dominance of the summer monsoon, during which only northern upwelling occurs, over the winter monsoon. Pathways of the thermocline flows related to the shallow overturning circulations are investigated here and estimates of subduction and upwelling are presented. From the observed mean northward flow of thermocline waters within the Somali Current and the interior southward cross-equatorial return flow the magnitude of the cross-equatorial cell is estimated at 6 Sv, with part of the thermocline waters being supplied by the Indonesian Throughflow. From observations we estimate that the northern upwelling occurs dominantly through the offshore outflows of the Somali Current by the Southern Gyre and Great Whirl and to a lesser degree off Oman. However, we also present model results suggesting a much lower role of Somali upwelling and a significant contribution from open-ocean upwelling in cyclonic domes around India and Sri Lanka. An interesting aspect of the Indian Ocean cross-equatorial cell is the mechanism by which the Ekman transport crosses the equator. Typically, Ekman transports during the summer (winter) monsoon are southward (northward) on both sides of the equator, while mean meridional winds on the equator are in the respective opposite direction. Earlier model evidence had suggested that this type of forcing should lead to an equatorial roll with northward surface flow and southward subsurface flow during the summer monsoon and reverse orientation during the winter monsoon. Observational evidence is presented here, based on shipboard ADCP sections, moored stations and surface drifters, confirming the existence of the equatorial roll. It is strongly developed in the western Indian Ocean during the SW monsoon where the wind conditions for the roll are best met. While in the central Indian Ocean and during the winter monsoon the roll appears to be a more transient phenomenon, superimposed by equatorial-wave currents. The evidence further suggests that the roll is mostly confined to the surface-mixed layer and is, therefore, of little consequence for the meridional heat transport.

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