Seasonal Heat Transport in a Primitive Equations Model of the Tropical Indian Ocean

Abstract

This work analyses seasonal heat transport in an ocean-only numerical simulation of the Indian Ocean forced by realistic seasonal winds and surface heat fluxes north of 15°S, assuming no Indonesian Throughflow. The seasonal changes in the model circulation and temperature structure are found to be overall consistent with observations, despite flaws in sea surface temperature and mixed layer depth. The simulation confirms that the reversal of the monsoons and of the associated Ekman transports plays an important role in reversing the sign of the ocean heat transport seasonally causing, in particular, the Arabian Sea's drastic annual cooling, but it suggests that, south of 10°N, deep boundary currents must reverse as well. Most of the model heat transport is carried by a deep downwelling cell during the northeast monsoon and by a shallower upwelling cell during the southwest monsoon. An analysis of the three-dimensional circulation reveals that, in boreal summer, the net −1.2 pW (1 pW = 1015 W) cross-equatorial model heat transport derives mostly from a 20 × 106 m3 s−1 northward boundary current at intermediate levels (12.5°C) returned over the interior at the surface (27.5°C). In boreal winter, the net +1 pW heat transport derives mostly from a 10 × 106 m3 s−1 northward interior surface flow (27.5°C) returned in several deep southward boundary currents (5°C). It is argued that the +1 pW February heat transport value is realistic and that a deep overturning cell must therefore exist, otherwise the return branch of the relatively small February Ekman transport would have to occur at a negative transport-averaged temperature. Moreover, deep downwelling during the northeast monsoon occurs in the model because of a pattern of flow convergence at intermediate levels of the Somali Current that is consistent with direct observations. An Approach toward assessing the location and the role of diabatic processes (which could be responsible for too deep a penetration of the downwelling cell) is tested, and a formal decomposition of the seasonal heat transport into diabatic and adiabatic components is suggested. Representing as a function of latitude and potential temperature an equivalent streamfunction associated with diffusion appears a promising step toward quantifying such diabatic heat transports a seasonal basis.