THE MONSOONAL HEAT-BUDGET OF THE HYDROSPHERE-ATMOSPHERE SYSTEM IN THE INDIAN-OCEAN SECTOR

The heat budget of the combined hydrosphere-atmosphere system in the Indian Ocean sector (30-degrees-N to 30-degrees-S, 30-degrees-120-degrees-E) is studied on the basis of surface ship observations and subsurface temperature casts to 400 dbar, analyses of the atmospheric heat and moisture transport in the layer surface to 100 mbar, published estimates of precipitation, and satellite measurements of net all-wave radiation at the top of the atmosphere. During boreal winter (November-April) the hydrosphere of the southern tropical Indian Ocean stores (+2 x 10(14) W) and exports (7 x 10(14) W) heat, of which about half (3 x 10(14) W) is carried northward across the equator; of similar magnitude (9 x 10(14) W) is the heat export in the atmosphere accomplished primarily in the upper tropospheric westerlies. In the northern portion of the basin the hydrosphere stores heat (+8 x 10(14) W), in part supplied by the hydrospheric heat import from the southern hemisphere; the atmospheric column exports heat (4 x 10(14) W), primarily owing to a southward moisture transport. In boreal summer (May-October) the water body of the northern Indian Ocean cools drastically (-8 x 10(14) W), with the net heat gain through the ocean surface being overcompensated by the larger (12 x 10(14) W) southward heat transport across the equator, which in tum results in a large heat import (8 x 10(14) W) to the hydrosphere of the southern Indian Ocean. This and the depletion of the oceanic heat content (-2 x 10(14) W) supply about a third of the energy required for the vigorous evaporation from the southern tropical Indian Ocean (34 x 10(14) W), which is further favored by the strong southeast trade winds peaking at this season of the year. The evaporation sustains the large latent heat export from the overlying atmospheric column (+14 x 10(14) W). The latent heat surplus from the southern portion of the basin is primarily carried northward across the equator by the lower atmospheric boreal summer monsoon airflow, and in the June-August core of the season this latent heat flux becomes as large as the southward cross-equatorial heat transport in the ocean (13 x 10(14) W). Associated with the summer monsoon airflow, latent heat import takes place in the northern portion of the basin (-2 x 10(14) W) and southern Asia (-6 x 10(14) W). In context, hydrosphere and atmosphere cooperate in the energetics of the Indian summer monsoon. The water body to the north of the equator cools (despite the summer season) and exports heat to the southern Indian Ocean, where this energy helps sustain the large evaporation associated with the strong winds, which further carry the moisture northward to supply the abundant monsoon rainfall and latent heat release that fuels the atmospheric portion of the monsoon machine. It is during boreal winter that the northern Indian Ocean water body replenishes its heat content, in preparation for the next summer monsoon.