On the Heat Budget of Hydrosphere and Atmosphere in the Indian Ocean

Abstract

The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N?30°S, 30?120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere. The hydrosphere to the north of the equator exports heat at rates of 5 ? 1014 W for the year as a whole, and more than 8 ? 1014 W during the northern summer (May?October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November?April and May?October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 ? 1014 W at 10?15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ?6 ? 10?7 m s?1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean. Residually determined heat export by the atmosphere north of the equator averages 18 and 4 ? 10?14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 ? 10?14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation. South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.