Observed subseasonal variability of heat flux and the SST response of the tropical Indian Ocean

Abstract

Abstract We develop an experimental daily surface heat flux data set based on satellite observations to study subseasonal variability (periods shorter than 90 days) in the tropical Indian Ocean. We use incoming shortwave and longwave radiation from the International Satellite Cloud Climatology Project, and sea surface temperature (SST) from microwave sensors, to estimate net radiative flux. Latent and sensible heat fluxes are estimated from scatterometer winds and near‐surface air temperature and specific humidity from Atmospheric Infrared Sounder (AIRS) observations calibrated to buoy data. Seasonal biases in net heat flux are generally within 10 W m −2 of estimates from moorings, and the phases and amplitudes of subseasonal variability of heat fluxes are realistic. We find that the contribution of subseasonal changes in air‐sea humidity gradients to latent heat flux equals or exceeds the contribution of subseasonal changes in wind speed in all seasons. SST responds coherently to subseasonal oscillations of net heat flux associated with active and suppressed phases of atmospheric convection in the summer hemisphere. Thus, subseasonal SST changes are mainly forced by heat flux in the northeast Indian Ocean in northern summer, and in the 15°S–5°N latitude belt in southern summer. In the winter hemisphere, subseasonal SST changes are not a one‐dimensional response to heat flux, implying that they are mainly due to oceanic advection, entrainment, or vertical mixing. The coherent evolution of subseasonal SST variability and surface heat flux suggests active coupling between SST and large‐scale, organized tropical convection in the summer season.