SST warming trend in the Indian Ocean: what causes it?

Basin-wide sea surface temperature (SST) in the Indian Ocean has been increasing steadily since early 1980s, the period beginning with satellite observations of SST. Over the past twenty years the sea surface in some regions has been warmed up by more than 0.5°C. Since SST is an important driver for climate change and variability, finding the mechanism that causes the SST warming trend has important implication for projecting long-term climate changes and for understanding the role of the Indian Ocean region as a primary heat source for the global circulation.

On the annual mean basis the Indian Ocean north of 15°S is a heat gain region; and the heat is transported southward by Ekman current during the summer monsoon. Hence, two scenarios can be used to explain the SST warming trend. One is based on surface energy flux theory, in which the Indian Ocean has been gaining more heat from the atmosphere so that the ocean temperature is being heated up. The other scenario is based on ocean transport theory, in which the southward heat transport has been slowed down so that less heat is being removed from the tropical Indian Ocean. However, the study found that the warming trend could not be explained by either of the theories based on newly developed heat flux products and satellite wind observations. The data suggested that the net heat gain in the Indian Ocean has been reducing due to the increase of oceanic latent heat loss; and that the southward heat transport has been increasing due to the strengthening of near surface atmospheric circulation. It is found that the SST upward trend appears to be caused by thinning the mixed layer. Mechanisms contributing to the mixed layer depth change and SST warming are discussed.