Ocean-atmosphere coupling and the regulation of the South Asian monsoon

Of all of the monsoon regimes, the monsoon of South Asia is the strongest and influences a larger proportion of the population than any other climate system. At the same time, the South Asian monsoon is unique because it is the most regular with a standard deviation in total monsoon rainfall that is far smaller than other monsoon regimes. The question of how the South Asian monsoon is regulated from year-to-year is addressed.

It is argued that the regulation process which damps out year-to-year variations in precipitation is the result of strong ocean-atmosphere coupling. Anomalies in SST in the North Indian Ocean (which are positively correlated to a strong monsoon) drive stronger than average southward Ekman ocean heat transports. Conversely, a cool SST produces weaker southward transports. In this sense, the wind driven ocean heat transports modulate the strength of the monsoon on a biennial time scale. Coupled to the modulation of the seasonal monsoon is an oscillation in the strength of upwelling in the eastern and western Indian Ocean associated with the anomalous monsoonal wind gyre producing anomalous east-west SST gradients producing alternatively the positive and negative phases of the Indian Ocean dipole. It is concluded that the biennial oscillation of the monsoon and the Indian Ocean dipole are results of the coupled ocean-atmosphere regulation of the South Asian monsoon.