Influence of the annual cycle of sea surface temperature on the monsoon onset

Differing from the traditional focus on land-sea thermal contrasts, this paper examines the influence of the annual cycle of sea surface temperature (SST) on the monsoon onset. It is found that SST is a major driver of tropical circulations in the atmospheric boundary layer. In the trade wind regions, the annual cycle of SST involves a shift in the warmest SST axis (WSSTA) between two local maxima on either sides of the equator or slight movement of WSSTA north of the equator; consequently, WSSTA has little effect on the regional meridional SST gradient and wind direction. However, in the monsoon regions, the annual cycle of SST is characterized by an abrupt northward jump of WSSTA, resulting in a marked change in regional meridional SST gradient and consequent onset of winds. The onset of the southwesterlies lags behind the abrupt northward jump of WSSTA. As an example, the climatological onset of the monsoon in the Bay of Bengal (BOB) shows that the abrupt northward jump of WSSTA is caused by an increase in SST in the BOB, leading to the monsoon onset after about 2 pentads. SST shows a maximum before the monsoon onset, contributing to destabilization of the atmosphere. A strong meridional SST gradient occurs between BOB and south of the equator after the abrupt northward jump of WSSTA, associated with cross-equatorial flow in the eastern Indian Ocean. Case studies indicate that an abrupt northward jump in WSSTA from the equator to north of 10 degrees N is a good omen for the monsoon onset.