A Mechanism of the Onset of the South Asian Summer Monsoon

Abstract

Using the European Centre for Medium-Range Weather Forecasts reanalysis data, we examine whether an onset mechanism of the Australian summer monsoon proposed by Kawamura et al., which incorporates possible air-sea feedback processes, can apply to the South Asian summer monsoon system as well. In their mechanism, a combination of the increase in sea surface temperatures and dry intrusion into the layer at 600-850 hPa level over the ocean off, the equatorial side of a continent, plays a crucial role in enhancing potentially convective instability prior to the onset. It is found that the onset mechanism is able to apply to the abrupt onset of the Indian summer monsoon at the beginning of June, rather than the earliest onset over the Indochina peninsula in middle May. This is consistent with the observational fact that abruptness of the monsoon onset is most evident over the coastal regions of the Indian subcontinent. The Indochina peninsula is characterized by a relatively slow onset, although its date is earliest. The asymmetry of the transition speed between the onset and retreat regimes also has similar regional features. If the mechanism operates efficiently on the Indian subcontinent and adjacent oceans, it is anticipated that the onset of the Indian summer monsoon is delayed, as compared to that over Southeast Asia because subsidence in the periphery of a sub-continental scale thermal low, resulting from intensification of land-ocean thermal contrast, inhibits convection. This may be one of the possible reasons why the two major onsets seen in the South Asian summer monsoon system are clearly distinguished from each other.