INVITED Genesis of the South Asian High in Spring and Its Impact on the Asian Summer Monsoon Onset

The formation of the South Asian High (SAH) in spring and its impacts on the Asian summer monsoon onset are studied using daily ERA–40 reanalysis data together with a climate-mean composite technique and potential vorticity–diabatic heating (PV–Q) analysis. Results demonstrate that, about two weeks before the Asian summer monsoon onset, a burst of convection over the southern Philippines produces a negative vorticity source to its north. The SAH in the upper troposphere over the South China Sea is then generated as an atmospheric response to this negative vorticity forcing with the streamline field manifesting a Gill-type pattern. Afterwards the persistent rainfall over the northern Indochina Peninsula causes the SAH move westward towards the peninsula. Consequently, a trumpet-shaped flow field is formed to its southwest, resulting in divergence-pumping just over the southeastern Bay of Bengal (BOB) and atmospheric ascent in the middle troposphere.

Near the surface, as a surface anticyclone is formed over the northern BOB, an SST warm pool is generated in the central-eastern BOB. This, together with SAH pumping, triggers the formation of a monsoon onset vortex (MOV) with strong surface southwesterly over the BOB. Enhanced air–sea interaction promotes the further development and northward migration of the MOV. Consequently, the wintertime zonal-orientated subtropical anticyclone belt in the lower troposphere splits, abundant water vapor is transported directly from the BOB to the subtropical continent, and heavy rainfall ensues; the atmospheric circulation changes from winter to summer conditions over the BOB, and Asian summer monsoon onset occurs. The whole processes are confirmed by the case investigations for 2003 and 2008.