Abstract
The onset process of Asian summer monsoon (ASM) is investigated based on diagnostic analysis of observations of precipitation and synoptic circulation. Results show that after the ASM commences over the eastern Bay of Bengal (BOB) around early May, the onset can propagate eastwards towards the South China Sea and western Pacific but is blocked on its westward propagation along the eastern coast of India. This blocking, termed the “monsoon onset barrier (MOB)”, presents a Gill-type circulation response to the latent heating released by BOB monsoon convection. This convective condensation heating generates summertime (wintertime) vertical easterly (westerly) shear to its east (west) and facilitates air ascent (descent). The convection then propagates eastward but gets trapped on its westward path. To the east of the central BOB, the surface air temperature (SAT) cools faster than the underlying sea surface temperature (SST) due to monsoon onset. Thus more sensible heat flux supports the onset convection to propagate eastward. To the west of the central BOB, however, the land surface sensible heating over the Indian Peninsula is strengthened by the enhanced anticyclone circulation and air descent induced by the BOB monsoon heating. The strengthened upstream warm horizontal advection then produces a warm SAT center over the MOB region, which together with the in situ cooled SST reduces the surface sensible heating and atmospheric available potential energy to prevent the occurrence of free convection. Therefore, it is the change in both large-scale circulation and air–sea interaction due to BOB summer monsoon onset that contributes to the MOB formation.
Highlights
Monsoon occurs as a consequence of the seasonal reversal of circulation induced by an atmospheric response to the seasonal transition of land–sea thermal contrast
The ERA-Interim reanalysis dataset and latest rainfall products derived from satellite observation were used to investigate the Asian summer monsoon (ASM) onset process
The results showed that the evolution of monsoon precipitation over India and the Arabian Sea is distinct from that over the Bay of Bengal (BOB) and the South China Sea (SCS) during ASM onset, i.e., there is a discontinuity between the monsoon onset process over the Indian subcontinent to the Arabian Sea region and that over the BOB and SCS
Summary
Monsoon occurs as a consequence of the seasonal reversal of circulation induced by an atmospheric response to the seasonal transition of land–sea thermal contrast. Monsoon onset is characterized by rapid changes in the prevailing wind direction and rainfall intensity. Rainfall is both the result of, and a driving force for, the atmospheric circulation (Eady 1950). The BOB summer monsoon onset is associated with an overturning of the meridional air temperature gradient (MTG) in the mid–upper troposphere (Mao and Wu 2007). The development of monsoon onset vortex (MOV) in early May over the BOB plays an important role in the onset (Lau et al 1998; Liu et al 2002; Vinayachandran et al 2007). All the above factors are strongly influenced by the Tibetan Plateau forcing (Wu et al 2011, 2012a)
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