Annual Cycle of Southeast Asia—Maritime Continent Rainfall and the Asymmetric Seasonal Transition

In general the maritime continent experiences a wet monsoon during boreal winter and a dry season during boreal summer. However, the complex distribution of land, sea and terrain results in significant local variations of the annual cycle. This study uses historical station rainfall data to classify the annual cycles of rainfall over land areas, the TRMM rainfall measurements to identify the monsoon regimes of the four seasons in the entire Southeast Asia, and the QuikSCAT winds to study the causes of the variations. The annual cycle is dominated largely by interactions between the complex terrain and a simple annual reversal of the surface monsoonal winds. Analysis of the TRMM data reveals a structure whereby the boreal summer and winter monsoon rainfall regimes intertwine across the equator. In particular the boreal winter regime extends far northward along the eastern flanks of the major island groups and landmasses.

A hypothesis is presented to explain the asymmetric seasonal march in which the maximum convection follows a gradual southeastward progression path from the Asian summer monsoon to the Asian winter monsoon but a sudden transition in the reverse. The hypothesis is based on the redistribution of mass between land and ocean areas during spring and fall that results from different land-ocean thermal memories. This mass redistribution between the two transition seasons produces sea- level patterns leading to asymmetric wind-terrain interactions throughout the region, and a low- level divergence asymmetry in the region that promote the southward march of maximum convection during boreal fall but opposes the northward march during boreal spring. AGCM simulation suggests that the atmospheric mass redistribution due to land-ocean thermal memories is a possible mechanism for the asymmetric monsoon transition.