Seasonal Heating Processes over the Indochina Peninsula and the Bay of Bengal Prior to the Monsoon Onset in 1998

Abstract

Using GAME reanalysis upper-air data, we attempt to reveal the seasonal evolution of the heat source (Q1), and moisture sink (Q2) over and around the Indochina Peninsula (IP), from April to June 1998. Pre-monsoon rainfall occurs inland of IP prior to the large-scale monsoon onset in middle May. In this period, positive Q1, in excess of 3 K day−1, appears around the middle-lower Mekong River basin (MLMRB) centered around 500-600 hPa, which is accompanied by positive Q2, slightly below the peak level of Q1. This suggests that the pre-monsoon rainfall is associated with a cumulus-type convection. The horizontal distribution of vertically integrated Q1 ( ) and Q2 ( ) over MLMRB is similar, suggesting that the contribution of latent heating is nearly equivalent to sensible heat supply from the land surface. In contrast, negative , less than −100 W m−2, is discernible over the Bay of Bengal (BoB), which is collocated with a strong downward motion.Once the large-scale monsoon commences, the value of in MLMRB becomes roughly the same magnitude as those in the pre-monsoon season. The vertical profile of Q1 and Q2 are indicative of the existence of stratiform and cumulus-type clouds. On the other hand, the whole troposphere over BoB is abruptly occupied by deep heating, associated with convective activities as implied by positive and .These atmospheric heat and moisture budget analysis indicate the presence of a large regional difference between MLMRB and BoB, especially in the pre-monsoon season. The qualitative heating difference, which may be attributable to the structure of the cumulus-type convection, is recognizable even after the monsoon onset.