The South Asian Summer Monsoon (SASM) has two major convective centers (the Bay of Bengal and the Philippines). The variations of monthly mean Outgoing Longwave Radiation in the two centers are poorly correlated, particularly in the decade of 1980s. The enhanced convection over the Bay of Bengal and Indian subcontinent is coupled with reinforced monsoon circulation (characterized by the vertical shears between 850 and 200 hPa) west of 80oE overIndia, the western Indian Ocean, and the tropical northern Africa. The enhanced convection in the vicinity of the Philippines, on the other hand, corresponds to intensified monsoon circulation primarily east of 80oE over Southeast Asia including the Indo-China peninsula, South China Sea,Philippine Sea, and the Maritime Continent". To better reflect regional monsoon characteristics, two convection indices (or associated circulation indices which are dynamically coherent with the convection indices) are suggested to measure the variability of the Indian Summer Monsoon (ISM) and the Southeast Asian Summer Monsoon (SEASM), respectively.
The change in the Bay of Bengal convection (the ISM) has global implications whereas the change in Philippine convection has only a regional impact including a linkage with the East Asia subtropical monsoon. The equatorial western Pacific winds exhibit a considerably higher correlation with the ISM convection than with the Philippine convection. During the summers when a major Pacific warm episode occurs (e.g., 1982/83, 1986/87, 1991/92, and 1997), the convection and circulation indices describing the ISM (or the SEASM) often diverge considerably, causing inconsistency among various normally coherent monsoon indices. This poses a primary difficulty for using a single monsoon index to characterize its interannual variability. The cause of the breakdown of the coherence between various convection and circulation indices during ENSO warm phase needs to be understood