Bull. Amer. Meteor. Sci. , 80, 629-638

Choice of South Asian Monsoon Indices

Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii

Z. Fan
Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii

In the South Asian region, two of the major precipitation maxima associated with areas of intensive convective activity are located near the Bay of Bengal and in the vicinity of the Philippines. The variations of monthly mean Outgoing Longwave Radiation in the two regions are poorly correlated, particularly in the decade of 1980s. The enhanced convection over the Bay of Bengal and Indian subcontinents is coupled with reinforced monsoon circulation west of 80oE over India, the western Indian Ocean, and the tropical northern Africa. In contrast, the enhanced convection in the vicinity of the Philippines, 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 planetary scale implications whereas the change in Philippine convection has primarily 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 often diverge considerably, causing inconsistency among various normally coherent monsoon indices. This poses a primary difficulty for using a single monsoon index to characterize the interannual variability of a regional monsoon. The cause of the breakdown of the coherence between various convection and circulation indices during ENSO warm phase needs to be understood.

Back to top | Back to menu