Dynamical and Boundary Forcing Characteristics of Regional Components of the Asian Summer Monsoon

In this paper, the authors present a description of the internal dynamics and boundary forcing characteristics of two major subcomponents of the Asian summer monsoon (ASM), that is, the South Asian monsoon (SAM) and the East–Southeast Asian monsoon (EAM). The description is based on a new monsoon-climate paradigm in which the variability of ASM is considered as the outcome of the interplay of a "fast" and an "intermediate" monsoon subsystem, under the influence of "slow" external forcings. Two sets of regional monsoon indices derived from dynamically consistent rainfall and wind data are used in this study. Results show that the internal dynamics of SAM are representative of a "classical" monsoon system in which the anomalous circulation is governed by Rossby wave dynamics, where anomalous vorticity induced by an off-equatorial heat source is balanced by the advection of planetary vorticity. On the other hand, the internal dynamics of EAM are characterized by a "hybrid" monsoon system featuring multicellular meridional circulation over the East Asian sector, extending from the deep Tropics to the midlatitudes. These meridional cells link tropical heating to extratropical circulation systems via the East Asian jet stream and are responsible for the observed zonally oriented anomalous rainfall patterns over East and Southeast Asia and the subtropical western Pacific. In the extratropical regions, the major upper-level vorticity balance is between the advection and generation by anomalous divergent circulation and basic-state circulation. A consequence of the different dynamical underpinnings is that EAM is associated with stronger extratropical teleconnection patterns to regions outside ASM compared to SAM. The interannual variability of SAM is linked to basin-scale SST fluctuation with pronounced signals in the equatorial eastern Pacific. During the boreal spring, warming of the Arabian Sea and the subtropical western Pacific may lead to a strong SAM. For EAM, interannual variability is tied to SST anomalies over the East China Sea, the Sea of Japan (East Sea), and the South China Sea regions, while the linkage to equatorial basin-scale SST anomaly is weak at best. A strong EAM is foreshadowed by a large-scale SST anomaly dipole with warming (cooling) in the subtropical central (eastern) Pacific. Comparison with the P. J. Webster and S. Yang (WY) monsoon index shows that WY is not significantly correlated with either the SAM or EAM regional-scale rainfall separately. It is demonstrated that WY can be considered as a measure of the large-scale atmospheric circulation state over the Indian/Pacific Ocean basin, including the integrated heat source over the ASM region. As such, the regional monsoon indices developed in this paper and WY provide a complementary description of the broadscale and regional aspects of the ASM.