Response of the Asian Summer Monsoon to Tropical SST: Results from Observations and AGCM sensitivity studies

The Asian Summer Monsoon (ASM) has three sub-components namely, the South Asian (SAM),East Asian (EAM) and West Pacific (WPM) monsoons. All these three components interact and interwine among themselves in a variety of time scales. The successful prediction of the seasonal mean rainfall from dynamical models has been a major challenge and still in its premature stage. The prediction of the monsoons is based on the premise that the Asiatic monsoon is a stable system and its seasonal mean precipitation and circulation is largely controlled by slowly varying boundary conditions such as SST, snow cover, soil moisture etc. Of all these boundary forcings, the impact due to SST has been the dominant, primarily associated with ENSO. However, the the physical mechanism through which the SST changes in the tropics influences the ASM and its three components are not yet clear.

During its evolution, the monsoon related convection (heat source) moves north/northwestwardfrom the equator while El Nino related SST/convection/heat source moves eastward along the equator. It is hypothesized that the nature of the interaction between these two major heat sources and the response of the atmospheric circulation have to be unearthed to advance the understanding between monsoon and ENSO. Apart from the direct influence of ENSO related heating, the competing impact by the regional SST anomalies over the west Pacific and Indian Oceans have to be understood, i.e., the impact of remote and local forcings.

In the present study, it is shown that the SAM and precipitation over North East China are reduced (increased) during the developing phase of a warm (cold) event in the Pacific, due to changes in the large scale circulation. These changes in the large scale circulation in April-May, play a significant role in suppressing the north/northwestward propagating convection over the SAM region. However, during the same time, low level cyclonic circulation is evident over the western tropical Pacific, primarily as a Rossby response due to the heating patterns over the equatorial central Pacific and favor the north/northwestward propagation of convection over thewest Pacific and hence WPM is enhanced (decreased) during El Nino (La Nina) events. Although ENSO is responsible for the out-of-phase relationship between the SAM and WPM systems, it is shown that the regional SSTA, particularly over the equatorial Indian Ocean, and west Pacific are responsible for the in-phase relationship between SAM and WPM systems. It is further demonstrated that these changes in the tropical monsoon systems (SAM/WPM), significantly influence the precipitation over the Meiyu rainfall regime of East Asia. The proposed hypothesis is validated first by diagnosing the NCEP/NCAR reanalyses products and second from a series of sensitivity experiments with the ECHAM4 Atmospheric General Circulation model.