Changes in the large-scale features associated with the Indian summer monsoon in the recent decades



The inter-annual variability of the Indian summer monsoon rainfall (ISMR) has been examined in association with the variability of surface temperature, the mean sea level pressure (MSLP), the tropospheric geopotential height, and wind patterns over the globe to study the changes in the large-scale features associated with Indian summer monsoon (ISM) in the recent decades. The data for the study have been considered from 1949–2005 (for a period of 57 years). It has been found that, during the recent decades, the El Nino-Southern Oscillation (ENSO) relationship over ISMR has weakened while the northwest (NW) of North Atlantic sea surface temperature (SST) relationship has increased. A physical mechanism for the relationship of North Atlantic SST with ISMR is proposed, by which during the excess years of ISMR, the SST over NW of North Atlantic is above normal. The rise in SST is the consequence of intensification and a cell-type shaped Azores high with isobars parallel to the east coast of North America, which drives a subtropical oceanic gyre (Gulf stream) transporting equatorial warm water NW of North Atlantic. The Azores high act as a source of downstream Rossby wave train over Eurasian region having barotropic structure consisting of a successive pressure trough and ridge. The Rossby wave may be associated with intensifying the low surface pressure anomaly over the Iranian landmass, which intensify the monsoon current over the Indian region. Copyright © 2008 Royal Meteorological Society