Changes in the relationship between Indian Ocean dipole and Indian summer monsoon rainfall in early and recent multidecadal epochs during different phases of monsoon

Abstract

AbstractIn this study, we explored the changes in the Indian Ocean Dipole‐Indian summer monsoon rainfall (IOD‐ISMR) relationship from early (1951–1980) to recent (1986–2015) multidecades during onset (June), peak (July–August) and withdrawal (September) phases of Indian summer monsoon. The analyses indicate that the IOD‐ISMR relationship is strengthening (weakening) during withdrawal (onset) phases over most of the Indian regions in the recent decades. During the peak phase, the relationship changes from out‐of‐phase (negative correlation) to in‐phase (positive correlation) in most of the Indian subcontinent. During positive IOD (pIOD) events, an increase in rainfall during all the phases is observed over the Indian region in the recent decades. Conversely, a general decrease in rainfall is observed over the Indian subcontinent during negative IOD (nIOD) events, except for the onset phase. The changes in sea surface temperature (SST), low level circulation and moisture transport over the equatorial Indian Ocean are linked to the changes in rainfall during both the positive and negative IOD events in the recent decades. However, most noticeable changes are observed during the peak and withdrawal phases. In the recent decades, during all the phases, a relatively strengthened low level jet (LLJ) with respect to the early decades is observed over the equatorial Indian Ocean and Arabian Sea associated with pIOD events. It causes a zone of moisture convergence over the northeast regions of India, which is conducive for above normal rainfall conditions in the monsoon trough zone and central Indian regions. In the case of nIOD events, the relatively weakened (strengthened) LLJ with respect to the early decades suppresses (enhances) the transport of moisture to the Indian region during the peak and withdrawal (onset) phases and cause reduced (enhanced) rainfall conditions. The observed results are more robust during the peak and withdrawal phases than that during the onset phase.