Individual and Combined Influences of ENSO and the Indian Ocean Dipole on the Indian Summer Monsoon

Abstract

The relative influences of the ENSO and Indian Ocean Dipole (IOD) events on the Indian summer rainfall were studied using observational data and an atmospheric general circulation model (AGCM). The composite analysis of rainfall anomalies demonstrates that the IOD, while significantly influencing the Indian summer monsoon rainfall, also significantly reduces the impact of ENSO on the Indian summer rainfall whenever these events with the same phase co-occur. The AGCM experiments have shown that during an El Nino event, the Walker circulation over the tropical Indo-Pacific region is modulated; a low-level anomalous divergence center over the western Pacific and an anomalous convergence zone over the equatorial Indian Ocean are induced. Furthermore, an anomalous zone of convergence over the Myanmar and south China regions is induced during an El Nino event. These zones of anomalous convergence are complemented by anomalous divergence over the Indian region, causing anomalous subsidence and weakened rainfall. When a strong positive IOD event simultaneously occurs with El Nino, the latter's influence on the Indian monsoon is reduced by both poles of the IOD through the following mechanism: an anomalous divergence center, as compared to the summers when an El Nino alone occurs, is introduced in the eastern tropical Indian Ocean. From this center, the anomalous divergent flow crosses the equator, and this air, while weakening the El Nino-induced divergence over the western Pacific, also leads to convergence over the Indian monsoon region. This results in the reduction of the ENSO-induced subsidence and the related rainfall deficit over the eastern flank of the Indian monsoon trough region and adjoining regions to the east. On the other hand, over the western part of the tropical Indian Ocean sector, part of the anomalous ascending motion from the warm pole of the positive IOD event subsides just to the north of the equator, moves northward, ascends, and causes surplus rainfall. This reduces the ENSO-induced rainfall deficit over western India, the western part of the monsoon trough, and parts of Pakistan. The AGCM experiments also demonstrate that positive IOD events amplify the ENSO-induced dryness over the Indonesian region.