Asymmetric Relationship between ENSO and the Tropical Indian Ocean Summer SST Anomalies

During the summer following El Niño, a basinwide sea surface temperature (SST) warming takes place over the tropical Indian Ocean (TIO), exerting profound influences on the Asian summer monsoon. This is an important source of seasonal predictability for the Asian summer monsoon. Based on observations, however, the present study finds that the relationship between El Niño–Southern Oscillation (ENSO) and the TIO SST anomalies during the decaying summer is asymmetric with a much weaker relationship between La Niña and the TIO SST anomalies relative to El Niño. The analyses show that this asymmetric relationship can be explained by the asymmetries in initial TIO SST, the oceanic Rossby wave in the southern Indian Ocean, and the ENSO decaying rate. In contrast to El Niño events, La Niña events tend to have a stronger initial TIO warming and a lower peak intensity with a weaker oceanic Rossby wave response in the southern Indian Ocean. On the other hand, La Niña events tend to decay more slowly with the persistent SST cooling over the central equatorial Pacific in the following summer. The equatorial Pacific SST cooling induces an anomalous anticyclone via a Gill-type Rossby wave response, weakening the positive feedback between the anomalous cyclone spanning the tropical northwest Pacific and north Indian Oceans and the TIO summer basinwide SST cooling. These results have important implications for the climate predictability of the Indian Ocean and Asian summer monsoon.