Impacts of Decay of Different El Niño Types on Boreal Summer Rainfall and Surface Air Temperature in South Asian Monsoon Region and Tibetan Plateau

In this paper, we extensively examine the impacts of El Niño events on boreal summer rainfall over the East Asian Monsoon and South Asian Monsoon (SAM) regions and their associated mechanisms. To date, the various impacts of an El Niño event on the Tibetan Plateau (TP) regional seasonal and monthly rainfall and circulation have not been systematically examined. Based on the timing of the El Niño decay with respect to the boreal summer season and 1950–2018 sea surface temperature (SST) data, the El Niño decay phases are classified into two types: (1) early decay and (2) late decay. If the El Niño decays to below the threshold before spring, a La Niña sea surface temperature anomaly (SSTA) pattern usually develops during summer with increasing anomaly amplitudes from June to September. This causes an enhanced westward shift in the Walker circulation with a strong ascending branch over the tropical Indian Ocean (TIO) and the SAM, and induces concurrent heavy rainfall over the SAM and southwestern TP areas from July to September. Meanwhile, the developing La Niña SSTA forces a response by the anomalous North Western Pacific anticyclone (NWPAC), an anomalous cyclonic circulation over the Arabian Sea, and anticyclonic circulation over the Western Asian region. These induce a strengthening southerly wind anomaly, enhance water vapor transport to the Indian and TP regions from the TIO, and thus increase summer precipitation over northern India and the southwestern TP.

In contrast, if El Niño decays below the threshold after September, the eastern Pacific El Niño SSTA pattern and the strong SST warming over the TIO persists into June to July, then gradually weakens from August to September. This causes an anomalous ascending branch of the Walker circulation over the eastern TIO with a weak ascending branch over the western TIO and SAM, an anomalous eastwardly extended NWPAC, an anomalous anticyclonic circulation over the Arabian Sea, and cyclonic circulation over the Western and Central Asian region, which induce a strengthening westerly wind anomaly and reduces water vapor transport over the TP. The above responses result in deficient rainfall and warm surface temperatures in the central and northern SAM regions in June, but relatively increased rainfall and cool surface air temperature over most of the SAM region during August and September. This coincides with dryness over northeastern India and the southwestern TP in June, and then increasing precipitation over northwestern India and the western TP in September.

Our results confirm that a decaying El Niño has a significant impact on summer seasonal and monthly precipitation and temperature over the TP, which may explain the positive correlation between Indian and southwestern TP precipitation recently discussed in some studies. Our results also suggest that differences in the El Niño decay phase have strong impacts on the seasonal and intraseasonal rainfall over the SAM region and the southwestern TP.