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Abstract:
Tropical SST anomalies are among the largest drivers of circulation regime changes on interannual time scales due to its characteristic heat capacity decay time scales. The circulation anomalies associated with extreme precipitation and the corresponding atmospheric response to SST anomalies are derived from ECMWF ERA-Interim reanalysis data by employing composite analysis and lagged maximum covariance analysis. Our results show that interannual variability of extreme winter precipitation in Southeast China is in close accordance with the interannual variability of total winter precipitation. Both are associated with similar abnormal circulation regimes, but for extreme precipitation events the circulation anomalies and moisture transport channels are significantly intensified. Two main moisture transport channels are captured: one extends from the North Indian Ocean through India and the Bay of Bengal to South China, and the other from the West Pacific Ocean through Maritime Continent and South China Sea towards South China, which are related to the preceding autumn SST patterns, El Niño and the Indian Ocean dipole (IOD), respectively. El Niño (La Niña) SST anomalies induce anomalous anticyclonic (cyclonic) circulation over Philippine Sea, which is favorable (unfavorable) to warm and humid air transport to South China from the tropical West Pacific by southwesterly (northeasterly) anomalies. Under these circulations, northeasterlies of East Asian Winter Monsoon are weakened (strengthened) resulting in extreme precipitation to be more (less) frequent in Southeast China. During the positive (negative) IOD phase, abundant (reduced) moisture transport to South China from tropical regions through India and Bay of Bengal is observed due to weakened (strengthened) Walker circulations and abnormal anticyclonic (cyclonic) circulation over India, leading to a higher (lower) likelihood for extreme precipitation events in Southeast China. The underlying physical mechanisms can be explained by a simplified one-dimensional vorticity equation (Sverdrup balance).
