Understanding the role of SST anomaly in extreme rainfall of 2020 Meiyu season from an interdecadal perspective

Extreme Meiyu rainfall in 2020, starting from early June to the end of July, has occurred over the Yangtze River valley (YRV), with record-breaking accumulated precipitation amount since 1961. The present study aims to examine the possible effect of sea surface temperature (SST) on the YRV rainfall in Meiyu season from the interdecadal perspective. The results indicate that YRV rainfall in June exhibits more significant variability on interdecadal time scale than that in July. The interdecadal-filtered atmospheric circulation in June, compared with the counterpart in July, shows a more predominant and better-organized Western North Pacific Anticyclone (WNPAC) anomaly, which could transport abundant moisture to the YRV by anomalous southwesterly prevailing in northwestern flank of anomalous WNPAC. Both observation and numerical experiment indicate that the interdecadal change of the SST anomaly in tropical western Indian Ocean (TWI) from preceding May to June can significantly affect the anomalous WNPAC, leading to enhanced YRV rainfall in June. The TWI SST anomaly shifts from a cold phase to a warm phase around the early 2000s, with a magnitude of 0.7 degrees C in 2020, which implies that such interdecadal warming might partly contribute to the heavy rainfall in June 2020 by providing a large-scale favorable background flow.