Two Physical Mechanisms Controlling the Interannual Variability of Baiu Precipitation

Abstract

 This work examines two physical mechanisms that control the interannual variability of Baiu precipitation from late May to mid-July, using objective analysis data of atmospheric parameters including precipitation from 1979 to 2010 and of sea surface temperature (SST) from 1982 to 2010. In late June, of this period, the mean atmospheric circulation that produces Baiu precipitation is altered. In the early Baiu season (May 26-June 24) after a warm El Niño/Southern Oscillation (ENSO) event, an anomalous anticyclone appears in the western North Pacific (WNP) through the Pacific-East Asian teleconnection, which enhances Baiu precipitation. The anomalous anticyclone is maintained by the Rossby wave response to negative SST anomalies (SSTAs) in the WNP, which persist from the preceding winter until this period. On the other hand, the effect of the Indian Ocean capacitor is essential for Baiu precipitation anomalies in the late Baiu season (June 25-July 19) after a warm ENSO event. Positive SSTAs in the tropical Indian Ocean (TIO) induce a specific atmospheric response with the Matsuno-Gill pattern, forcing an anomalous anticyclone in the WNP that increases Baiu precipitation. This effect occurs only in the late Baiu season, because it is necessary for the anomalous tropospheric heating to extend northeastward from the western TIO with intraseasonal development of the Indian summer monsoon.
 Thus, the physical processes producing the interannual variability of Baiu precipitation differ between the early and late Baiu seasons. Based on these differences, more detailed consideration can be given for the indicators of the interannual variability of Baiu precipitation, i.e., the SSTAs in the tropical Pacific and Indian Oceans. The detailed monitoring of SSTAs in these oceans from the preceding winter has the potential to improve predictions of the interannual variability of Baiu precipitation.