Interannual Variation of the Leading Modes of Summertime Precipitation in the Sichuan Basin

Based on precipitation observation data from 134 stations in the Sichuan Basin, ERA-Interim reanalysis datasets, sea surface temperature (SST), and sea ice datasets from the Met Office Hadley Centre, this study investigates two major modes of summer precipitation over the Sichuan Basin using empirical orthogonal function (EOF) method and linear regression technique. Results showed that the first mode of EOF (EOF1) exhibits a consistent pattern throughout the region, whereas the second mode of EOF (EOF2) exhibits an inverse pattern between the east and west of the Sichuan Basin. In the 1980s, the dominant mode was an east–west reversed pattern, which remained consistent from the 1990s to early 2000s. The inverse pattern has been restored to the dominant position in recent years. Further investigation shows that the EOF1 of precipitation is substantially influenced by the low-latitude circulation in 500-hPa geopotential height, particularly in the western Pacific subtropical high. Convergent or divergent fields control the 850-hPa wind over the basin. The water vapor budget is influenced by water vapor channels in the Bay of Bengal, South China Sea, and western Pacific. The vapor channel in the South China Sea could be crucial. The amount of precipitation of EOF1 corresponds to the consistent input/output of water vapor at the North–South boundary of the Sichuan Basin. The pre-signal for most cases in EOF1 may come from the decaying phase of the El Niño–Southern Oscillation. However, the EOF2 is greatly related to the mid and high latitude circulation at 500 hPa, similar to the Polar/Eurasia pattern. The EOF2 mode, which has been found more rain in the west than in the east of the Sichuan Basin, is linked to the input of the southern border and the output of the northern border, and vice versa. The EOF2 pattern is closely related to the water vapor channel in the western Pacific. The variability of Arctic sea ice may be the source of the pre-signal in most EOF2 cases.