Differences between decadal decreases of boreal summer rainfall in southeastern and southwestern China in the early 2000s

Abstract

On decadal time scale, the boreal summer (June–August) rainfall over southern China significantly decreased during the epoch 2003–2012 compared to that in the preceding epoch 1993–2002. This study investigates different processes involved in the decadal decreases in summer rainfall over southeastern China (SEC) and southwestern China (SWC) since the early 2000s. Possible mechanisms for these changes are also explored. Results show that the daily total amount of rainfall in light, moderate and heavy (light and moderate) grades during 2003–2012 is significantly lower than that in the preceding period 1993–2002 by 14.4% (8.2%) over SEC (SWC) region. The epochal rainfall change caused by changes in the light, moderate and heavy (light and moderate) grades accounts for 90% (86%) of total summer rainfall difference between the epoch 1993–2002 and 2003–2012 over SEC (SWC). Based on regression and composite analyses, it is found that the reasons for decadal changes in summer rainfall in the early 2000s are different for the two regions. The rainfall decrease over SEC is attribute to reduced moisture transport by the southwesterly flow from the South China Sea, which is caused by the northward-shift of the western Pacific subtropical high (WPSH) in association with the sea surface temperature (SST) cooling over the western North Pacific (WNP) to the east of Japan. The rainfall decrease over SWC can be explained by reduced moisture transported by the southwesterly/westerly from the Bay of Bengal, which induced by the weakened South Asian monsoon since the early 2000s that is plausibly related to SST warming over the northern Indian Ocean (NIO). Furthermore, the anomalous anticyclonic circulations connected to the NIO warming and WNP cooling make a southwest-northeast-oriented high pressure band. WPSH shifts northward, extending from east of Japan to the Bay of Bengal. SEC and SWC are located along the southern flank of the “dumbbell-shaped” subtropical high band, where the northeasterly prevails. As a result, moisture transports from the South China Sea and the Bay of Bengal to SEC and SWC both decrease on the decadal time scale since 2003. Although different physical-dynamical processes are involved in rainfall changes over the two different regions, the summer rainfall decreases on the decadal time scale are closely related to the shifts in SSTA pattern and background atmospheric circulation on global-scale that occurred in the early 2000s.