Reversal of decadal trend in West China autumn rainfall in the late 1990s and possible causes

As a complex regional weather and climate phenomenon in China, the highly variable West China autumn rainfall (WCAR) causes severe flooding and related disasters that can have major impacts on socioeconomic and environmental sustainability. This study investigated both the reversal of the decadal trend in the WCAR during 1961-2020 and the related atmospheric circulation and oceanic backgrounds. The results showed that the decadal trend in the WCAR underwent distinct reversal in the late 1990s, with a significant trend of decline (increase) before (after) this reversal, which was directly linked to variations in the large-scale Eurasian atmospheric circulation, local moisture transport and dynamic conditions, and sea surface temperature (SST) in the subtropical South Pacific. Further investigation indicated that reduced sea level pressure over Asia, an enhanced European blocking high, and a deepened Lake Balkhash trough during the latter period favored southward movement of high-latitude cold air into West China. Moreover, a stronger southerly flow in the southern West China was observed in the latter period, which tended to transport more moisture from low-latitude ocean areas. Northward displacement of the East Asian jet (EAJ) also played a strong dynamic role in the reversal of the WCAR trend. Additionally, the reversal of the decadal trend in WCAR might also be attributed to a change in the trend of SST in the subtropical South Pacific from warming to cooling in the late 1990s. This resulted in southward displacement of the EAJ, which generated an anomalous descent and reduction in the WCAR.