The persistent heavy precipitation that occurred in most of West China (WC) during August 11–18, 2020, generated the highest rainfall record since recording began in 1961 and was selected as one of the top 10 worst national natural disasters of 2020 in China. Favorable circulation was sustained during August; WC was positioned between two anomalous high-pressure centers over the Tibetan Plateau and Sea of Japan and an anomalous low-pressure center over Mongolia located on its north side, which created a stable and long trough and formed a low-pressure center over WC. At 200 hPa, the subtropical westerly jet was much stronger than average and southward, and the South Asian High (SAH) was strong and extended eastward to 150°E. At 500 hPa, the western Pacific subtropical high (WPSH) was westward and exceptionally strong, which helped abundant water vapor reach the southeastern part of WC and provided favorable dynamic and thermodynamic conditions for precipitation in this region. In addition, the eastward extension of the SAH promotes the westward extension of WPSH, which collectively enhanced the precipitation in WC. At 850 hPa, the low-level jet corresponding to the west-extending subtropical high from the Sea of Japan to WC further enhanced and guided the water vapor transport to WC. In addition, the Mei-Yu front over the Yangtze River Basin in June and July strengthened the northwestward spread of diabatic heating, transient energy, and wave activity fluxes, which likely influenced the large-scale circulation factors and reinforced the precipitation in WC in August 2020.
The characteristics and related mechanisms of the interannual variability of late summer (August) extreme precipitation in West China (WC) were investigated from 1961 to 2021. Precipitation and extreme precipitation (defined as the 99th percentile) generally decreased in the southeast-northwest direction, with a maximum in the Sichuan Basin. The non-linear trends in extreme precipitation have increased since the 1980s. Therefore, we further found that the interannual increase in extreme precipitation in the WC was significantly related to the eastward-strengthened South Asian high, western-stretched Western Pacific Subtropical high, enhanced westerly jet, anomalous cyclone in Mongolia, and anomalous anticyclone in the western Pacific. The anti-cyclonic anomaly is a Gill-type response to increase the sea surface temperature in the western Pacific. A mid-high latitude barotropic Rossby-wave train can be induced and has essential effects on the above key circulation patterns, further cooperating with the strong updrafts rather than strengthening and maintaining extreme precipitation in the WC.
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