The causes of the extreme precipitation event in the Sichuan Basin in August 2020: A perspective from anomalous inter-monthly climate differences and weather processes

During August 2020, record-breaking precipitation occurred over the Sichuan Basin (SCB), the highest since 1979, exceeding both the historical August average and the rainfall of July 2020. This observed inter-month precipitation evolution contrasts with the typical climatological pattern, in which precipitation rises from July to August. During this period, two extreme rainfall episodes occurred—11 to 13 August (E1) and 15 to 18 August (E2)—and E2 exhibited a more northerly-located precipitation center and higher precipitation compared to E1. These two extreme precipitation episodes accounted for 60.1 % of the total rainfall in August 2020. This study investigates the causes of inter-monthly precipitation anomalies in the SCB during July–August 2020 from the perspective of the inter-monthly difference (August minus July) and further reveals the causes of two extreme precipitation episodes. Compared to July, the enhanced and eastward-shifted South Asian High (SAH) and the northeastward-shifted Western Pacific Subtropical High (WPSH) collectively contributed to the record-breaking precipitation over the SCB in August. The enhanced western Pacific convection and suppressed Indian Ocean convection influence precipitation through its effects on WPSH and SAH indirectly. During both extreme episodes (E1 and E2), abundant water vapor was transported from the tropical western Pacific to the SCB by the intensified WPSH. Concurrently, more moisture from the Indian Ocean resulted in more precipitation in E2. Moreover, non-adiabatic heating anomalies on the Tibetan Plateau facilitated the development of a southwest vortex, which enhanced upward motion over the SCB. All key processes can be validated by the Linear Baroclinic Model.