Understanding the 2022 Extreme Dragon-Boat Rainfall in South China from the Combined Land and Oceanic Forcing

Abstract

The most frequent and concentrated rainfall in the pre-flood season in South China usually occurs around the Dragon Boat Festival every year, locally known as ‘Dragon-boat Rainfall (DBR)’. In 2022, a record-breaking DBR attacked South China, causing disastrous flooding. We suggest that this extreme DBR was jointly regulated by the tropical convective forcing and Tibetan Plateau (TP) heating. Distinctly strong low-level southwesterlies and ascending motions over South China were the key atmospheric conditions. And the abnormal low-level southwesterlies were contributed by both the anomalous anticyclone over the western North Pacific and the anomalous westerlies at the southern side of the TP. On the one hand, during the period of 2022 DBR, stronger-than-normal convective forcing over the Maritime Continent induced the low-level anomalous anticyclone over the western North Pacific through triggering the meridional vertical circulation and further promoted the upward motions over South China. On the other hand, positive diabatic heating over the TP forced abnormal warm anticyclone in the mid-upper troposphere, more warm air advected downstream by the background westerlies, intensifying the upward motions over South China. Meanwhile, the TP heating could induce the anomalous low-level westerlies at the southern side of the TP, which further merged into and intensified the southwesterlies over South China and greatly enhanced the moisture transport and convergence there. Therefore, we highlight the strong thermal forcing over the TP, exerting a combined and amplified effect with the convective forcing over the Maritime Continent, dominated the record-breaking DBR in 2022.