Land-atmosphere coupling speeds up flash drought over South China in a changing climate

Flash drought is a new type of drought with rapid onset, which occurred frequently in recent years over the world. Compared with the traditional drought, the rapid onset makes it difficult to predict in time, and it poses a serious threat to agriculture and ecosystem. However, causes of the rapid onset and underlying mechanisms are still unclear. Considering that the land-atmosphere coupling can regulate the evolution of extreme drought, here we investigate the coupling characteristics during flash droughts over South China, and carry out the attribution by using the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulations. Through the synthetic analysis of flash drought onset, it is found that extreme precipitation deficit and strong evapotranspiration provide favorable conditions for flash drought onset, and the dry coupling between land and atmosphere further aggravates the decline in soil moisture, and increases the onset speed. In addition, with the increase of onset speed, the contribution of evapotranspiration increases accordingly, and the dry coupling between land and atmosphere further dominates the evolution. This suggests that the land-atmosphere coupling plays a key role in increasing the onset speed of flash drought. Furthermore, the impact of climate change on the onset speed of flash drought also can’t be ignored. The results of detect and attribution show that anthropogenic climate change (caused by the emissions of greenhouse gases and aerosols, etc) has increased the likelihood of flash drought onset speed over South China in 2019 by 24±16%, which is closely related to anthropogenically increased evapotranspiration.