The evolution process of a prolonged compound drought and hot extreme event in Southwest China during the 2019 pre-monsoon season

The latest IPCC AR6 pointed out that some climate extremes, such as temperature extremes and droughts have increased in frequency and/or intensity significantly under global warming background. Based on in situ observations, ERA5 hourly reanalysis and CRA-Land surface reanalysis data, this study described the occurrence and evolution process of compound drought and hot extreme event (CDHEE) in Southwest China in the pre-monsoon season (March–June) in 2019. The spatio-temporal evolutions of key physical variables (soil moisture, temperature, precipitation, etc) were diagnosed from the perspective of local land-atmosphere interaction. The Liang-Kleeman Information Flow was firstly used to reveal the interactions between the key variables in CDHEE. In order to find the physical processes which may exist in this event, the CDHEE was divided into three development stages based on the soil moisture reduction. Further diagnostic analysis shown that the trigger variable contributing to CDHEE are not consistent in these three stages. At the first stage, CDHEE was induced by extreme high temperature, which promoted the evaporation, leading to the decrease of soil moisture. Meanwhile, increase of sensible heat promoted the rise of temperature further. Consequently, a positive feedback cycle formed and CDHEE intensified. At the second stage, CDHEE was triggered by precipitation deficit, which broke the water budget, thus soil moisture was decreased by evaporation and CDHEE deteriorated further. At the third stage, precipitation deficit and extreme high temperature are both triggers. Combined effects of high temperature and drought lead to stronger land-atmosphere interaction and feedback. Thus, based on this study, it reveals that extreme high temperature and drought had an obvious interact with each other in the whole process, and extreme high temperature not only was an external environment of others variables, but also involved in local processes.