Research On The Process And Mechanism Of The High Temperature And Drought Composite Event In Yunnan Province From March To June 2019

The IPCC AR6 pointed out that in the background of global warming,the frequency and/or intensity of extreme temperature and drought events have increased significantly,and the probability of compound drought and hot extreme events(CDHEE)has also increased.At present,many scholars have made a lot of research on individual hot extreme events and drought events,but the research on CDHEEs is still insufficient,especially on its mechanism.To understand the physical process and occurrence mechanism of CDHEEs will help us to attribute and predict such compound events in the future,and improve the ability and level of disaster prevention and mitigation.Based on the ERA5,CRA?LAND reanalysis datasets and the station daily observation datasets,this study analyzed the overall characteristics of the CDHEEs in Yunnan Province,and diagnosed the temporal and spatial evolution characteristics of the CDHEEs from March to June 2019.The large-scale mechanism of compound event formation was analyzed from the perspective of the abnormal ocean and the circulation.The sensible heat advection tracking method was used to explore the contribution of local and advective sensible heat to compound event in Yunnan Province.Finally,focusing on the local land-atmosphere interaction,with the help of the land-atmosphere coupling index(TCI_SM-LE),Liang-Kleeman information flow,lag correlation coefficient,the changes in the physical process of the local development of compound event and the interaction between elements were revealed.The results are as follows:(1)The CDHEEs increased since 1981 during the period of March-June in Yunnan Province,which is mainly caused by the increase of hot extreme events.From March to June in 2019,Yunnan Province experienced the lowest precipitation and highest temperature since1961.The EOF results of meteorological drought index(SPI)and extreme temperature showed consistent temporal and spatial characteristics,indicating that hot extreme events and drought events appeared in combination,then,developed rapidly in April,and reached its peak in May.(2)Because the abnormal high pressure controlled the Yunnan Province from March to June,the divergence and subsidence occurred under the control of the abnormal anticyclone,and the lower layer heated up adiabatically,creating conditions for the compound event.In addition,the positive phase of Indian Ocean Dipole in the spring of the same period,and the reduction of sea ice in the Barents-Kara Sea in the previous winter and the current spring stimulated the sea-atmosphere interaction,the high-latitude factors had an effect through wave energy propagation,and the low-latitude factors had an effect through changing the regional circulation,which further affects the formation of CDHEE in Yunnan Province.(3)During the CDHEE from March to June in 2019,three stages were divided according to the soil moisture decline process.In the first stage,extreme temperature was the trigger for soil moisture decline,which further caused land-atmosphere interactions.In the second stage,the precipitation deficit was the trigger for the decline of soil moisture in this stage.In the third stage,extreme high temperature and precipitation deficit are both triggers,they caused a decrease in soil moisture.In general,in the three stages,the precipitation deficit and the extreme high temperature triggered the decline of soil moisture,which caused the land-atmosphere interaction and thus finally led to the development of the CDHEE.By tracking the sensible heat advection,it was found that the increase of temperature in this compound event was mainly caused by local sensible heat.The land-atmosphere coupling index showed that the land-atmosphere coupling effect was enhanced in the three stages.Since Yunnan Province locates at the junction of the tropics and subtropics,the solar radiation and heat are sufficient;at the same time,there are dry and wet seasons.March-June is the transition period from spring to summer and dry season to wet season.The state of soil moisture is conducive to the occurrence and strengthening of land-atmosphere coupling and feedback.Therefore,combined with the reduction of water vapor transport and sensible heat advection caused by circulation conditions(such as anticyclone control),it will lead to abnormal precipitation or temperature,which can trigger the land-atmosphere feedback mechanism,leading to the CDHEE easily.The CDHEE in 2019 developed rapidly through the feedback between the atmosphere and land.We found that this event was triggered by the initial extreme high temperature,and the impact of high temperature on soil moisture was significant throughout the process.High temperature was not only an element participating in the local process,but also an important influencing factor in the environment.This research revealed that the CDHEE in Yunnan Province in 2019 was first affected by favorable large-scale circulation conditions,and developed under the land-atmosphere interaction.The effects and mechanisms of ocean factor anomalies from low latitude to high latitude on high temperature and drought were discussed in order to reveal the large-scale background of composite events.The Liang-Kleeman information flow causal analysis gave the triggers of the event process,and revealed how the interaction between local soil moisture,air temperature and precipitation led to the occurrence and development of compound events.