Spatiotemporal Evolution Of Compound Drought And Heatwave Events And The Dependence Of Drought And Heatwave

Under the background of climate warming,the frequency and intensity of extreme climate events in China continue to increase,which seriously threatens the sustainable development of society and economy.Compared with single extreme climate events,compound extreme climate event has caused more serious economic losses and more catastrophic environmental impacts.The compound drought and heatwave event refers to the compound extreme climate in which drought and heatwave occur simultaneously in the same area.In recent years,the compound drought and heatwave event has caused increasingly severe negative impacts on China’s agricultural production,ecosystems and social economy.Due to the catastrophic impact of compound drought and heatwave event and their potential increase under climate change,it is necessary to further explore their spatiotemporal evolution characteristics and the dependence of drought and heatwave in order to provide a basis for adapting and mitigating drought and heat wave composite events.Based on the meteorological data of 639 stations in mainland China from 1961 to 2021,this paper divides China into seven sub-regions,namely,northwest desert region(sub-region I),Inner Mongolia region(sub-region II),Qinghai-Tibet Plateau(sub-region III),northeast humid and semi humid temperate region(sub-region IV),humid and semi humid temperate regions of North China(sub-region V),humid subtropical regions of central and southern China(sub-region VI)and humid tropical region of South China(sub-region VII).Firstly,the nonstationary characteristics of meteorological drought were considered,and the nonstationary standardized precipitation evapotranspiration index(NSPEI)on multiple timescales was calculated.Based on historical disaster data and typical drought events,the applicability of NSPEI was evaluated,and the spatial distribution and periodic changes of drought frequency,intensity and duration on seasonal and annual scales were discussed by using run-length theory and wavelet analysis.Based on the relative threshold method,the heatwave was defined,the evaluation system of heatwave properties was constructed,the spatiotemporal distribution pattern of heatwave properties was studied,and the influence of climate warming on heatwave was further discussed.Based on the respective properties of drought and heatwave,the compound drought and heatwave event was defined,and the spatiotemporal evolution rules and trends of the frequency,intensity and duration characteristics of compound drought and heatwave event in different climatic sub-regions were analyzed.The contribution of the dependence of drought and heatwave to compound drought and heatwave event was quantified,and the nature of the intensity dependence of drought and heatwave(DH_i)at different levels and different timescales of drought was further analyzed to check the robustness of DH_i results.Finally,the impact of single extreme weather event heatwave and drought variations on the growth of compound drought and heatwave event was diagnosed.The main results are as follows:(1)The variation patterns of NSPEI and SPEI at different time scales are similar,which are generally dry and wet alternation,and the peaks are slightly different.In the characterization of historical drought data and typical drought events,NSPEI has better performance than SPEI and is more in line with actual drought conditions.The results based on NSPEI show that a certain degree of drought occurred throughout the country from 1961to 2021.The frequency of drought decreased significantly with the increase of drought level,showing the characteristics of high frequency of light drought and low frequency of heavy drought.At the annual scale,the characteristics of dry-wet alternation are more obvious.The frequency,total intensity and total duration of drought have a consistent spatial distribution pattern.Drought has more frequency,stronger intensity and longer duration in the northern part of sub-region V,sub-region VI and VII.There are obvious periodic characteristics in spring,summer and autumn,and the periods are about 14 years,18 years and 10 years respectively.On the annual scale,the periodic oscillation on the time scale of 22-29 years is the most obvious.Starting from 1995,the period is 15 years,and it is expected that the drought will show an increasing trend in the next few years.(2)The heatwaves started from May and lasted until the end of September,mainly concentrated in July and August.The spatial evolution was generally from the west of sub-region VII to sub-regions II,IV and V,and then to sub-regions I and VI.The high value areas of heatwave frequency and intensity are mainly concentrated in sub-regions I and VI,indicating that these two sub-regions are seriously affected by heatwave.Although the intensity of other sub-regions is low,the temperature rise trend during its occurrence is faster.The shorter the duration of heatwave is,the larger the proportion is.The proportion of heatwave with duration of 3-4 days and 7 days and above shows the opposite spatial distribution characteristics of“low in central and high in north and south”and“high in central and low in north and south”respectively.From the perspective of trend variation,the frequency of heatwave in each sub-region from 1961 to 2021 gradually increased,the temperature gradually increased,and the duration gradually prolonged.In addition,under the influence of climate warming,the frequency,number of days,effective accumulated temperature and duration of heatwave have increased,indicating that more frequent,more severe and more persistent heat waves are closely related to climate warming.(3)The frequency,effective accumulated temperature and duration of compound drought and heatwave events have a consistent spatial distribution pattern.The high value areas are located in sub-regions I,IV and VI,indicating that compound drought and heatwave events are more,stronger and more persistent in sub-regions I,IV and VI.The positive dependence of drought and heatwave has a cumulative amplification effect on the disaster situation.The positive dependence of drought and heatwave has led to an almost half increase in the number of compound drought and heatwave events in sub-region IV and VI,and sub-region I is less dependent as a hotspot for compound drought and heatwave events.Under the stricter definition of drought,the intensity dependence of drought and heatwave is stronger,and the DH_i results under the longer time scale drought definition prove that the sub-region IV may be more prone to extreme high temperature in summer and long-term drought from spring to summer.The spatial distribution pattern and data distribution characteristics of the frequency variations of heatwave and compound drought and heatwave events are basically the same.In sub-regions II,III and VII,the increase of heatwave frequency is an important driving factor for the increase of frequency of compound drought and heatwave events,while in sub-regions V and VI,the decrease of frequency of compound drought and heatwave events is mainly attributed to the decrease of drought frequency.In addition,98.93%of the stations with increased compound drought and heatwave events were accompanied by increased heatwave,while 55.87%of the stations with increased compound drought and heatwave events were accompanied by increased drought.Compared with the increase of drought,the increase of heatwave(excess heat)contributed more to the increase of compound drought and heatwave events.This study can provide scientific support for the early establishment and implementation of compound drought and heatwave events response system in different regions of China,and has certain practical significance.