Observed And Projected Changes In Heat Waves With Different Severities And Types Over China

In the context of global warming,the occurrence of heat waves(generally referring to sustained processes of anomalous high temperatures)has become more frequent and more intense,and exerted profound impacts on natural system,human health as well as economic society.Recently,the variety of heat waves has also been highlighted,with more extreme and compound events.Therefore,based on the observations,reanalysis data and climate model simulations,this research addresses a comprehensive picture of heat waves over China in terms of their spatiotemporal changes,atmospheric backgrounds,future projections,and associated impacts.We expect these results can provide valuable information for adaptation and mitigation,and call for urgent actions to address heat wave risks.The main findings are summarized as follows:(1)Based on the station observations,the high frequency of heat waves mainly appears in East China(EC),Central China(CC),South China(SC)and Northwest China(NWC),and the distributions of severe and extreme heat waves are more spatially homogenous than those of mild and moderate heat waves.Since 1960 s,the frequencies of heat waves with different severities have increased significantly.Moreover,a decadal difference is found before and after the late 1990 s,the frequencies of moderate,severe and extreme heat waves during the latter period are almost twice as many as those during the former period.Spatially,the occurrence of extreme heat waves has significantly increased most in North China(NC)and NWC during the latter period,with 68% and 59% of stations experiencing significant increase compared with the former period.(2)The ensemble of RegCM4 dynamic downscaling simulations indicates substantial increases in frequency and magnitude of heat waves over China toward the end of the 21 st century.Relative to the current climate(1986–2005),the frequency and magnitude are projected to increase by 2.9-fold(3.9-fold)and 29%(42%)during the mid-century(end-century),respectively,with considerable changes occurring in SC and Southwest China(SWC2).The prolonged duration of heat waves plays a dominant role in the enhancement of heat wave severity(relative contribution is over 65%).It is noted that the risks of the occurrence of extreme heat waves would aggravate.The heat wave frequency above the maximum of the current climate would appear once almost every year,and SC,CC,EC(NC,Northeast China)would experience more severe heat waves than the most extreme of the present every 1–2(2–4)years.(3)Heat waves can be divided into daytime(occurring only in daytime),nighttime(occurring only in nighttime)and compound(occurring simultaneously in daytime and nighttime)heat waves.Since 1960 s,nighttime and compound heat waves in China are observed to increase significantly,while the frequency of daytime heat waves has remained basically unchanged.Take the the middle and lower reaches of the Yangtze River basin(MLYR)as an example,it is found that the occurrences of three heat wave types are all associated with anticyclonic circulation anomalies from the upper troposphere to the lower troposphere,whereas their locations and intensities determine the configuration of atmospheric conditions.The resultant less(more)cloud cover and humidity as well as increased downward shortwave(longwave)radiation at the surface favor the warming of daytime(nighttime),conducive to the occurrence of daytime(nighttime)heat waves,reflecting a dry-hot(moisture-hot)nature.The occurrence of compound heat waves corresponds to the combination of conditions for daytime and nighttime heat waves,which provides beneficial background for the persistence of high temperatures from daytime to nighttime,concurrent with slight above-normal humidity.Moreover,the occurrence of compound heat waves is associated with the northwestward extension of the western Pacific subtropical high,and is closely related to the wave train propagating from the Scandinavian Peninsula to East China.After the early 2000 s,the prominent interannual variation of compound heat waves in the MLYR appeared,which is related to the combined effect of sea surface temperature(SST)anomalies both in the North Atlantic Ocean(NAT)and around the Maritime Continent(MC).The NAT tripole SST anomaly(positive anomaly of MC SST)is favorable for the waves train over the mid and high latitudes(abnormal meridional vertical circulation),contributing to the aggravation of anomalous high in the upper troposphere over the MLYR(anomalous anticyclonic anomaly over the northwestern subtropical Pacific).(4)The projection based on the Coupled Model Intercomparison Project phase 6(CMIP6)simulations indicates that the compound heat waves are projected to increase persistently toward the end of the 21 st century,with larger increase under SSP5-8.5 than that under SSP2-4.5,while the frequency of daytime(nighttime)heat waves is projected to increase first and then decrease after the 2050s(2070s)under SSP5-8.5.Accordingly,the proportion of compound HWs to the total HW events tends to increase and that of daytime HWs tends to decrease toward the end of the 21 st century.The projected increase in the frequency of compound heat waves generally scales linearly with the climate warming up to 2100,while the projected frequency of daytime(nighttime)heat waves shows a parabolic response to the change in seasonal maximum(minimum)temperature.Due to the significant increase of compound heat wave,the population and GDP exposures will also considerably increase over China.Western China and southern China are projected to experience the largest increases of population exposure.The increases in GDP exposure are more spatially comparable over China,but are also higher in southwestern China.