The Spatiotemporal Variations Of Heat Waves In China And The Underlying Mechanisms

Under the background global warming,the occurrences of heat waves have increased globally.The heat wave is one of the typical extreme weather events,which are characterized by consecutive days of high temperatures.China has suffered a lot from the frequent occurrences of heat waves and the disastrous impacts in recent decades.Therefore,it is of great significance to understand the driving mechanism underlying heat waves.However,there is not an objective and universal heat wave definition yet.Based on the temperature criterion,the heatwave definitions can be classified into two categories:one is based on the absolute threshold,which applies a fixed temperature value,and the other is based on the relative threshold by considering the local climatology.Few efforts were made to understand the comparison of two types of heat waves and the uncertainty arising from the definitions,which could jeopardize the accurate description of the magnitude,duration of heat waves.Thus,in this study,using the daily maximum air temperatures(Tmax)in 587 observational sites,the spatiotemporal characteristics of heatwaves over China during 1959-2013 are investigated based on two types of heatwave definitions.Based on reanalysis data and model simulation,we link the variability of heat waves with the large-scale circulation,SST and soil moisture,aiming to provide some theoretical foundation for the prediction of heat waves in China.Moreover,using WRF model,we project the future changes of heat waves in the coming decades under global warming.The main results are as follows:1.Characteristic analysis of heat wave events in ChinaHeat waves are detected using both absolute and relative definitions.The spatiotemporal characteristics of heat wave occurrence/duration/amplitude are compared between the two definitions.The abosulte heat waves(AHWs)primarily occur in the Southeast and Xinjiang regions of China,while relatve heat wavs(RHWs)evenly occur in most of China.Both the AHWs and RHWs show stronger intensities in North China,consistent with the observed fact that North China has stronger warming than South China in recent decades.Although there are differences between the spatial distributions of AHWs and RHWs,the annual variations of regional averaged absolute and relative heatwave indices over China are quite similar,i.e.,a weak decreasing trend before late 1980s and an increasing trend afterwards,indicating that more frequent and longer lasting heatwaves are found for both the AHWs and RHWs since the 1990sWith the consideration of spatial extension of heat wave events,two kind of regional heat waves using absolute and relative thresholds,namely RHWs-A and RHWs-R,are investigated during 1959-2013.By utilizing the cluster analysis,several typical spatial distributions of RHWs-A/RHWs-R are obtained.For RHWs-A,there are three clusters covering the southeastern,northwestern China and the lower reaches of Yangtze River,of which the southeastern cluster groups the most heat waves.For RHWs-R,there are seven clusters distributed throughout the whole regions of China.The northern clusters are of larger intensity than that of the southern ones.Considering the significant differences in regional climatolog,we believe that the relative threshold is more meaningful to detect the local extremes over China.2.The spatiotemporal variability of heat waves in China and the underlying mechanismsBy utilizing the empirical orthogonal function,the integral index heat wave total intensity is decomposed into three dominant modes:interdecadal(ID),interannual-tripole(IA-TR),and interannual-dipole(IA-DP)modes.The ID mode shows uniform anomalies over the whole China,with the maximum in north,and its corresponding time series depict notable interdecadal variations with a turning point around mid-1990s.The IA-DP mode exhibits opposite-signed anomalies over north and south China.The IA-TR mode shows an anomalous tripole pattern with negative anomalies over central China and positive anomalies over north and south China in its positive phase.Both the IA-DP and IA-TR patterns are more obvious since mid-1990s with mainly year-to-year variations before that.All the three modes are controlled by anomalous high-pressure systems,which are accompanied by local-scale dry land conditions.The diabatic heating associated with anomalous convective activities over tropical western Pacific triggers Rossby wave trains propagating northward along the East Asia,which causes abnormal heat waves through descending motion over the high-pressure nodes.In turn,the severe convections are generated by enhanced Walker circulation in the tropical Pacific due to warming and/or cooling sea surface temperature(SST)anomalies in the tropical western and eastern Pacific,respectively.Moreover,through composite ananlysis,it's found that all RHWs-A/RHWs-R are locally accompanied by the anomalous high systems along with the reduced soil moisture.The southern clusters are controlled by Northwestern Pacific subtropical high(WPSH),and the northern ones are influenced by the mid-latitude high systems.3.WRF simulations of heat wave variability in ChinaUsing the Weather Research and Forecasting(WRF)model,we analyze investigates the capability of WRF to reproduce the observational spatiotemporal features of heat waves in China.Five continuous simulations are conducted from 1989 to 2008,of which four runs apply the interior spectral nudging(SN)technique with di erent wavenumbers,nudging variables and nudging coe cients.WRF can reasonably reproduce the spatiotemporal features of heat waves in China.However,the application of SN is effective on improving the skill of the model in simulating both the spatial distributions and temporal variations of heat waves.The WRF model shows advantages in reproducing the synoptic circulations with SN and therefore yields better representations for heat wave events.Besides,the SN method is able to preserve the variability of large-scale circulations quite well,which in turn adjusts the extreme temperature variability towards the observation.Among the four SN experiments,those with stronger nudging coeffcients perform better in modulating both the spatial and temporal features of heat waves.Also using WRF model,we investigate the sensitivity to ISM for the three severe heat wave events that dominated eastern China in 2003,2007,and 2013.The decreased ISM could cause an enhancement on heat waves with increased amplitudes,extents and intensities,while it has insignificant influence on the spatial distributions and temporal variations.The responses of heat waves are generally decreasing with the increasing ISM.Through enhanced sensible flux as well as reduced latent cooling,the initial soil dryness locally strengthens the surface warming and the further drying of the soil.Besides,the reduced ISM forces positive anomalies of geopotential height atmid-troposphere and negative anomalies at lower levels,leading to an enhanced thickness of the atmosphere.Such a thickened atmosphere can strengthen the anomalous high-pressure systems,favoring the maintenance of severe heat waves.4.Future projection of heat waves over China under global warmingDriven by four global climate models(GCMs)from the Coupled Model Intercomparison Project Phase 5(CMIP5)(i.e.,CNRM-CM5,EC-EARTH,GFDL-ESM2M and MPI-ESM-LR)under the Representative Concentration Pathway 8.5(RCP8.5)scenario,projections for future changes in heat waves over China are performed by Weather Research Forecasting(WRF)model simulations for future(FTR,2031-2055)and present(1981-2005)periods.Six heat wave indices are applied to characterize heat waves based on their frequency,duration,magnitude,intensity,accumulated occurrence days and severity.Analyses show that notable increases in heat wave indices cover all of China.More areas will endure more frequent,longer lasting and more severe heat waves in the coming decades.The increasing tendencies of heat wave indices in the FTR period are more significant than those at present,indicating that heat waves will intensify more rapidly in the future.The impacts of climate changes on the accumulated properties of heat waves are more substantial than those on the individual aspects of heat waves.It is also projected that stronger heat waves with prolonged durations and more severe magnitudes will occur more often in the FTR period,whereas relatively weaker heat waves would occur less often.Hence,the occurrence of extreme heat waves shows a more remarkable increase than the occurrence of moderate heat waves.The changes in heat waves can be largely explained by the changes in the dominating high-pressure systems.