The Mechanisms And Prediction For The Interannual/Interdecadal Variability Of Summer Extreme High-temperature Frequency Over East China

Eastern China is densely populated and has developed industry and agriculture.The occurrence of extreme high temperature events has a serious impact on human health,ecological environment and social economy.It is of great scientific value and social significance to study the characteristics and mechanisms of extreme high temperature events in this region and improve the prediction for it,which will be conducive to disaster prevention and reduction.Based on the gridded daily maximum temperature observation dataset over China(i.e.,CN05.1),the reanalysis dataset of atmospheric circulation from National Center for Environment Prediction(NCEP),the reanalysis dataset of sea surface temperature provided by National Oceanic and Atmospheric Administration(NOAA),the data of snow depth and soil moisture provided by the fifth set of reanalysis dataset from European Centre for Medium-Range Weather Forecasts(ECMWF)and the forecast data from the NCEP coupled forecast system model version 2(CFSv2),the study analyzed the characteristics and mechanisms of the interannual and interdecadal variability of summer extreme high temperature over eastern China,the prediction skill of CFSv2 for it and established physical-empirical models to improve the prediction.Some major conclusions are shown as follows:(1)The characteristics and mechanisms of interannual variability of summer extreme high temperature frequency over eastern ChinaThe dominant mode of the interannual variability of summer extreme high temperature frequency(FEHE)over eastern China exibited a meridional dipole pattern,with opposite anomalies over Northeast China and South China.The anomalous atmospheric circulation associated with the positive phase of the dominant mode,was characterized by anomalous positive geopotentional height,descending and divergent water wapor over Northeast China and opposite anomalies over South China.The atmospheric circulation anomalies could influence the occurrence of extreme high temperature events through inducing anomalous atmospheric vertical adiabatic heating and surface radiation and contributed to the dominant mode of FEHE.The anomalous atmospheric circulation related to the dominant mode of FEHE,was influenced by the dominant mode of 500 h Pa geopotential height over Eurasia and the western tropical Pacific Sea surface temperature(SST).The dominant mode of 500 h Pa geopotential height over Eurasia could contribute to the dominant mode of FEHE via a middle-to highlatitude atmospheric wave train.The anomalous SST over the western tropical Pacific could influence the convection in the overlaying troposphere and induce opposite atmospheric circulation anomalies over South and Northeast China via stimulating a meridional atmospheric wave train,which contributed to the dominant mode of FEHE.(2)The interdecadal change in the dominant mode of summer FEHE over eastern ChinaThe interannual variability of the dominant mode of FEHE underwent an interdecadal increase after the early 1990 s and the spatial characteristics of the dominant mode of FEHE also changed,which was influenced by the western tropical SST and air-sea interaction over the North Atlantic.The interannual variability of the western tropical Pacific SST was increased and the meridional atmospheric wave train associated with the western tropical Pacific SST was enhanced after the early 1990 s,which contributed to the interdecadal increase in the interannual variability of the dominant mode of FEHE.The dominant mode of FEHE was slso influenced by different SST anomalies over North Atlantic before and after the early 1990 s.The anomalous SSTs in the North Atlantic could induce anomalous atmospheric circulation via air-sea interaction and exerted influence on the dominant mode of FEHE via middle-to high-latitude atmospheric wave trains.The interannual variability of the dominant mode of FEHE over eastern China,induced by the associated anomalies of SST in the North Atlantic,was siginificantly increased after the early 1990 s,indicating that the air-sea interaction over the North Atlantic also contributed to the interdecadal increase in the interannual variability of the dominant mode of FEHE after the early 1990 s.The interdecal change in the anomalous atmospheric circulation associated with the western tropical Pacific SST and North Atlantic SST also contributed to the different spatical characteiristics of the dominant mode of FEHE before and after the early 1990 s.(3)The interdecal variability of summer FEHE over southern China in the early2000 s and associated mechanismsThe FEHE over southern China underwent an interdecadal increase after the early 2000 s,which was closely connected to the interdecadal variations of atmospheric circulation over East Asia and Pacific.After the early 2000 s,the Walker circulation over the tropical Pacific was enhanced and induced anomalous divergent water vapor and descending motion over southern China,which provided important climatic conditions for the interdecal increase of FEHE over southern China.The anomalous Walker circulation over Pacific had a close relationship with the Pacific SST anoamlies.Around 2000,the Pacific decadal oscillation(PDO)shifted from positive phase to negative phase.Under the modulation of negative phase of the PDO,there were negative(positive)SST anomalies over the eastern tropical Pacific(the western tropical and northern Pacific),which induced strengthened convection over the western tropical Pacific,suppressed convection over the eastern tropical Pacific,anomalous east wind over the equatorial Pacific and led to the enhanced Walker circulation over the tropical Pacific.Model simulations further demonstrated that the negative phase of PDO was conductive to the enhancement of the Walker circulation over Pacific and the increase of FEHE over southern China.(4)The prediction for the dominant mode of the interannual variability of summer FEHE over eastern ChinaThe prediction capability of the CFSv2 for the dominant mode of summer FEHE was poor.Based on the mechanisms of the dominant mode of summer FEHE,three predictors physically connected to the dominant mode of summer FEHE were found,including the tropical and extropical Pacific SST in the previous April,the snowmelt over the middle-to high-latitude Eurasia from previous April to May and the North Atlantic SST in the previous May.The SST and snow factors could lead to anomalous atmospheric circulation in summer over East Asia through air-sea interaction and land processes respectively,which contributed to the dominant mode of summer FEHE.Finally,the above three predictors were used to establish physicalempirical prediction models.Compared with the prediction of CFSv2,the physical-empirical prediction models could significantly improve the prediction for the dominant mode of summer FEHE over eastern China.