Objective Identification And Multi-scale Control Mechanism Of Extreme Heat Waves In Southern China

Southern China(SC)is often subjected to the impacts of extreme heat-wave(EHW)events with large-scale extended-period hot days during the boreal summer.The present study explores new objective identification methods of the EHW events and investigates the controlling mechanisms of different temporal variations in shaping the EHW events over SC from 2000 to 2017 with in-situ observations and latest reanalysis.According to the selection method of heat wave events in this study,43 heat wave events occurred in SC from 2000 to 2017,and the start time of the events was almost July.A compound index of EHW(with impact area,duration and magnitude)has been defined to quantify the overall intensity of EHW events in SC.The larger the compound index,the more extreme the heat wave events,which can objectively and effectively identify the EHW events in SC.The analysis illustrates that the synoptic variability and10-30-day intra-seasonal variability(ISV)control the onset of the long-term EHW events,while 30-90-day ISV shapes the durations;the 10-30-day ISV is the key to the maintenance in short-term EHW events.An innovative daily compound index of the EHW has been introduced,which is found to well track the outbreak of EHW events.The occurrence of EHW in SC is coincident with the arrivals of intra-seasonal signals(e.g.,the anomalies of 500 h Pa geopotential height and surface air temperature)propagating from the east and south.About 12 days before the onset of the EHW in SC,the 10-30-day positive anomalies of velocity potential and OLR appear near the Maritime Continent,which then propagate northwestward.At the same time,the30-90-day positive anomalies propagate eastward from the Indian Ocean to South China sector to influence the development of the EHW events.With the positive anomalies of30-90-day ISV spreading eastward and far away from SC,the EHW event gradually weakened and finally ended.On interannual time scale,it is found that the EHW events in SC occurred in years with robust warming of the West Pacific in early summer(MJ)and warming of the East Pacific in preceding winter(DJF).The interannual SST anomalous index,which adds the SST anomalies over the above two regions together,serves as a very useful seasonal predictor of the EHW occurrence in Southern China at least one-month ahead.