| Global warming is one of the most challenging environmental problems for human beings in the 21 st century.As temperatures continue to rise,extremely high temperatures have become more frequent and constantly break historical records.The number of days with continuous heat waves has also significantly increased.Climate change has been greatly affecting societal development,human health,agriculture,ecosystems,and infrastructure.This study focused on summer heat waves in Zhejiang,China.We used historical daily maximum temperature records and monthly Western Pacific Subtropical High(WPSH)area and intensity indices during the period of 1971–2017 from 71 observing stations in the study province.Based on the monthly NCEP/NCAR reanalysis data from 1971 to 2016,we used the data of vertical velocity,vertical wind,zonal wind and geopotential height to screen the hot days by adopting the method of trend analysis,synthetic analysis,and singular value decomposition(SVD).We then investigated the local summer heat waves from four aspects: spatial distribution,interannual variations,processes of development,and impact factors.The main findings are as follows:(1)Zhejiang province consists of six topographical subareas,including the northern plain,western hills,eastern hills,central Jinqu Basin,southern upland,and southeastern coastal plain and island.Using an annual average scale,we detected the highest numbers of hot days and heat wave events in southwestern Zhejing.In particular,annual average number of hot days(heat wave events)was 24.8–40.3(3.34–5.53),24–31.1(2.87–4.04),and 26.3–35.3(3.4–4.53)in the southern upland,the western medium-slope hills,and the central Jinqu Basin,respectively.Since the year 2000,we found that both the number of hot days and annual mean maximum temperature have been significantly increasing in the northern plain,the eastern hills,and the southeastern coastal plain and island.Within these subareas,annual mean maximum temperature increased at a rate of 4.5 °C per century in the northern plain,which was significantly higher than the increase rate for the entire Zhejiang province(1.17 °C per century).(2)During the study period of 1971–2017,the number of hot days in Zhejiang was the highest in year 1971,followed by the four years of 2013,1994(tied with year 2007),2003,and 2017.The annual mean maximum temperature was the highest in year 2013(37.20 °C),followed by the four years of 2003(37.14 °C),2017(36.99 °C),2016(36.79 °C),and 1971(36.73 °C).When combining the two above indices,we defined the top five hot summer years as 1971,2003,2017,2013,and 1994;we also found from the June 500 hpa circulation field that the subtropical high ridge point was to the west of 125°E,and the ridge line was in the vicinity of 20°N ridge for these five years.Meanwhile,the average June 500 hPa height field for the bottom five hot summer years showed that the subtropical high body was southward and eastward,and the westward ridge point was in the vicinity of 135°E,which was away from the mainland China.(3)In Zhejiang,the correlation coefficient between the number of hot days in summer and the WPSH area and intensity indices was generally high in the eastern region and low in the western region.Relatively high correlation coefficient(above 0.4,p ? 0.05)mainly showed in the eastern coastal area,Hangzhou Bay surrounding area,Ningbo,Taizhou,and most of Wenzhou.The highest correlation coefficient(above 0.5,p ? 0.01)was in the south of Taizhou and north central area of Wenzhou,especially in the three observing stations of Rui'an(0.599),Yongjia(0.585),and Wenling(0.568).In addition,the correlation coefficient was relatively high in the south of Jinhua.We concluded that the number of hot days in summer on the east coast was closely related to the WPSH area and intensity indices,while this correlation was relatively weak for the west inland.(4)In Zhejiang,the mean maximum temperature during the hot days in summer in 2017 was 37 °C(ranked No.3 in the study period),which was only 0.2 °C lower than the historical record in 2013.During the hot days in summer in 2017,July mean maximum temperature was 37.4 °C(ranked No.2 in the history),which was 1.0 °C higher than the historical average;August average number of hot days was 28,which was much higher than the historical average of 12.1d and,what's more,this broke the historical record of 26 d in 1998.There were four typical high temperature processes in the summer of 2017,including July 5th–July 8th,July 10th–July 29 th,August 2nd–August 11 th,and August 13th–August 30 th.Of these,the high temperature process during July 10th–July 29 th was the strongest.From July 21 st to 25 th,the average daily maximum temperature in Zhejiang was consecutively above 38°C(above 39°C from 24 th to 25th),which is extremely rare,historically speaking.According to the 500 hpa circulation field during the high temperature process of July 10th–July 29 th,the WPSH was particularly strong and moved northwestward as compared to the previous process.At that time,Zhejinag was located in the vicinity of the subtropical high,which led to the intense and long high temperature process. |
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