2018: The Hottest Summer in China and Possible Causes

In 2018,China experienced the hottest summer since 1961.The maximum,mean,and minimum temperatures all reached the highest.Air temperatures in most regions were much higher than normal;in northern China especially,the temperature anomalies were above double of the standard deviations.Consistent variations of temperature anomalies appeared in the national mean and in northern China on different timescales from intraseasonal to annual,indicating that the above normal temperature in northern China contributed significantly to the record-breaking hot summer of entire China.Relationships among the high temperature in summer 2018,the tropospheric circulation,and the global sea surface temperatures(SSTs)are further analyzed.It is found that the intensified and more northward western Pacific subtropical high(WPSH),weakened Northeast China cold vortex(NECV),and positive geopotential height anomaly from northern China to the Sea of Japan resulted in the abnormally high temperature in summer 2018.From late July to mid August,the WPSH was stronger than normal,with its ridge line jumping to north of 40°N;meanwhile,the NECV was much weaker and more northward than normal;both of the two systems led to the persistent high temperature in northern China during this period.In addition,the SSTs in Kuroshio and its extension area(K–KE)in summer 2018 were also the highest since 1961 and the greatest positive SST anomaly in K–KE was favorable for the above normal geopotential height over North China–Northeast China–Japan at 500 hPa,giving rise to the exceptionally high temperature in northern China.