Summer air temperature variation from 1951 to 2007 in northern China

Based on the monthly mean air temperature data in China from 29 weather stations during 1951—2007 and NCEP/NCAR reanalysis data from January 1951 to December 2007,summer air temperature was analyzed in northern China.The temporal-spatial distributions of summer air temperature and its forming reasons were discussed by EOF,REOF,Morlet wavelet analysis,Mann-Kendall mutation testing,tendency analysis and correlation analysis.The results show that the spatial distributions of mean air temperature in northern China display obvious differences during recent 57 years.The consistent characteristics become main spatial distribution mode of summer air temperature,and the second spatial mode also displays the difference of summer air temperature between the south and the north areas in northern China.Summer air temperature in northern China is divided into four spatial distribution types,i.e.circum-Bohai Sea type,the southern type,the northeastern type and the western type.The periodic oscillations of summer air temperature are not well consistent in every division during recent 57 years,while each one exists a period of quasi-18 years.Except that areaⅡ,the other three divisions all exist abrupt changes from 1990 to 1995.There are five hot summer years after abrupt changes.The reasons are as follows: based on the general atmospheric circulation of 500 hPa in hot summer year,there is the most positive advection in Mongolia areas,which indicates that the height field in mid latitude areas of East Asia is abnormally high,in favor of making subtropical high in west Pacific moving to the north and making summer air temperature higher in North China.On the other hand,based on the precipitation anomaly field in flood season of northern China,less precipitation can also lead the hot summer.