Variation Of Extreme Temperature In Northest China And Its Relations With Atmospheric Circulation During Summertime

Based on the analysis of extreme temperature in China during summertime using the daily maximum, minimum and average temperature data of314stations from1960to2010, the spatial and temporal features and regional variation of extreme temperature in Northest China are studied. Based on the NCEP/NCAR monthly mean reanalysis data, the relations between the atmospheric circulation and the extreme temperature in Northest China in summer are analyzed. The results show that:(1) The first EOF eigenvector of the frequency of the summer extreme maximum temperature in China shows the consistent change overall, and20.13%of total variance is explained. The time coefficient indicates that the frequency of extreme maximum temperature increases, in which the increasing trend is obvious after1993, and keeps on high level during1997-2010. The first EOF eigenvector of the frequency of summer extreme minimum temperature explains35.99%of total variance, which shows consistent change all over China. The time coefficient indicates that the extreme minimum temperature frequency decreases obviously, with the significant mutation taking place in the later1980s and the frequency declines more obviousy after the mutation.(2) The first EOF eigenvector of the frequency of summer extreme maximum temperature in Northest China reflects the main variation character, which is the consistent change over the region, with60.3%of total variance. The interdecadal variation trend shows a smooth-decline-rise-decline variation. It can be divided into three variation areas:the southern part, the northestern part and the northwestern part. Their variation trends show much more different before early1980s, afterwards, it shows the decline before early1990s and obvious decline after that. The first EOF eigenvector of the frequency of summer extreme minimum temperature explains35.99%of total variance in Northest China, which shows that the distribution is as same as that of the extreme maximum temperature. The time coefficient takes place mutation in1983, and rise obviously from the late1970s to the early1980s. It can be divided into three variation areas:the southern and central part, the northwestern part, and the central and eastern of Heilongjiang. The variation trends of them are obvious decline. It shows that the frequency decreases obviously from late1970s to early1980s in the central and eastern of Heilongjiang with the mutation taking place in1979, and in other two areas, the frequency decreases during early1980s with the mutation taking place in1983.(3) During the high frequency years of extreme maximum temperature or the low frequency years of extreme minimum temperature, the zonal circulation prevails at500hPa geopotential height field, with negative anomaly in northwestern of Novaya Zemlya polar leading to weakening of the ridge, and with positive anomaly in Northest China leading to strengthening of the trough. There is an equivalent barotropic structure in vertical direction, with a positive anomaly from the surface extending to the tropopause in Northest China, which is useful for the stable maintaining of zonal circulation. Asian continental low shows to be weakened significantly and the sea level pressure decreases in polar region, in such a way that the North-South pressure gradient decreases. The temperature above Northest China is obviously positive anomalies with stronger warm air, which leads to cold air difficult to affect Northest China, and v.v..