Summer Climate Warming And Drying Trend In The Middle Reaches Of The Yarlung Zangbo River In Recent 57 Years And Its Possible Causes

Based on the observation data of the monthly mean temperature,precipitation and relative humidity in summer(June-August)during the recent 57 years(1961-2017)at4 stations(Lhasa,Shigatse,Tsedang and Gyangze),as well as the NCEP/NCAR rean alysis datasets during the same period,variation characteristics of the summer climate on multiple time scales and potential impact mechanism were analyzed using various statistical methods in the middle reaches of Yarlung Zangbo River where is the hinte rland of the Tibetan Plateau taken as the typical research area in this study.The resul ts as follows:(1)In the past 57 a,the characteristics of summer climate in this region exhibited a warming and drying trend in which the temperature(relative humidity)increase(de crease)significantly,while the precipitation trend is unapparent.Moreover,there is an inter-decadal change in the trend of warming and drying in that from the early 1960 s to the early 1980 s,it was in the cold and wet period;at the beginning of this centu ry,the temperature and relative humidity experienced a significant climate change,and then it entered the high temperature and low humidity period,with the trend of war m and dry climate being more prominent.(2)In addition,the significant warming in summer in this region has resulted in the continuous increase of the saturated water vapor pressure in the air,while the pre cipitation has not changed obviously,that is,the variation of actual water vapor press ure is indistinctive,leading to a significant decline in the relative humidity.This also means that the warming and drying of the summer climate in this region in the past57 a is the result of long-term changes in the two major climate factors of precipitatio n and temperature.(3)What's more the potential height of 500 h Pa over the plateau has significant ly increased in the past 57 a,and the plateau is in a relatively high-pressure system wi th continuous enhancement,resulting in an increase in events in the relatively clear sky,a significant reduction in cloud cover and a significant increase in temperature.Wh en the upper Ural mountain and the Mongolian plateau high-pressure are relatively str ong(weak)at 500 h Pa at the same time,the two high-pressure anticyclonic shear force s the abnormal cyclone(anticyclonic)circulation between them,and the abnormal cycl one(anticyclonic)circulation inhibits(guides)the cold air invading the plateau,resulti ng in high(low)temperatures.(4)Last but not least,the anomalous anticyclone circulation over the northern par t of the Indian peninsula caused by the north Indian monsoon and the west wind ano maly at the upper reaches of the plateau caused by the NAO(North Atlanta Oscillatio n),which cooperatively impact on the water vapor transport,leading to the annual fl uctuation of summer precipitation in the region.The west wind anomaly is an importa nt medium for NAO to influence the summer precipitation in this region through the upstream and downstream effects,and the west wind in the upper plateau can modulat e the Indian monsoon.