Temporal And Spatial Variation Characteristics Of Snow Depth On The Qinghai-Tibet Plateau And Its Impact On Summer Intraseasonal Oscillations

The snow cover over the Qinghai-Tibet Plateau(TP)is an important physical factor affecting climate.It plays an important role in regional water and energy balance,and influences Asian monsoon system through atmospheric circulation.The Boreal Summer Intraseasonal Oscillation(BSISO)is a significant part of Asian monsoon system,whose convection and circulation anomalies can change the background conditions of weather systems.BSISO includes BSISO1 mode with a period of 30-60 days and BSISO2 mode with a period of 10-30 days.BSISO1 and BSISO2 can be divided into 8 different phases according to the changes of their convective activities.In this study,the temporal and spatial variation characteristics of snow depth in winter and spring in different regions of TP were analyzed by using the 1979-2016 long-term snow depth data set provided by Cold and Arid Region Environmental and Engineering Research Institute,Chinese Academy of Sciences.Based on the changes of snow depth over the TP(TPSD),years with more and less snow were identified;the relationship between TPSD and BSISO was analyzed by statistical methods.Furthermore,the physical mechanism of snow cover over the TP affecting BSISO is discussed.The main results are as follows:(1)The snow cover of the TP mainly exists from October to May of the next year,and the peak of snow depth appears in January except the western TP over which the peak of snow depth exists in February.The result of EOF analysis shows that the first three modes of TPSD in spring and winter all pass the significance test.The first mode of TPSD shows a uniform change of snow depth for the entire TP.The second mode of the TPSD in the spring shows a opposite change of snow depth for the eastern TP and the western TP.However,the second mode of winter TPSD indicates an opposite change of snow depth over the northern TP and the southern TP.From the time coefficient of first mode,the TPSD has changed from snowy to snowless status in the interdecadal time scale since 1980 s.(2)During 1981-2016,the two kinds of modes of BSISO generally show the transition of "inactive-active-inactive".The inactive period of BSISO1 is 1981-1995 and 2010-2016,and the active period is 1996-2009;while the inactive period of BSISO2 is 1981-1992 and2008-2016,and the active period is 1993-2007.(3)The strong convective regions of intraseasonal oscillation in the northern hemisphere are mainly distributed in the Indian summer monsoon region and the East Asian summer monsoon region.The BSISO1 is active from May to October and reaches the maximum in August.The strength of BSISO2 reaches the maximum in June,and it is active throughout the year.The results obtained by correlation analysis show that there is a negative correlation between spring TPSD and BSISO1 intensity with a correlation coefficient of-0.3.There is a positive correlation between spring TPSD and the frequency of the fourth phase of BSISO1 with a correlation coefficient of 0.55.(4)The snow depth anomaly over the TP can influence the BSISO by affecting the atmospheric heat source and monsoon circulation.The fourth phase of BSISO1 is associated with active convection in the Marine Continent(MC).When there is more snow over the TP in spring,it can weaken the heating source over the TP,and change the circulation,and strengthen the vertical easterly shear in India and the equatorial Indian Ocean in summer,which is conducive to the BSISO convection spreading eastward and northward to the MC region.Therefore,the BSISO1 convection is more likely to appear in the fourth phase when the snow cover increases in spring over the TP.