Spatial-temporal Characteristics Of The Snow Depth Over The Tibetan Plateau And The Cause Of Its Interannual Variability

Snow cover across the Tibetan Plateau(TP),as a sensitive factor to climate change,the interannual variation of which directly influence water and energy balance and then influence larger scale atmospheric circulation.It is necessary to study the spatial and temporal feature of snow cover over the TP and the cause of its interannual variation.In this study,we used ground meteorological station and SMMR-SSM/I derived observation data to analysis the spatial and temporal characteristics of snow depth over the TP in early winter(November-following January)and late winter(February-April),and then revel anomalous circulation and water vapor patterns.Circulation anomaly and Singular value decomposition pattern of snow depth and horizontal wind speed indicate that interannual variation in TP snow depth is closely associated with the inverse change of North Tibetan Plateau jet(NTPJ)and South Tibetan Plateau jet(STPJ).This paper mainly discusses the associated physical mechanism,external forcing and internal atmospheric dynamic process.Snow depth over the TP shows significant interannual variation.Spatial heterogeneity of snow cover is notable: snow depth in western TP dominates the interannual variation in snow depth across the TP;interannual variation of snow depth in the high altitudes is more notable.In positive snow depth years,Lake Baikal ridge strengthens and cold air tends to move southward;subtropical high moves northward and becomes stronger,with water vapor flux in its peripheral area strengthens;southern branch trough and STPJ intensify.Meanwhile,water vapor flux from low latitudes increases on the southern boundary of the TP,and water vapor flux from western and northern TP boundary decrease,reducing the net loss of water vapor in TP.Distribution of this circulation pattern is conducive to the confluence of cold air and warm air.Situation in negative TP snow depth years are opposite to positive TP snow depth years.Strength and north-south shift of NTPJ and STPJ change in verse.Coordinate variation in strength of STPJ and NTPJ significantly modulates interannual variation of TP snow depth.For the conjunction of intense STPJ and weak NTPJ,an anomalous cold low-pressure vortex circulation pattern is observed over the TP.The vortex extends across the TP and spans from the ground surface to the upper troposphere.There is anomalous ascending motion above the TP due to secondary circulations immediately south and north of STPJ,with increased moisture flux from the southwest.These circulation structures cause significant cooling and increased precipitation,thus promoting snowfall and snow accumulation.Temperature is a more important influence than precipitation on snow accumulation.Cooling over the TP is caused by cold temperature advection due to intensely cold air and weakened descending adiabatic heating due to anomalous ascending motion.Local moisture and evaporation reduce,and anomalous ascending moisture advection leads to more precipitation and snowfall over the TP.From the perspective of atmospheric teleconnection,AO dominates interannual variation in strength of NTPJ and STPJ and modulates snow depth variation through NTPJ and STPJ.ENSO is a significant signal modulating interannual variation of NTPJ and STPJ in late winter.ENSO in early winter is likely to be a predictor forecasting TP snow depth in February and March,however,the connection between them notably decrease in April.From the perspective of internal atmospheric dynamic process,when Eddy growth rate significantly decrease to the north side of TP in late winter,baroclinic eddies decrease and the strength of NTPJ weakened notably,where cold air tends to move southward toward TP.When Eddy growth rate significantly increase to the south side of TP in late winter,baroclinicity increases and the strength of STPJ intensifies notably.Besides,when synoptic-scale transient eddy activity significantly decrease to the north side of TP,energy converses from mean flow to transient eddy through convergence of (?) vector,causing the decrease of NTPJ.When synoptic-scale transient eddy activity significantly increase in the mid and north TP,energy converses from transient eddy to mean flow through divergence of (?) vector,intensifying STPJ.