Analysis Of Spatial And Temporal Variations Of Snow Depth Over Tibetan Plateau And Its Cause Under The Background Of Elevation-dependent Warming

Snow depth over the Tibetan Plateau(TP,hereafter)is very sensitive to climate change,and its change will affect the material cycle and energy flow process,and thus have a greater impact on climate,hydrology and ecosystem.Therefore,it is very essential to study the spatial and temporal characteristics of the snow depth over TP.Snow is significantly affected by temperature,precipitation and topography.At the same time,the significant warming of the TP in recent decades has an obvious elevation effect,so it is one of the crucial problems to be solved to explore the response of snow depth to elevation-dependent warming.The variation of snow depth and its causes,as well as its response to elevation-dependent warming are studied by using surface observational snow depth,temperature and precipitation,as well as SMMR-SSM/I remote sensing passive microwave snow depth data,combined with CRU climatic dataset.The main results are as follows.(1)In recent decades,the surface temperature of TP has been warming obviously,and the isotherm of 0? has been decreasing to the high-elevation area.The TP has experienced a general warming phenomenon.Additionally,a significant phenomenon of elevation-dependent warming is observed over TP.The warming in winter is more obvious than that in summer,and the warming in high elevation area is more obvious than that in low elevation area.The phenomenon of warming is more obvious in winter than that in summer,and in high-elevation area than that in low-elevation area.(2)Based on the datasets from surface observation and remote sensing passive microwave,the spatial distribution of snow depth over the TP is very uneven,with high values in the south of the TP(south slope of the Himalayas),the high-elevation areas in the river source area in the center of the TP(Nianqing Tanggula Mountain-Tanggula Mountain-Bayankala mountain-Animaqing mountain),the Qilian Mountain area in the northeast of the TP and the western edge of the TP.And the high value area of snow depth often corresponds to the severe interannual fluctuation.The average snow depth in winter reaches the maximum within the year,which can lead the change of snow depth within the year.The lower value area of snow depth is located in the Qaidam Basin and low-elevation area around the edge of the TP.(3)There are spatial differences in the annual variation trend of snow depth.In the long run,the trend is mainly decreasing,especially in the center and east of the TP in summer and autumn.The alpine areas in the northeast and middle of the TP are the main areas of snow depth reduction.At the same time,under the background of general warming and wetting over the TP,the snow depth changes show the inter-decadal characteristics,which is not continuous and consistent,and is more obvious in the southeastern TP.In terms of monthly changes,the northeast of the TP is dominated by single peak,while the southeast is dominated by double peak.The passive microwave data shows the snow depth mainly decreased in the past decades,accounting for 74.27% of the total TP area.The snow depth decreased faster in winter than that in other seasons,and the decreasing trend was the fastest in Qiangtang plateau and the southwestern TP.(4)The influence of elevation on snow depth is that snow depth increases with elevation,but the deepest snow depth zone does not appear at the highest elevation.The influence of temperature and precipitation on snow depth is that the snow depth in spring,summer and autumn is dominated by temperature,while that in winter is dominated by precipitation.Different combinations of temperature and precipitation will form different spatial patterns of snow depth.The significant decrease of snow depth in summer and autumn from 1961 to 2014 is mainly related to the significant surface warming over the TP in recent decades.The increase of snow depth in winter and spring from 1961 to 1998 is related to the significant increase of precipitation,and the increase of temperature and the weak decrease of precipitation after 1998 together lead to the decrease of snow depth over the TP.The passive microwave snow depth data shows the annual average snow depth over the TP decreased significantly from 1980 to 2014(-0.24cm/decade),which was obviously related to the significant surface warming(0.30 ? /decade)in the same period.(5)On the sub-region-scale average,with the increase of elevation,the decreasing rate of snow depth is accelerated,existing an obvious elevation-effect,and the decreasing rate in cold season is faster than that in warmer season.Similarly,the temperature also has an obvious elevation effect,which explains the elevationdependent reduction of snow depth.The response of snow depth to elevation-dependent warming is that the rate of snow depth decrease is faster in the high elevation areas,which is related to the faster warming in the high elevation areas.The precipitation has no obvious phenomenon of elevation-dependent changes.In the southeast of the TP,there are significant elevation dependent warming phenomena in sub-region IC1(Southern Qinghai high-cold meadow steppe zone),?C1(Southern Xizang montane shrub-steppe zone),IB1(Golog-Nagqu high-cold shrub-meadow zone)and ?AB1(Western Sichuan-eastern Xizang montane coniferous forest zone).However,the decreasing rates of the trends of snow depth in ?B1,?AB1,OA1(Southern slopes of Himalaya montane evergreen broad-leaved forest zone)and ?C2(Eastern QinghaiQilian montane steppe zone)are accelerating with the increasing elevation.