Effects Of Winter Circulation Anomalies On Springtime Snow Cover And Land Surface Temperature Over Tibetan Plateau

Subseasonal to seasonal(S2S)prediction,especially that of extreme events as droughts and floods,is one of the major challenges of climate predictions.Recent research suggests the great potentials of land surface temperature(LST)and snow cover on Tibetan Plateau(TP)as new sources of predictability to improve forecast skill for downstream summer precipi-tation.Therefore,study of springtime snow cover,LST on TP and their relationship with atmospheric circulation has important scientific and practi-cal significance for understanding of land-atmosphere interaction on TP and improving of climate prediction on S2S time scale.In this study,statistical analysis and dynamic diagnosis are used to analyze the land processes and interaction with atmosphere over TP through comparison of various datasets such as stations and satellite snow cover data,global reanalysis data and pre-cipitation data.Beyond the research on climatic characteristics of snow cover and climate trends over TP,we further investigate the relationship between springtime LST on TP and previous snow cover and atmospheric circulation.As a result,the connection and mechanism between the mid-high latitude circulation,snow cover and the LST over TP was discovered.We also ex-plored the relationship between the atmospheric circulation pattern causing springtime LST and snow cover over TP and the external forcing such as sea ice and SST.The main conclusions obtained are as follows:(1)The snow season of TP is October to May,with the increase period from October to January or February,and the decrease period from February to May.For the spatial distribution,snow is mainly concentrated in the central,eastern,and southern fringe areas of TP with the number of snow cover days(NSCD)for climatic state about 10-15 days/month in winter.These areas are also where with large interannual variability of snow cover,showing the standard deviation about half of climatic state.Generally,the climate state of snow cover on TP show a downward trend,but with strong heterogeneity in time and space.The most obvious trend of snow reduction period is June to September,while in winter,there are increase trends of snow in the eastern,northeast and southern slopes of TP.(2)It is found that previous snow cover in winter and spring is the main factor affecting the spring LST on TP.When NSCD in middle and eastern TP showing positive anomaly from February to April,the overall surface temperature of TP appeared a significant cold anomaly.We further uncover the most important atmospheric circulation patterns affecting the anomaly of snow cover in winter and spring.It is shown that when the February atmospheric circulation shows negative-positive-negative meridional quasi-tripolar distribution from polar to mid-latitude,there will be two wave trains from polar regions propagate and reach TP through northern and southern paths.That will bring dynamical and moisture conditions for the TP snowfall.As a result,the NSCD from February to April in the central and eastern TP increase significantly.(3)This study reveals that the innate character of above-mentioned atmospheric circulation pattern establishing Arctic-Tibetan connection is a combined circulation of Arctic Oscillation and West Pacific teleconnection pattern.But the wave train generated by the positive AO alone cannot prop-agate to TP.Only when the Pacific exhibits a negative phase of WP,the Arctic-Atlantic wave train can propagate along the north and south paths to TP and form a cyclonic circulation providing dynamical conditions for TP precipitation.At the same time,the negative WP pattern supports water vapor transport to TP in the Bay of Bengal,providing moisture conditions for precipitation.Resultant snow anomaly on TP in February caused by cir-culation pattern can last to springtime through soil memory and feedback,and further influence the downstream summer precipitation through the LST anomaly on TP.(4)The study further shows that the above-mentioned atmospheric cir-culation pattern establishing the polar-tibetan connection has a significant relationship with the early autumn Arctic sea ice.When the circulation pat-tern on positive phase,the early sea ice shows a significant positive anomaly and vice versa.At the same time,the circulation pattern is also significantly associated with the North Atlantic SST tripole and AMO.