Impact Of Tibetan Plateau Snow Cover On Eurasian Climate:from The Winter NAO And Developing ENAO Perspectives

As global warming continues to intensify,more extreme climate events emerge in Euransia.The Tibetan Plateau snow cover(TPSC)shows high altitude and premium boundary and thermal condition,whose climate effects over Eurasia have received most of the research and media attention.Although breakthrough progresses about TPSC have been made by the preceding studies,comprehensive understandings about Eurasian climate change still need further research.As the most important signals on interannual timescale,the winter North Atlantic Oscillation(NAO)and the El Ni?o Southern Oscillation(ENSO)can not be ignored.By using the monthly dataset of the TPSC provided by the Global Snow Lab(Rutgers University)(http://climate.rutgers.edu/snowcover)during 1970-2014,we analyzed its seasonal variation pattern and found out the key role of TPSC in Eurasian climate change(winter surface temperature and summer precipitation)from the winter NAO and developing ENSO perspectives.During fall and winter,the preceding TPSC may impact the Eurasian climate change through modulating the winter NAO variability since the late 1990 s.In addition,based on the close relation between the TPSC and the developing ENSO during spring and summer,we suggested that the western TPSC may act as a “capacitor” that helping ENSO developing signal to modulate the summer Yellow River precipitation.Our study introduces such possible relationships and the underlining mechanisms are investigated through the ECHAM5.4 models.The main conclusions are as follows:Since 1999,relatively weak background westerly wind prevails in high latitudes over Eurasia,the more TPSC in late autumn is usually accompanied by a local anomalous high in Mongolian region.Thus,the westerly anomaly discovered to its northern side gradually extends over the entire Eurasia from the surface all the way up into the stratosphere.When the troposphere and stratosphere are closely coupled,the strengthened polar vortex and the upstream positive NAO in winter may triggere warm anomalies over Eurasian mid-high latitudes.The opposite situation occurs when less TPSC dominates.With the recently rapid emissions of greenhouse gases,Arctic regions are losing more sea ice.Such situation may decrease the temperature gradient between the polar and mid-high latitude regions and cause the decelerate of the subpolar westerly wind,which can modulate the intensity and amplitude of the TPSC's impact on the Arctic regions.Thus,the preceding autumn TPSC would become one of the possible precursors controlling the winter NAO and then the Euransian winter climate in the coming decades.Besides,our study shows that the spring and summer western TPSC(WTPSC)may also act as a “capacitor”,helping ENSO signal to reach the Yellow River region.During the El Ni(?)o developing spring,the associated diabatic heating in Pacific region can excite an anomalous cyclone over the plateau and anomalous upward flows over the western plateau.Such circulation configuration favors an excessive WTPSC anomaly in spring.The more WTPSC may increase the surface albedo,decrease the absorbed net shortwave radiation and in turn intensify the WTPSC.Through such snow-albedo feedback process,the excessive WTPSC anomaly may strengthen and persist through summer,which may induce two noticeable wave trains in the upper and lower troposphere propagating northeastward to the Yellow River region.Associated with the wave trains,a low pressure anomaly prevails over northeast China.To the southwest side of the anomalous low pressure,the abnormal northerly wind may bring large volumes of dry cold air with little moisture to the Yellow River region,leading to the anomalous drought there.During the La Ni?a developing summer,the situation tends to be opposite.As such,the ENSO associated influence is tied to the interannual variations of the following summer Yellow River precipitation,with the development of ENSO from spring.