Influence Of Northern Hemisphere High-latitude Forcing On Variability Of East Asian Winter Climate

East Asia is located over the east of Eurasian continent,eastward conjected to the North Pacific,inducing large land-sea thermal contrast over East Asia,thus its climate variation in cold season is under control of the East Asian winter monsoon(EAWM)system.While previous studies have demonstrated the influence from tropical forcing on the EAWM,the increasingly enhanced climate response over the high-latitude region in recent decades,also implies an intensified impact from high-latitude forcing.The competition between tropical and high-latitude forcing in cold season makes a complex climate variation over East Asia,which owes robust subseasonal diversity.This study explores the influence from high-latitude forcing such as Northern Hemisphere snow and the Atlantic Multidecadal Oscillation(AMO)based on observational and reanalysis datasets,revealing dynamic processes and physical mechanisms associated with land-atmosphere coupling,synoptic eddy and low-frequency flow(SELF)interaction that are responsible for the subseasonal climate variation over East Asia,and enriching our knowledge to East Asian climate prediction in cold season.The main conclusions are as follows:(1)This study reveals the impact of North American(NA)snow cover in December on surface air temperature(SAT)over the midlatitude Asia in the following January,based on the period of 1979/80-2014/15.In December,the extensive snow cover decreases SAT over NA and sea surface temperature(SST)over the western North Atlantic,the former further intensifying the Atlantic jet stream by enlarging the meridional gradient of the 1000-to 300-h Pa layer thickness.Meanwhile,we found enhanced transient eddy activity over the eastern North Atlantic due to anomalous net energy conversion from the mean flow via SELF interaction.In January,a zonally-oriented dipole of transient eddy anomalies straddles the North Atlantic,with the negative-positive(west-east)anomaly centers attributing to local oceanic cooling and the persistence of the SELF interaction,respectively.Such dipole of transient eddy activity in turn favors the northward displacement of the Atlantic jet stream and induces a northeastward-southwestward dipole of potential heigh anomalies.Besides,the changes in surface turbulent heat flux over the North Atlantic caused by overlying circulation anomalies,further trigger a Rossby wave train that propagates eastward across the Euraisan continient.The Rossby wave train strengthens the Siberian High and shifts the East Asian polar-front jet stream more southward in January,which decreases SAT over the midlatitude Asia.Hence our analysis indicates that December NA snow cover could potentially be exploited for sub-seasonal predictability over Asia.(2)This study reveals a distinguished teleconnection between February North Atlantic Oscillation(NAO)and March SAT over the Tibetan Plateau(TP),which is out-of-phase and regulated by warm phase of the Atlantic Multidecadal Oscillation(AMO+),based on the study period of 1920-2017.The results show that,in March during the AMO+,the negative phase of February NAO largely maintains.It is then accompanied by a quasi-stationary Rossby wave train trapped on the subtropical westerly jet stream across Eurasia.As a result,the westerly jet over the TP decelerates and moves northward,inducing anomalous adiabatic descent that warms the TP.Whereas in March during the cold phase of AMO,February NAO hardly persists,and there is no aforementioned NAO-TPSAT teleconnection.Further investigation suggests the positive SST anomalies during the AMO+ promote turbulent heat flux into the air over the Gulf Stream,which disturbs overlying atmospheric baroclinicity and shifts the storm track more southward.Accordingly,SELF interaction over the North Atlantic in February and March is enhanced during the AMO+,and help maintain the NAO anomaly pattern via positive eddy feedback.This study provides a new detailed perspective on the decadal variability of the North Atlantic-TP teleconnections.(3)This study shows that snow water equivalent(SWE)over the Ural region in early(1-14)November is in positive association with precipitation over southern China in 15-21 November and 6-15 January,based on the period of 1979/80-2016/17.In early November,increased Ural SWE decreases local air temperature via diabatic cooling,indicative of significant land-atmosphere coupling over the Ural region.Meanwhile,a stationary Rossby wave train originates from the Urals and propagates along the polar-front jet stream.In mid(15-21)November,this Rossby wave train propagates downstream toward East Asia,integrated with the weakened East Asian Trough,increasing precipitation over southern China by enhanced water vapor transport.Afterward,in 22 November-5 January,there is little obvious circulation anomaly due to the weak land-atmosphere coupling over the Urals.In early(6-15)January,the negative SWE anomaly expands southward to the north of the Mediterranean Sea and warms the overlying atmosphere,suggestive of land-atmosphere coupling occurred over western Europe.A stationary Rossby wave train trapped in the subtropical westerly jet stream appears along with anomalous anticyclonic circulation over Europe,and again with the weakened East Asian Trough and abundant precipitation over southern China.The current findings have implications for winter precipitation prediction over southern China on the subseasonal timescale.