A Study Of The Impacts Of ENSO And EAWM On The Winter Climate Over East Asia

The impacts of El Nino-South Oscillation(ENSO)and the East Asian winter monsoon(EAWM)on the winter climate over East Asia(EA)are investigated using observational data,the output of seasonal forecasts of air-sea coupled numerical models and a simple linear baroclinic model.The main results are summarized as follow.1)Clear shifts of the winter precipitation over southeastern China are observed around the mid-1980s and late 1990s,respectively.Therefore,the data are divided into three subperiods,i.e.,1969-1987(P1),1988-1997(P2)and 1998-2014(P3).It reveals that in these three subperiods,the winter precipitation associated with EAWM and ENSO shows similar pattern over southeastern China.Corresponding to weaker-than-normal EAWM and El Nino events,the positive precipitation anomalies moves northward from P1 to P3,with positive precipitation anomalies centered over South China in P1,over Jiangnan region in P2 and over the mid-to-lower Yangtze region in P3,respectively.Partial regression analysis further indicates that,without the contribution of ENSO,the EAWM-related positive precipitation anomalies over southern China are significant only in P1.In constrast,corresponding to an EAWM-independent El Nino event,significant negative precipitation anomalies are observed over Jianghuai and Huanghuai region in P1 and P2,wheres significant positive precipitation anomalies are noticed over South China.2)Diverse impacts of ENSO on the winter climate over EA are investigated in this paper.ENSO events are divided into four categories according to their amplitude.The impacts of moderate La Nina events on the winter surface air temperature(SAT)variability are more pronounced compared to those of the other three kinds of ENSO events.During moderate La Nina winters,a strong anomalous high system is observed over the mid-to high-latitude Eurasian continent,with strong northeasterly anoamlies along its southeastern flank,which brings intense cold air from the high-latitude continental interior and penetrates all the way down to the subtropical region,leading to significant negative SAT anomalies over EA and the surrounding oceans.Moreover,when the extreme EAWM years are removed,the moderate La Nina-related significant negative SAT anomalies still can be seen over the mid-to high-latitude EA.Further analysis of individual moderate La Nina events and idealized numerical experiments suggest that the anomalous sea surface temperature(SST)distribution with narrow band-shaped negative SST anomalies over the central tropical Pacific and narrow band-shaped positive SST anomalies over the subtropical western North Pacific aroud 20°N can generate an anomalous low system over the mid-latitude North Pacific.This anomalous low system enhances the anomalous northlies along its western flank over the EA continental area,resulting in a colder-than-normal winter over EA.3)The modulation effects of the Pacific Decadal Oscillation(PDO)to the impacts of moderate ENSO events on the winter climate over EA are further analyzed.It shows that when PDO is in its warm phase,moderate La Nina events have a remarkable influence on the winter SAT variability over EA,whereas the relationship between moderate La Nina events and East Asian wintertime SAT becomes less significant during a cold PDO phase.Two factors may contribute to the pronounced impacts of moderate La Nina events during a warm PDO phase.First,the cold PDO background is more favourable to the negative sea level pressure anomalies over the mid-latitude North Pacific.Second,during a warm PDO phase,the moderate La Nina-related SST anomalies are characterized as narrow band-shaped negative SST anomalies over the central tropical Pacific and narrow band-shaped positive SST anomalies over the subtropical Pacific,which are also favourable to the formation of the anomalous low over the mid-latitude North Pacific.