Co-mingled Effects Of Mega-ENSO And The North Hemisphere Mid-high Latitude Atmospheric Circulation Leading Modes

By using newly defined mega-ENSO index,we re-classified the mature phase ENSO events,and then discussed their climatic impacts as well as the physical mechanism.In addition,on the interdecadal timescale,ENSO and North Atlantic Oscillation(NAO),ENSO and two leading modes of North Pacific Atmospheric circulation(the West Pacific teleconnection(WP)and Aleution Low(AL))experienced unstable relations during the past several decades.This study introduced such unstable relationship,as well as the related physical mechanism by using SGCM and ECHAM5.4 models.The main conclusions are as follows:(1)Based on the SSTA intensity in the Nino3.4 region and the gradient in the eastern-western Pacific,El Nino events are classified in the 3 groups.As a response to the weak mega-El Nino,the North Atlantic(NA)sector Eurasia(EA)continent experiences the atmospheric anomaly resembling a negative phase of North Atlantic Oscillation(NAO),which is accompanied by the eastward elongation of a strengthened East Asian subtropical jet and a weakened Atlantic jet.The weakened Atlantic jet benefits the cold and dry air coming down from Arctic to NA-EA,leading to a cooler and drier than normal winter over EA.However,the strong and equatorial El Nino tend to have an unnoticeable impact on the EA-EA climate.Observational and numerical evidences indicate that the prolonged subtropical jet from the North Pacific to the North Atlantic act as a waveguide may enhance the probability of trapping the planetary Rossby waves during their poleward propagation.In contrast,accompanied by the equatorial El Nino with weaker gradient,a relatively weak subtropical jet occurs and impacts NA-EA insignificantly.For the strong El Nino,the absence of the NAO signal may be attributed to the counteracting of the teleconnections excited by Pacific and Indian Ocean.(2)Mega-ENSO has experienced a notable inter-decadal change in its linkage with the winter NAO during the past 56 years:mega-ENSO is significantly correlated with the NAO during 1957-1981(or synchronous epoch),while such correlation has broken down since 1982(or out-synchronous epoch).The synchronous epoch is concurrent with the anomalous tropical North Atlantic(TNA)SST forcing,whereas the out-synchronous epoch is associated with the anomalous extra-tropical North Atlantic(XNA)SST forcing.There is a positive feedback between the TNA SSTA and the NAO-like atmosphere anomalies,which helps to"prolong" the NAO impacts from the developing phase through mature phase of mega-ENSO.Additionally,the TNA SSTA itself may induce a NAO-like atmosphere anomaly.(3)The relationship between ENSO and the West Pacific(WP)teleconnection-Aleutian Low(AL)patterns change notably around the late 1970s.From 1957 to 1975,significant sea surface temperature anomalies(SSTAs)occur mainly in the equatorial eastern Pacific Ocean,the associated atmospheric circulation anomaly pattern resembles the negative phase of WP teleconnection pattern.In contrast,for the 1978-2011 period,SSTA pattern highly resembles the defined areas of mega-ENSO.The associated atmospheric circulation anomaly pattern shows an AL-like mode.Further analysis suggests that a negative-positive anomaly pattern in the 500 hPa geopotential height throughout the entire North Pacific,is possibly triggered by the SSTAs in the extratropical North Pacific associated with the mature phase of ENSO.This circulation pattern in North Pacific is responsible for modulating the relationship between ENSO and the North Pacific atmospheric circulation.