Decadal And Subseasonal Variations Of ENSO Impacts On The East Asian Winter Climate And Their Mechanisms

The East Asian winter monsoon(EAWM)is a primary atmospheric circulation system controlling the weather and climate conditions in East Asia and surrounding areas.As the dominant low-frequency climate phenomenon resulting from coupled ocean-atmosphere interactions in the tropical Pacific,ENSO provides the prominent source for EAWM skillful predictability on the seasonal-to-interannual timescale.However,it has been pointed out by many recent studies that the EAWM responses to ENSO are not stationary,which significantly limits the advance of EAWM predictability in dynamical and statistical climate models.Based on a suit of diagnosis analyses for reanalysis dataset and the Atmospheric General Circulation Model(AGCM)experiments or simulations,from perspectives of both interdecadal and sub-seasonal timescales,this work investigates the complexity and its associated mechanisms for ENSO impacts on the East Asian winter climate.Three main conclusions are summarized as follows:(1)The decadal variability of the ENSO-EAWM relationship is investigated.It is found that the AMO modulation is the critical reason for the multi-decadal variations of the ENSO-EAWM relationship.Based on diagnosis analyses for reanalysis dataset,we find the ENSO-EAWM relationship displays prominent multi-decadal variations.A stable ENSO-EAWM relationship is found during the positive Atlantic Multidecadal Oscillation(AMO)phase but not during the negative phase.While the impact of El Nino events on the EAWM is stable and independent of the AMO phase,a different picture is observed for La Nina events.The La Nina boreal winter season coincides with a strengthened EAWM during a positive AMO phase and a weakened EAWM during a negative AMO phase.We suggest that the AMO's modulating effect mainly comprises two pathways that influence ENSO's impact on the EAWM.On one hand,when La Nina coincides with a positive AMO,the warm SST anomalies over the western North Pacific(WNP)are amplified both in intensity and spatial extent,which favors strengthened WNP cyclonic anomalies and an enhanced EAWM.During La Nina with a negative AMO,only very weak SST anomalies occur over the WNP with reduced WNP cyclonic anomalies that are confined to the tropics,thus having little effect on the EAWM.On the other hand,an eastward-propagating Rossby wavetrain across the mid-high latitudes of Eurasia during a warm AMO phase strengthens the Siberian high and thus leads to a strengthened EAWM,while during a cold AMO phase the Siberian high is weakened,leading to a reduced EAWM.In contrast,El Nino and its associated atmospheric responses are relatively strong and stable,independent of the AMO phase.GFDL AGCM simulations also realistically capture the above features and validate our conclusions.(2)A new method for interpreting nonstationary running correlations is proposed and applied to mathematically demonstrate the AMO modulation effects.The Pacific Decadal Oscillation(PDO)has been widely considered as an important factor that could modulate the ENSO-EAWM relationship.In order to reconfirm the AMO's dominant modulation effects,we here propose a new statistical method to interpret nonstationary running correlations by decomposing them into a stationary part and a first-order Taylor expansion approximation for the nonstationary part.Then,a multiple linear regression framework can be utilized to explore possible controlling factors.We apply this method to investigate the nonstationary behavior of the ENSO-EAWM relationship.It is demonstrated that the first-order approximation of the nonstationary part can be expressed to a large extent by the impact of the nonlinear interaction between the AMO and ENSO(AMO*Nino3.4)on the EAWM.In contrast,the PDO can hardly explain the multidecadal variations of the ENSO-EAWM relationship.Therefore,the nonstationarity in the ENSO-EAWM relationship comes predominantly from the impact of an AMO modulation on the ENSO-EAWM teleconnection via this key nonlinear interaction.This general method can be applied to investigate nonstationary relationships that are often observed between various different climate phenomena.(3)It is revealed that ENSO could also exert significant impacts on the sub-seasonal variability of the East Asian winter surface air temperature.In the 2015/16 boreal winter,East Asian surface air temperature exhibited remarkable sub-seasonal variations and a super strong cold event was detected,with record-breaking cold temperatures across the region.We find that this large-scale cold spell can be attributed to the concurrent super El Nino event in the tropical Pacific.Neither moderate El Nino nor La Nina winters underwent this similar sub-seasonal variation.Further analysis reveals that all super El Nino winters(1982/83,1997/98,and 2015/16)were accompanied by a rapid sub-seasonal North Atlantic Oscillation(NAO)/Arctic Oscillation(AO)phase reversal from a positive to a negative state during early January,which was largely caused by the interaction of these super El Nino events with the subtropical jet movement annual cycle.The NAO/AO phase transition leads to a rapidly strengthened Siberian High,which favors southward intrusions of cold air to East Asia and thus causes severe local cooling.Due to the weaker amplitude of the ENSO forcing,these sub-seasonal atmospheric responses cannot be detected for moderate El Nino events.GFDL AGCM simulations also realistically capture the East Asian cold spells associated with the fast sub-seasonal NAO/AO phase reversal,which further convinces our conclusions.