Interdecadal Change In The Relationship Between The El Ni(?)o-Southern Oscillation And The North/South Pacific Meridional Mode

The El Ni(?)o–Southern Oscillation(ENSO)is the most prominent interannual climate variability in the tropical Pacific.It has profound impacts on weather and climate worldwide via atmospheric teleconnections.Therefore,improvements in our understanding and ability to predict ENSO are of great practical value.Traditional theories of ENSO emphasize the role of air–sea coupling processes in the tropical Pacific.As the diversity of ENSO increases,extratropical sea–air variability may be an important deterministic factor in terms of ENSO predictability.The North and South Pacific Meridional Modes(NPMM and SPMM)are subtropical ocean–atmospheric coupling variabilities that are significantly related to the ENSO,but the influence of the NPMM and SPMM on the following ENSO is not constant.Using observation and reanalysis data and 35 Coupled Model Intercomparison Project Phase 6(CMIP6)models,this paper analyzed the interdecadal relationship between the NPMM/SPMM and ENSO,which is helpful to improve the understanding and prediction of ENSO.Results show that there has been a clear interdecadal change in the lagged relationship between NPMM/SPMM and ENSO by analyzing sea surface temperature data from Hadley center and sea surface wind from NCEP(National Centers for Environmental Prediction)during 1948–2018.By calculating the 15-year sliding lead–lag correlations of Ni(?)o3.4 index with NPMM/SPMM indices,it can be found that there is a significant and close relation between SPMM and ENSO during 1956–1980(period1,PD1),with SPMM leading by 10 months;while the significant and close correlation between NPMM and ENSO appears during 1981–2005(period 2,PD2),with the former leading by 10 months.The thermodynamic coupling plays an important role in the interdecadal relationship of SPMM–ENSO.The coupling strength of SPMM is calculated as the correlation coefficient between the wind index and the 1-month lagged SST index.The atmosphere–ocean coupling processes sustain the sea surface temperature anomalies induced by the subtropical sea surface wind anomalies.These signals migrate from the subtropical to the central tropical Pacific through wind–evaporation–SST(WES)feedback,resulting in anomalous westerly winds along equator that are conducive to the initiation of the subsequent El Ni(?)o event.The SPMM coupling strength shows a character that is similar to that of the SPMM–ENSO relationship.An intensified sea surface temperature variability results in a stronger air–sea coupling strength for SPMM before 1980 than that after 1980,in which WES feedback plays an important role.Consequently,the influence of the SPMM on the following ENSO is stronger in PD1 than in PD2.The ocean dynamics plays an important role in the interdecadal relationship of NPMM–ENSO.The trade wind charging(TWC)mechanism details ocean dynamics in linking NPMM and ENSO,in which that NPMM-related subtropical Pacific windstress curl can induce subsurface temperature anomalies during boreal winter,their eastward and upward propagation along the thermocline favors the development of El Ni(?)o events.Mixed layer heat budget analysis shows that the stronger zonal advective and thermocline feedbacks associated with the NPMM contribute to ENSO onset in PD2.At the same time,subsurface ocean temperature evolution induced by the TWC mechanism in the equatorial Pacific associated with NPMM in PD2 is more significant than that in PD1,which is responsible for the better relation between NPMM and ENSO in PD2.Changes in the interdecadal connection between NPMM/SPMM and ENSO can be attributed mainly to the phase change of the Pacific Decadal Oscillation(PDO).During the positive phase of PDO,anomalous westerly winds reduce the intensity of Pacific trade wind.The relaxation of Pacific easterly wind give rise to less warm water accumulation in the western equatorial Pacific,inducing a shallower thermocline over here.At the same time,it also weakens the upwelling in the eastern Pacific,resulting positive thermocline depth anomalies generated here.A shallower thermocline in the central Pacific is responsible for the stronger TWC mechanism,which leads to a stronger equatorial subsurface temperature evolution.This dynamic process strengthens the connection between NPMM and ENSO.Associated with the negative phase of PDO,a shallower thermocline over southeastern Pacific allows an enhanced WES feedback,strengthening the connection between SPMM and ENSO.Using 35 CMIP6 models,we also examine NPMM/SPMM performance and its connection with ENSO in the historical runs(1948–2014).The great majority of CMIP6 models can well reproduce the pattern and the seasonality of NPMM and SPMM,but reveal discrepant NPMM/SPMM and ENSO relationship.The intermodel uncertainty for the connection of NPMM–ENSO is due to different TWC mechanism.A stronger TWC mechanism will enhance NPMM forcing.For SPMM,few models can simulate a good relationship with ENSO.The intermodel spread in the relationship of SPMM and ENSO owing to sea surface temperature bias in the southeastern Pacific,as WES feedback is stronger when the southeastern Pacific is warmer.The NPMM and SPMM have made important contributions to ENSO diversity.The NPMM tends to be associated with the onset of Central Pacific ENSO events,while the SPMM is considered to favor the development of Eastern Pacific ENSO events.Through a simple statistical prediction model,it can be found that combining the NPMM and SPMM can significantly enhance forecast skill of sea surface temperature anomalies in the Pacific basin.