The Study Of The Interdecadal Relationship Between Victoria Mode And ENSO In The North Pacific And Its Oceanic Channels

Previous studies have demonstrated that the Victoria mode(VM),defined as the second empirical orthogonal function mode(EOF2)of monthly sea surface temperature anomalies(SSTAs)in the North Pacific poleward of 20°N,can trigger the onset of the El Ni?o/Southern Oscillation(ENSO)event through the processes of the surface air–sea coupling and the evolution of subsurface ocean temperature anomalies along the equator associated with the VM.Previous studies mainly focused on the interannual relationship between the VM and ENSO,and the subsurface mechanism of VM affecting ENSO remains unclear.In this study,we use some common statistical methods such as empirical orthogonal function(EOF)decomposition and correlation analysis to explore the interdecadal change in the lagged relationship between the VM and ENSO and the ocean channel in which the VM affects ENSO.Main conclusions are as follows:(1)We find that the VM–ENSO relationship exhibits a distinct interdecadal variation with significant positive correlations mainly during two periods(1957–1964 and1981–2004),but with weak or negative correlations during four periods(1907–1924,1926–1956,1965–1980 and 2005–2008).To investgate possible drivers of this interdecadal change in the VM–ENSO relationship,we compared surface air–sea coupling and subsurface ocean temperature evolutions associated with the VM during high correlation(HC)and low correlation(LC)periods.During the HC periods,significant positive sea surface temperature anomalies associated with the VM can extend sufficiently into the western-central tropical Pacific,resulting in the anomalous westerlies there and leading to an increase in the zonal SSTA gradient.Then it can warm the central-eastern equatorial Pacific during the following winter.In addition,significant positive subsurface temperature anomalies associated with the VM propagate eastwards and upwards subsequently reaching the central-eastern equatorial Pacific and warming the surface ocean.Ultimately,these two processes make the central-eastern equatorial Pacific warm enough to trigger an ENSO event.On the contrary,these two processes are too weak,hence causing a weakened VM-ENSO relationship during LC periods.The interdecadal change in the VM-ENSO relationship may be mainly attributed to the interdecadal change of the intensity of the VM,which has been found to be influenced by the NPO.(2)This paper finds that STC,as an ocean channel,plays an important role in the process of the VM affecting ENSO.We find that the impact of the VM on shallow STC is greater than that of deeper level.The VM has significant lead-lag correlation with STC of different depths.This correlation is getting better and better when VM leads STC for 12 months.After removing the STC signal in the VM index,it is found that the impact of VM on ENSO is significantly weakened,and we speculate that VM affects ENSO events through STC.At the same time,the seasonal evolution of SSTAs and 850 h Pa wind anomalies associated with STC is consistent with the surface air-sea coupling process of VM affecting ENSO.Therefore,it is speculated that VM affects ENSO through STC,and STC can be used as a oceanic channel to connect VM and ENSO.These results investigate the relationship between the VM and ENSO,and the deeper understanding of the relationship between VM and ENSO will help to improve the prediction skills of ENSO and further help to improve the theoretical framework of ENSO prediction.It has certain practical application value.