Dynamical Analysis Of Interdecadal Changes Around Year 2000 In Properties Of Two ENSO Types

The El Ni?o-Southern Oscillation(ENSO)experienced an interdecadal change around year 2000,and the ENSO properties changed significantly: Compared to 1980-1999,after 2000,the ENSO amplitude weakened,the ENSO period shortened,the occurrence of the central Pacific(CP)El Ni?o increased,the eastern Pacific(EP)El Ni?o became suppressed and the forecasting skill of ENSO reduced.Based on the reanalysis data,the changes in characteristics and dynamical diagnoses of ENSO were thoroughly examined in this study by using statistical and diagnostic methods.And the possible mechanism underlying the observed interdecadal ENSO change was discussed.Such significant changes in ENSO properties,including amplitude,frequency and spatial pattern,were related to the two types of ENSO,which correspond to the two coupled ENSO modes,the quasi-quadrennial(QQ)mode and quasi-biennial(QB)mode.The essence of theinterdecadal change of ENSO is that the relative activity of the two ENSO modes has changed since 2000.During 1980-1999,the QQ and QB modes actively coexisted in the tropical Pacific.But the QQ mode,which is linked to the EP ENSO,was significantly strong.Meanwhile,the ENSO events were mainly EP type.After 2000,the QQ mode became much weaker,and the QB mode became the dominant interannual mode in the tropical Pacific,and more CP ENSO events occurred.After theinterdecadal change of ENSO,ENSO transited from lower frequency mechanism to higher frequency mechanism,and the mainly oceanic dynamic processes of ENSO changed significantly,including the thermocline feedback(TH),the zonal advective feedback(ZA)and the Ekman feedback(EK).The QQ mode is a canonical “recharge oscillator” mode,whose primary mechanism for the phase transition is due to the slow oceanic dynamic adjustment of equatorial heat content,whereas the zonal advective feedback plays a dominant role in the phase transition of the QB mode.With theinterdecadal change of ENSO,the TH termof EP ENSO type weakened,and made less contribution to the phase transition,while the ZA termof CP ENSO type enhanced.Thepositive area of EK termmainly located east of 120°W before 2000,and turned to negative after 2000.The changes of mean states are suggested to be likely responsible for theinterdecadal change of ENSO around 2000.Compared to 1980-1999,the equatorial thermocline tilt steepened,the strength of zonal trade winds weakened in the eastern equatorial Pacific,but strengthened in the western equatorial Pacific.These changes in mean states might lead to changes in TH and ZA terms,and then make the QQ mode weaken,and the QB mode enhance.