Favorable Wind Stress Structure And Mechanism For The Development Of The Central Pacific El Ni(?)o

ENSO(El Ni(?)o-Southern Oscillation)is the largest interannual climate variability mode in the tropical Pacific,and has important impacts on the global climate,marine and terrestrial ecosystems,and social economy.In the past few decades,the complexity of ENSO has become more and more prominent,mainly manifested by the frequent occurrence of CP-El Ni(?)o(central Pacific El Ni(?)o)events.Compared with the traditional EP-El Ni(?)o(eastern Pacific El Ni(?)o),CP-El Ni(?)o,as a new type of El Ni(?)o event,has great difference in its spatio-temporal process and impact on global climate.Therefore,the in-depth study of the generation process and mechanism of CP-El Ni(?)o is of great guiding value and practical significance for better prediction of ENSO in the future.Many previous studies have shown that wind stress anomalies play an important role in the generation and evolution of CP-El Ni(?)o.However,the wind stress anomalies corresponding to different CP-El Ni(?)o events are also different,which makes it hard for previous studies to avoid overgeneralizing the timing and location of the winds that indeed matter.Based on the adjoint sensitivity method of GECCO(German Estimating the Circulation and Climate of the Ocean),the favorable wind perturbations(FWPs)which can theoretically increase sea surface temperature(SST)in Ni(?)o-4 region are given for the first time,including direction,horizontal structure,location,the specific month before the peak month of CP-El Ni(?)o(December),and their mechanism of promoting the generation of CP-El Ni(?)o.In the first half of the developing year of CP-El Ni(?)o,FWPs are mainly easterly perturbations over the central equatorial Pacific and westerly perturbations over the northern and southern sides of the equator and the western tropical Pacific;FWPs are mainly westerly perturbations over the western and central tropical Pacific in the second half of the developing year of CP-El Ni(?)o.This study innovatively reveals that the easterly perturbations with decreasing intensity along the north-south direction of FWPs over the central equatorial Pacific in the first half year is conducive to the generation of CP-El Ni(?)o.Easterly perturbations with decreasing intensity along the north-south direction in the central equatorial Pacific can produce strong anticyclonic wind perturbations curl,which is conducive to the excitation of offequatorial downwelling Rossby(DR)waves.DR waves propagate westward and reflect as downwelling Kelvin(DK)waves at the western boundary,and reach the central equatorial Pacific around the peak period of CP-El Nino.The zonal advection feedback generated by the accompanying zonal flow anomalies can effectively promote the sea surface warming in the Ni(?)o-4 region.Meanwhile,the ocean-atmosphere coupling experiments results show that,on the one hand,the unique easterly wind structure over the equatorial central Pacific in the first half year results in cold anomaly and triggers Bjerknes feedback.On the other hand,due to Gill’s response,the strong wind stress curl of the easterly wind structure off the equator results in warm SST anomalies off the equator,which triggers the DR waves propagating westward,causing the SST warming in the northwestern tropical Pacific.DR waves reflect as DK waves at the western boundary,causing SST warming in the equatorial western-central Pacific.Those warm SST anomalies in the northwestern tropical Pacific and western-central equatorial Pacific induce westerly anomalies,which develop eastward through the Bjerknes feedback.Since the Gill’s response-(and the following Bjerknes feedback-)induced SST warming in the western tropical Pacific is stronger than Bjerknes feedback-induced SST cooling in the eastern tropical Pacific,the CP-El Nino is ultimately generated.In addition,by comparing with the real wind anomalies in CP-El Ni(?)o events,we find that many CP-El Ni(?)o events have wind anomalies similar to FWPs in the first half of the year,that is,easterly anomalies in the central equatorial Pacific or strong positive wind anomalies gradient in the north and south sides of the central equatorial Pacific.This further indicates that the unique easterly wind structure in the first half of the year may be an important factor in the occurrence of CP-El Nino.Meanwhile,the westerly anomalies of CP-El Ni(?)o events over the western and central tropical Pacific in the second half of the year are consistent with FWPs.Therefore,FWPs obtained in this study can provide useful guidance for analyzing the generation and evolution of wind-driven CP-El Ni(?)o.