Research On The Asymmetric Characteristics And Mechanism Of ENSO Zonal Winds About The Equator

El Ni?o-Southern Oscillation(ENSO)is the strongest interannual variability in the global climate system.It is an important source of predictability for global climate prediction and seasonal forecast.Based on dynamic diagnosis of reanalysis data and numerical model of Atmospheric General Circulation Model(AGCM),the mechanism for the zonal wind anomaly asymmetry during mature phase of ENSO is analyzed.Also,the difference of zonal wind anomaly between three types of ENSO is compared.In addition,the simulation of ENSO related anomalous zonal wind in CMIP6 models is evaluated.Finally,the decadal change of ENSO intensity is investigated.The main conclusions are summarized as follows:(1)The cause of southward shift of anomalous zonal wind in the central equatorial Pacific(CEP)during ENSO mature winter was investigated through observational analyses and numerical model experiments.Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data,a two-step physical mechanism is proposed.The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind.The second step involves the development of an antisymmetric mode in the CEP,which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces.Two positive feedbacks are responsible for the growth of the antisymmetric mode.The first involves the moisture–convection–circulation feedback,and the second involves the wind–evaporation–SST feedback.General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift,and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.(2)A composite analysis of observational data reveals that maximum westerly anomalies associated with all three kinds of ENSO(EP El Ni?o,CP El Ni?o,La Ni?a)shift southward to5°S during their mature phase(boreal winter),with different zonal locations.A zonal momentum budget analysis indicates that leading factors to cause the southward shift of the zonal wind anomaly for all three types of ENSO composites are anomalous pressure gradient force and anomalous meridional advection,while anomalous Coriolis force has positive contribution for EP El Ni?o but negative for CP El Ni?o and La Ni?a.The difference in the longitudinal locations arises from their zonal shift of maximum SST anomaly centers.(3)The ENSO related zonal wind response in 21 CMIP6 models is evaluated.Almost all models can catch the characteristics that the maximum anomalous zonal wind locating in the south of the equator.While the zonal position of anomalous wind in models shows significant difference.The ENSO wind response locates more westward than observation.The different meridional and zonal positions of wind response attribute to the mean precipitation and mean wind.The stronger precipitation dipole in western equatorial Pacific and tropical southwestern Pacific causes larger cross-equatorial northerly,leading to the stronger meridional advection of anomalous wind and thus more southward maximum zonal wind center.The stronger precipitation dipole in western equatorial Pacific and eastern equatorial Pacific accompanying weaker easterly trade wind favoring more eastward maximum zonal wind,due to zonal advection and more active convection in eastern Pacific.In all models,the intensity of zonal wind response to unit SSTA is weak.It is because the mean precipitation in the central equatorial Pacific to the western Pacific is weak,the mean water vapor is weak correspondingly.Therefore,response to unit SSTA shows weak anomlous precipitation and wind.(4)The impact of the Atlantic Multi-decadal Oscillation(AMO)on the ENSO amplitude was investigated through observational analyses.During the past 100 years the interdecadal variability of ENSO intensity is significantly correlated with the AMO.ENSO variability was strengthened(weakened)during a negative(positive)AMO phase.An ocean mixed layer heat budget analysis reveals that the thermocline feedback is the main process regulating AMO negative phase dependent growth characteristic.A further examination indicates that a strengthened atmospheric response to unit SST anomaly,an enhanced thermocline response to unit wind stress forcing and a strengthened subsurface temperature response to unit thermocline variation all contribute to the enhanced thermocline feedback during the negative phase of AMO.Such changes are attributed to the increase of background moisture,the weakening of mean subtropical cell(STC)and increase of upper ocean vertical temperature gradient respectively.