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Will the Historic Southeasterly Wind over the Equatorial Pacificin March 2022 Trigger a Third-year La Nia Event?Xianghui FANG1,Fei ZHENG*2,3,Kexin LI2,Zeng-Zhen HU4,Hongli REN5,Jie WU6,Xingrong CHEN7,Weiren LAN8,Yuan YUAN6,Licheng FENG7,Qifa CAI8,and Jiang ZHU21Department of Atmospheric and Oceanic Sciences&Institute of Atmospheric Sciences,Fudan University,Shanghai 200438,China2International Center for Climate and Environment Science(ICCES),Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029,China3Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters,Nanjing University of Information Science&Technology,Nanjing 210044,China4Climate Prediction Center,NCEP/NWS/NOAA,College Park,MD 20740,USA5State Key Laboratory of Severe Weather,Chinese Academy of Meteorological Sciences,Beijing 100081,China6National Climate Center,Beijing 100081,China7National Marine Environmental Forecasting Center,Beijing 100081,China8Mailbox 5111,Beijing 100094,China(Received 25 May 2022;revised 15 June 2022;accepted 26 June 2022)ABSTRACTBased on the updates of the Climate Prediction Center and International Research Institute for Climate and Society(CPC/IRI)and the China Multi-Model Ensemble(CMME)El Nio-Southern Oscillation(ENSO)Outlook issued in April2022,La Nia is favored to continue through the boreal summer and fall,indicating a high possibility of a three-year LaNia(202023).It would be the first three-year La Nia since the 19982001 event,which is the only observed three-yearLa Nia event since 1980.By examining the status of airsea fields over the tropical Pacific in March 2022,it can be seenthat while the thermocline depths were near average,the southeasterly wind stress was at its strongest since 1980.Here,based on a quaternary linear regression model that includes various relevant airsea variables over the equatorial Pacific inMarch,we argue that the historic southeasterly winds over the equatorial Pacific are favorable for the emergence of thethird-year La Nia,and both the anomalous easterly and southerly wind stress components are important and contribute50%of the third-year La Nia growth,respectively.Additionally,the possible global climate impacts of this event arediscussed.Key words:El Nio-Southern Oscillation,three-year La Nia,strongest southeasterly wind,airsea interactionCitation:Fang,X.-H.,and Coauthors,2023:Will the historic southeasterly wind over the equatorial Pacific in March 2022trigger a third-year La Nia event?Adv.Atmos.Sci.,40(1),613,https:/doi.org/10.1007/s00376-022-2147-6.El Nio-Southern Oscillation(ENSO)is the largest interannual climate variability in the tropics.Although its evolutiontakes place in the tropical Pacific region,it has far-reaching impacts on climate and society around the world(McPhaden etal.,2006).Meanwhile,ENSO signals also provide very important information for other short-term climate predictions.There-fore,a successful ENSO forecast is of great importance.In fact,both statistical and physical forecast models could successfullypredict ENSO evolution with lead times of 6 to 12 months by the late 20th century(Latif et al.,1994).Despite this progress,the complexity of ENSO has always been an important obstacle restricting ENSO forecasting(Zheng et al.,2016;Timmermannet al.,2018;Fang and Xie,2020;Chen et al.,2022).ENSO exhibits a high degree of complexity both spatially and tempo-rally.For example,there are central Pacific(CP)and eastern Pacific(EP)types of El Nio(Ashok et al.,2007;Kao and Yu,*Corresponding author:Fei ZHENGEmail:ADVANCES IN ATMOSPHERIC SCIENCES,VOL.40,JANUARY 2023,613 News&Views Institute of Atmospheric Physics/Chinese Academy of Sciences,and Science Press and Springer-Verlag GmbH Germany,part of Springer Nature 20232009),there are differences in amplitude and spatial asymmetry between El Nio and La Nia(An and Jin,2004),and LaNia events can span two or three consecutive years(Hu et al.,2014;Zheng et al.,2015;DiNezio et al.,2017;Wu et al.,2021).These complexities undoubtedly bring significant challenges to simulating and predicting ENSO(DiNezio et al.,2017;Wu et al.,2021).By analyzing historical data,it has been noted that,compared with El Nio,the complexity of La Nia is morereflected in time than space.For example,of the 16 La Nias that have occurred from 1980 to the present,13 have lastedfor at least two years,and 19982001 was a three-year La Nia event.Recently,202122 has been officially identified as aLa Nia year,which makes 202022 a two-year La Nia event(Li et al.,2022).Furthermore,this La Nia did not quicklydecay after its peak.Instead,it persisted as a moderate La Nia and even recently rebounded.According to the latest ClimatePrediction Center and International Research Institute for Climate and Society(CPC/IRI)ENSO Outlook issued in April2022 (Fig.1a;http:/iri.columbia.edu/climate/ENSO/currentinfo/update.html),La Nia is favored to continue throughboreal summer(59%chance during JuneAugust 2022),with a 50%55%chance of continuing through fall.A similar advisory can also be found in the China Multi-Model Ensemble(CMME;https:/cmdp.ncc- platform organized by the National Climate Center of China Meteorological Administrationto release monthly forecasts from various ENSO forecast models in China since March 2020.The corresponding predictionsfor the same period are also shown in Fig.1b.It can be seen that there are large differences among the models in CMME,inwhich both the intermediate coupled models(ICMs)and statistical models(SAMs)call for a La Nia advisory,the coupledgeneral circulation models(CGCMs)indicate a neutral state,and the hybrid dynamic and statistical models(HDSMs)predictan El Nio state.To sum up the predictions of the two platforms,it seems another three-year La Nia(202023)willemerge,being the first since 19982001.Based on an ENSO statistical forecast model,the possible reasons for this potentialLa Nia occurrence will be investigated.Another obstacle of ENSO prediction is the so-called spring predictability barrier(SPB),i.e.,the prediction skill willdrop significantly when it strides over the spring regardless of when a prediction is made(Webster and Yang,1992;Mu etal.,2007;Zheng and Zhu,2010;Fang et al.,2019;Hu et al.,2019).In addition,another important feature associated withENSO seasonal variation is seasonal phase locking,i.e.,ENSO events normally mature in winter and decay quickly in the fol-lowing spring.Using the empirical orthogonal function analysis method to quantitatively analyze this characteristic from1980 to 2018,Fang and Zheng(2021)pointed out that a typical ENSO evolution can explain 90%of the total variance.Thecorresponding principal component is nearly identical to the OctoberDecember(OND)mean Nio-3.4(5S5N,170120W)sea surface temperature(SST)index.Thus,an accurate prediction of ENSO evolution can potentially be providedif the OND mean Nio-3.4 index is well predicted(Fang and Mu,2018;Fang and Zheng,2021).Based on this hypothesis,Fang and Zheng(2021)explored the possibility of using March data to predict the subsequent ENSO evolution using a statisticalmodel,which can also quantitatively reflect the relative contributions of the predictors.In the present study,their modelwill be utilized to explore the important contribution of the historic southeasterly winds over the equatorial Pacific in March2022 to the possible third-year La Nia event.For this purpose,four physically oriented variables are used to establish the connection between the March airseafields and the subsequent ENSO evolution.They are the equatorial mean thermocline depth(TCD)anomalies(TCDa_M),the zonal wind stress anomaly in the western Pacific(Tauxa_W),the zonal gradient of the TCD anomalies in the equatorialPacific(TCDa_G),and the mean meridional wind stress anomalies over the eastern equatorial Pacific(Tauya_E).The firsttwo variables are associated with the recharge oscillator theory(Jin,1997).TCDa_G reflects the see-saw oscillation of thethermocline between the western and eastern Pacific(e.g.,Kumar and Hu,2014)or the eastward propagation of Kelvinwave-like signals(Tseng et al.,2017).Tauya_E is linked to the meridional processes that have been emphasized in studyingENSO complexity(Hu and Fedorov,2018;Xie et al.,2018).Specifically,the meridional processes in the eastern Pacificregion,which were less often considered in classical ENSO theories,were verified to be important in depicting the followingENSO evolution from March(Hu and Fedorov,2018;Xie et al.,2018).Based on the above physical parameter selection,a quaternary linear regression equation is used to study the relationshipbetween the March airsea fields and the OND mean ENSO property as follows:Nio34ONDp=aTCDa_MMo+bTCDa_GMo+cTauxa_WMo+dTauya_EMo+e,(1)Nio34ONDpTCDa_MMoTauxa_WMoTCDa_GMoTauya_EMoa b c d,ewhere is the OND mean Nio-3.4 index and,and are theTCDa_M,Tauxa_W,TCDa_G,and Tauya_E indices in March,respectively.,and are the regression coefficients.The subscripts“P”and“O”mean the prediction and observation,respectively.Details of the method can be found in Fangand Zheng(2021).Figures 1cd show the anomalous surface wind stress,SST,and TCD in March 2022.The equatorial central to easternPacific still maintains cold conditions,with a large area colder than 1C.Correspondingly,the thermocline depth shows gen-erally shallow anomalies in the east and deep anomalies in the west,a so-called tilt or dipole mode that is in phase with theJANUARY 2023FANG ET AL.7ENSO cycle(Kumar and Hu,2014).Meanwhile,the southeasterly wind over the equatorial Pacific is appreciable.Suchwind field distributions favor the induction of La Nia-like evolution through zonal Bjerknes feedback and meridional physicalprocesses,such as the incursion of off-equatorial subsurface cold water(Zheng et al.,2015;Zhu et al.,2016).Nino 3.4 SST Anomaly(C)Nino 3.4 SST Anomaly(C)(a)Model Predictions of ENSO from Apr 2022CPC CONSOLDYN MODELS:NCEP CFSv2CMC CANSIPNASA GMAOCS-IRI-MMJMALDEOAUS-ACCESSECMWFCOLA CCSM4MetFRANCEUKMOGFDL SPEARKMASAUDI-KAUBCC_CSM11mSINTEX-FIOCAS ICMDYN AVGSTAT MODELS:CPC MRKOVCPC CACSU CLIPRIAP-NNUCLA-TCDBCC_RZDMNTU CODASTAT AVGJFM-OBSMar-OBSMAMAMJMJJJJAJASASOSONONDNDJDJF-3.00-2.50-2.00-1.50-1.00-0.5000.501.001.502.002.503.00(b)CMME Model Predictions of ENSO from Apr 2022120E150E180150W120W90W20S10S010N20N(c)Taua and SSTa of March 2022-1-1-1-1-1-1-1-0.5-0.5-0.5-0.5-0.5-0.5-0.50.05 N/m2-3-2-10123120E150E180150W120W90W20S10S010N20N(d)TCDa-40-30-20-1001020304050Fig.1.(a)Prediction of the three-month mean Nio-3.4 index from April 2022 initial conditions of the IRI/CPC ENSOplume.(b)Prediction of the monthly mean Nio-3.4 index from April 2022 on CMME.(c)Anomalous surface wind stress(Taua,N m2)and SST(shading,C)of March 2022.The contour interval of SST is 0.5C.(d)Anomalous thermoclinedepth(TCD,m)in March 2022.8A NEW THIRD-YEAR LA NIA EVENT IN 2022VOLUME 40To quantify the contributions to the possible third-year La Nia,Figs.2ad show the time series of TCDa_M,TCDa_G,Tauxa_W,and Tauya_E in March during 19802022,which are normalized by their standard deviations.For comparison,March 2000(i.e.,the last year of the latest three-year La Nia)is marked by blue dots,while March 2022 is marked by redFig.2.Normalized indices for(a)TCDa_M,(b)TCDa_G,(c)Tauxa_W,and(d)Tauya_E in March.In eachpanel,the blue and red dots represent 2000 and 2022,respectively.(e)The relationships between thereconstructed and observed OND mean Nio-3.4 index during 19802021.Their correlation coefficient(R)is 0.86.The reconstructed indices are obtained by Eq.(1)utilizing the TCDa_M,TCDa_G,Tauxa_W,andTauya_E indices in March.The red,blue,and black dots represent El Nio,La Nia,and neutral years,respectively.(f)The coefficients(bars)and their 10%significance intervals by Students t test(error bars)ofthe four variables.They are calculated by the product of the regression coefficients and the correspondingstandard deviations.JANUARY 2023FANG ET AL.9dots.As seen,neither of the thermocline variables(TCDa_M and TCDa_G)are significantly abnormal this year,indicatinga challenge in predicting the subsequent ENSO evolution from the ENSO rechargedischarge theory and other classicalENSO theories that emphasize the variations in thermocline depth,i.e.,zonal oceanic dynamics.In contrast,the atmosphericvariables exhibit appreciable anomalies,with the easterly wind stress being comparable to that of 2000 and the southerlywind stress reaching its largest amplitude since 1980.Except for the strong easterly wind,the airsea fields from 2000 arequite different from those in 2022.Namely,the former exhibits a large negative TCDa_G(i.e.,the TCD was deeper in thewest and shallower in the east)and anomalous northerly wind stress.To quantify the contributions of each variable to ENSO,Fig.2e presents the reconstructed OND mean Nio-3.4 indexby utilizing the quaternary regression and the four variables from March in 19802021.The correlation coefficient betweenthe reconstructed and observed indices is 0.86.Additionally,all La Nia events can be accurately captured(i.e.,the bluedots are in the third quadrant).Moreover,the coefficients (Fig.2f)of the regression suggest that the TCDa_M andTauxa_W indices play dominant roles in ENSO development.The negative coefficients of the other two variables(i.e.,TCDa_G and Tauya_E)indicate that the positive(negative)zonal gradient of the TCD or the northerly(southerly)windstress in the eastern Pacific favors the warming(cooling)of the eastern equatorial Pacific,illustrating the phase transitionand the importance of the meridional processes in the eastern Pacific region,respectively.The latter factor(Tauya_E)hasreceived less attention than the zonal Bjerknes feedback in understanding its influence on ENSO evolution from March(Xie et al.,2018;Fang and Xie,2020;Fang and Zheng,2021).Based on this model,the relative contribution of each variable in March(i.e.,TCDa_M,TCDa_G,Tauxa_W,andTauya_E)to the OND mean Nio-3.4 index can be quantitatively estimated(Fig.3).It can be seen that for 2000,it is thestrong easterly wind stress that overcame the opposite effects from all the other three variables.It suggests that although theoceanic dynamics (characterized by the thermocline depth distribution)does not support more cooling based on therecharge oscillator and other classical ENSO theories,the abnormally strong easterly wind in the western Pacific can lead tocooling through exciting the upwelling oceanic Kelvin waves and driving the anomalous westward surface currents.How-ever,the situation is quite different in 2022.The anomalous easterly and southerly wind stresses are both important,andeach may contribute 50%toward the pending La Nia evolution,while the variables related to the TCD have little effect.The southerly wind is argued to be able to intensify the ocean upwelling south of the equator(Xie et al.,2018)and enhancethe incursion of the subsurface cold water on the off-equator into the equatorial area(Zheng et al.,2015),thus acting as anextra cooli