Understanding...cific El Ni_o_Chengyang GUAN.pdf

Understanding the Development of the 2018/19 Central Pacific El NioChengyang GUAN1,Xin WANG*2,3,4,and Haijun YANG51College of Ocean Science and Engineering,Shandong University of Science and Technology,Qingdao 266590,China2State Key Laboratory of Tropical Oceanography,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 510301,China3Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou 511458,China4Innovation Academy of South China Sea Ecology and Environmental Engineering,Chinese Academy of Sciences,Guangzhou 510301,China5Department of Atmospheric and Oceanic Sciences,Fudan University,Shanghai 200438,China(Received 4 November 2021;revised 17 May 2022;accepted 18 May 2022)ABSTRACTA central Pacific(CP)El Nio event occurred in 2018/19.Previous studies have shown that different mechanisms areresponsible for different subtypes of CP El Nio events(CP-I El Nio and CP-II El Nio).By comparing the evolutions ofsurface winds,ocean temperatures,and heat budgets of the CP-I El Nio,CP-II El Nio,and 2018/19 El Nio,it isillustrated that the subtropical westerly anomalies in the North Pacific,which led to anomalous convergence of Ekman flowand surface warming in the central equatorial Pacific,played an important role in the 2018/19 El Nio event as well as inthe CP-II El Nio.Although the off-equatorial forcing played a vital role,it is found that the equatorial forcing acted as adriving(damping)term in boreal spring(summer)of the 2018/19 El Nio.The 2018/19 El Nio provides a timely andvivid example that helps illustrate the proposed mechanism of the CP El Nio,which could be leveraged to improve ElNio predictability.Key words:El Nio,subtropical Pacific,westerly anomalies,Ekman transportCitation:Guan,C.Y.,X.Wang,and H.J.Yang,2023:Understanding the development of the 2018/19 central Pacific ElNio.Adv.Atmos.Sci.,40(1),177185,https:/doi.org/10.1007/s00376-022-1410-1.Article Highlights:The 2018/19 El Nio was a CP El Nio,and the warm SST anomalies originated in the subtropical North Pacific.Ekman transport caused by the westerly anomalies in the northern subtropical Pacific was vital for the 2018/19 El Nioas well as the CP-II El Nio proposed in a previous study.Different from the composite CP-II El Nio,equatorial airsea coupling processes could have contributed to thedevelopment of the 2018/19 El Nio in spring.1.IntroductionEl Nio Southern Oscillation(ENSO)is the dominantinterannual variation of sea surface temperature(SST)in thetropical Pacific region.The event-to-event diversity of indi-vidual ENSO events can lead to different climate impacts(Ashok et al.,2007;Wang and Wang,2013,2014;Capotondiet al.,2015;Yu et al.,2017).El Nio events are often classi-fied as Eastern Pacific(EP)or Central Pacific(CP)El Nioevents(Yu and Kao,2007;Kao and Yu,2009;Yu and Kim,2011).An EP El Nio first exhibits surface warming in thecold-tongue region in the eastern Pacific,while a CP ElNio first exhibits surface warming in the central tropicalPacific.The latter is also referred to as an El Nio Modoki(Ashok et al.,2007)or a dateline El Nio(Larkin and Harri-son,2005).Recent studies have revealed that different types of ElNio events are dominated by different physical processes(Kug et al.,2009;Yu et al.,2010;Chen et al.,2015;Lai etal.,2015;Wang et al.,2019b).Kug et al.(2009)demonstratedthat a CP El Nio arises mainly from zonal advective feed-back,while warming in the eastern tropical Pacific is sup-pressed by enhanced upwelling and evaporation caused byequatorial easterly anomalies.Yu et al.(2010)pointed outthat during the development of a CP El Nio,SST anomaliesfirst appear in the northeastern subtropical Pacific and*Corresponding author:Xin WANGEmail:ADVANCES IN ATMOSPHERIC SCIENCES,VOL.40,JANUARY 2023,177185 Notes&Letters Institute of Atmospheric Physics/Chinese Academy of Sciences,and Science Press and Springer-Verlag GmbH Germany,part of Springer Nature 2023extrend towards the central equatorial Pacific throughwindevaporationSST(WES)feedback(Xie and Philander,1994).There are also studies suggesting that the forcingsfrom the northern subtropical Pacific could trigger the devel-opment of a CP El Nio via the seasonal footprinting mecha-nism(Vimont et al.,2001;Yeh et al.,2015;Yu and Fang,2018;Fang and Yu,2020).Chen et al.(2015)and Lai et al.(2015)emphasized that the diversity of El Nio events is acombined effect of both zonal wind anomalies and subsurfacetemperature anomalies in the equatorial Pacific.Wang and Wang (2013)investigated the diversity inSST evolution and climate impact of CP El Nio events andfurther classified them into two subtypes,Modoki-I andModoki-II(referred to as CP-I and CP-II hereafter).Despitethe similarity of both CP types warming in the central tropicalPacific,they have distinctly different spatial patterns andregional climatic effects(Liu et al.,2014;Wang and Wang,2014;Tan et al.,2016;Liu et al.,2017;Chen et al.,2019;Wang et al.,2020b;Kim et al.,2021).A CP-I(CP-II)ElNio tends to induce positive(negative)SST anomalies inthe South China Sea during developing autumn,mainly byaffecting the latent heat flux(Tan et al.,2016).Associatedwith a weaker Walker circulation in the IndoPacific region,a CP-I(CP-II)El Nio favors a positive(negative)IndianOcean Dipole via the Bjerknes feedback(Wang and Wang,2014).As for the western Pacific subtropical high duringthe decaying summer,a CP-I(CP-II)El Nio imposes little(strong positive)impact(Chen et al.,2019,2021a).Wang etal.(2018)came up with an index to identify the two subtypesof CP El Nio events and showed that CP-II eventsoccurred the most often after 1990.Wang et al.(2019c)inves-tigated CP El Nio events occurring from 1900 to 2010 byanalyzing the heat budget of the mixed layer water in theNio-4 region (5S5N,160E150W)and discoveredthat zonal advective(Ekman pumping)feedback is the leadingcontributor to CP-I(CP-II)El Nio events.Chen(2021b)con-cluded that a CP-I El Nio is triggered by the weakening ofthe Australian winter monsoon,while a CP-II El Nio ismainly forced by the Pacific Meridional Mode(PMM)viaWES feedback.In 2018/19,an El Nio event occurred,with the warmingcenter located at the Date Line.The SST anomalies firstappeared in the northern subtropical Pacific and thenextended towards the central tropical Pacific.The surfacewarming was stronger in the Nio-4 region (5S5N,160E150W)than in the Nio-3 region(5S5N,15090W;Fig.1).Based on observations and model forecasts,Liu et al.(2020)argued that the central tropical Pacific warm-ing in the 2018/19 El Nio,along with tropical Atlantic warm-ing and interdecadal variation,is one of the major factors lead-ing to the extremely wet winter of 2018/19 in the lowerreach of the Yangtze River.Wang et al.(2020a)suggestedthat the 2018/19 El Nio induced a remote teleconnection pat-tern with pronounced low-level southerly anomalies overEast China,which transported moisture from oceans to thecontinent and caused persistent rainy days in the 2018/19 win-ter in Shanghai,China.The 2018/19 El Nio event presents an opportunity toexamine the genesis mechanisms of a CP El Nio.In thiswork,the physical processes associated with the 2018/19 ElNio are analyzed.It is demonstrated that the off-equatorialforcings in the northern subtropical Pacific are the primarycause of the 2018/19 El Nio.The contributions of the equato-rial forcings,however,can be opposing during differentstages.2.Data and methodMonthly SSTs from the Extended Reconstructed SeaSurface Temperature,version 5(ERSST v5)dataset with aN m2Fig.1.Evolution of SST(shading,C)and wind stress(vector,)anomalies in the tropical Pacific during thedevelopment of the 2018/19 El Nio.(a)MAM,(b)JJA,and(c)SON indicate the periods of MarchAprilMay,JuneJulyAugust,and SeptemberOctoberNovember in2018,respectively.(d)DJF indicates the period fromDecember 2018 to February 2019.Black vectors indicate windstress anomalies exceeding 1.5 standard deviations.Thedotted area indicates where SST anomalies exceed 1.5standard deviations.178UNDERSTANDING THE 2018/19 EL NIOVOLUME 40resolution of 2.0 2.0(Huang et al.,2017),monthly sealevel pressures,surface winds and heat fluxes from theNational Centers for Environmental Prediction-National Cen-ter for Atmospheric Research(NCEP/NCAR)Reanalysis 1with a resolution of 2.5 2.5(Kalnay et al.,1996),andmonthly wind stress,ocean current,and temperature datafrom the Global Ocean Data Assimilation System(GODAS)(Behringer and Xue,2004)with a horizontal resolution of0.333 latitude 1.0 longitude are used to analyze the evolu-tion of the 2018/19 El Nio.Ocean current and temperaturedata at 40 vertical levels from GODAS are used.Thesemonthly variables cover the period from 1950 to January2019.Monthly oceanic current and temperature data fromthe Simple Ocean Data Assimilation(SODA)2.2.4(Cartonand Giese,2008;Giese and Ray,2011)and monthly sea sur-face wind data from the Twentieth Century Global ReanalysisVersion 2(20CR v2)(Compo et al.,2011),with a longertime span covering 19002010,are used for the compositeanalyses of CP-I and CP-II El Nio events.The 20CR v2data are on a global T62 Gaussian grid.Pentad wind stressdata from GODAS are also analyzed for the development ofthe 2018/19 El Nio.The precipitation rate data is from theGlobal Precipitation Climatology Centre(GPCC)MonitoringProduct Version 6 (2014present;Schneider et al.,2011)and Version 7(19012013;Schneider et al.,2016),with a res-olution of 1.0 1.0.The classification of EP or CP El Nio is based on theEl Nio Modoki index (EMI)proposed by Ashok et al.(2007):EMI=SSTAC0.5SSTAW0.5SSTAE,(1)where the subscripts C,W,and E indicate the SST anomaliesaveraged in the central(165E140W,10S10N),western(125145E,10S20N),and eastern (11070W,15S5N)Pacific,respectively.The El Nio event is consid-ered a “typical”CP event when the EMI is equal to orgreater than 0.7,where is the climatological standard devi-ation.CP El Nio events,as mentioned above,can be furtherclassified as CP-I or CP-II El Nio events,with reference totheir opposite influences on the precipitation in southernChina(Wang and Wang,2013).Additionally,Wang et al.(2018)developed a new index(CP-II index in this paper)toidentify the subtypes of CP El Nio events.The CP-II indexis defined as the leading principal component of multivariateempirical orthogonal function analysis of the normalizedEMI,Nio-4 index,and 850-hPa relative vorticity anomaliesaveraged near the Philippine Sea(115145E,1025N)during boreal autumn.CP El Nio events with a CP-II indexlarger than one standard deviation are designated as a CP-IIEl Nio.Following Wang and Wang(2013),seven CP-I El Nioevents (1914/15,1940/41,1941/42,1963/64,1987/88,1990/91,and 2002/03)and six CP-II El Nio events(1968/69,1979/80,1991/92,1992/93,2004/05,and 2009/10)areselected as the historical collections.Monthly data are utilizedto conduct the heat budget analysis of the ocean mixed-layer temperature based on the equation from Huang et al.(2010):Tt=Qu+Qv+Qw+Qq+R,(2)where the primes indicate temporal anomalies.Tt representsthe temperature tendency.Qu,Qv,and Qw represent thezonal,meridional,and vertical advective feedback.Qq repre-sents the net surface flux.R represents the residuals.Theocean mixed-layer depths,defined as the depth where thebuoyancy difference with respect to the surface level isequal to 0.03 cm s2,are from the GODAS dataset.The advection terms on the right side of Eq.(2)are furtherdecomposed asQu=uTxuTxuTx+uTx,(3)Qv=vTyvTyvTy+vTy,(4)Qw=wTzwTzwTz+wTz,(5)Qq=Qnetcph,(6)where T is the ocean mixed-layer temperature.u,v,and w rep-resent the zonal,meridional,and vertical current,respec-tively.is the seawater density.cp is the specific heat capacityof seawater under constant pressure.h is the depth of themixed layer.The overbars indicate temporal average.3.Results3.1.Classification of the 2018/19 El NioFirstly,the EMI in 2018 is calculated,which exceeds0.7 from June to November Fig.S1 in the electronic supple-mentary material(ESM).Thus,the 2018/19 El Nio is identi-fied as a CP El Nio.Secondly,there are two ways to separateCP-I and CP-II El Nio events.The southern China rainfallduring the boreal autumn of 2018 was significantly positive,the same as that of a CP-I El Nio(Fig.S2 in the ESM).The CP-II index of 2018 is 0.87 (less than one standarddeviation).So,the 2018/19 El Nio should be categorizedas a CP-I El Nio event under either classification approach.The 2018/19 El Nio exhibited equatorial westerly anomaliesduring the developing February and March(Fig.2c).Theequatorial westerly anomalies generated downwellingKelvin waves that propagated eastward and caused positivezonal advective feedback (Fig.3b),leading to positiveocean heat content(OHC)anomalies in the central Pacific dur-ing the developing February to March(Fig.2c).All of theseJANUARY 2023GUAN ET AL.179features are the precursors of a CP-I El Nio,and the zonaladvective feedback is supposed to support the warming inthe central equatorial Pacific until the peak.The subsequentdevelopment process,however,tells a different story.3.2.Developing processesEasterly anomalies spread across the equatorial Pacificin late boreal spring and summer(April to August),inducednegative zonal advective feedback(Fig.3b),and suppressedthe increase of oceanic subsurface temperature in the centralequatorial Pacific.Yet,the oceanic subsurface temperaturein the central equatorial Pacific still increased notably inlate boreal summer without significant westerly anomaliesin the western equatorial Pacific (Figs.2b,c).It will bedemonstrated below that this notable warming in the centralequatorial Pacific is mainly ascribed to the Ekman feedbackinduced by the subtropical westerly anomalies.During the spring of 2018,westerly anomalies weremainly located in the central subtropical Pacific(510N,170E130W),and positive SST anomalies(exceeding 1C)were in the northeastern subtropical Pacific(Fig.1a).Underthe influence of WES feedback,the surface warmingextended from the northeastern Pacific to the central equato-rial Pacific.When the central Pacific SST anomalies were ade-quately positive,westerly anomalies arose in the westernequatorial Pacific(Fig.1c).The westerly anomalies and thesurface warming in the central Pacific enhanced themselvesthrough Bjerknes feedback(Bjerknes,1969)and peaked inthe boreal winter of 2018/19.Particularly,the developing pat-terns of SST anomalies and wind anomalies resembledthose of a CP-II El Nio(Figs.S3eh in the ESM).Evolutions of composite OHC anomalies in the top 300m and surface zonal wind anomalies in the equatorialPacific during CP-I and CP-II El Nio events are illustratedin Fig.2a and Fig.2b,respectively.For the CP-I El Nioevents,the significantly positive OHC anomalies in the east-ern-central Pacific and westerly anomalies in the western-cen-tral equatorial Pacific persist during the whole developingyear,indicating that the equatorial ocean and the atmosphereare well coupled.For the CP-II El Nio events,OHC anoma-lies in the central equatorial Pacific begin to increase