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