Processes Leading to Second-Year Cooling of the 2010–12 La Ni˜na Event,Diagnosed Using GODAS

FENG Licheng,ZHANG Rong-Hua,WANG Zhanggui,and CHEN XingrongNational Marine Environmental Forecasting Center,State Oceanic Administration,Beijing 0008

2Key Laboratory of Ocean Circulation and Waves,Institute of Oceanology,Chinese Academy of Sciences,Qingdao 266071

3Earth System Science Interdisciplinary Center(ESSIC),University of Maryland,College Park,Maryland,USA,20740

1.Introduction

The El Ni˜no–Southern Oscillation(ENSO)is the leading mode of interannual variability in the tropical Pacif i c climate system,signif i cantly impacting global weather and climate.In the past several decades,extensive studies have led to substantial progress in understanding,modeling and predicting El Ni˜no events(e.g.,McCreary and Anderson,1984;Cane and Zebiak,1985;Zebiak and Cane,1987;Philander,1992;Wang et al.,2011,2013).The delayed oscillator mechanism has been proposed to explain ENSO dynamics and its interannual oscillation within the tropical Pacif i c climate system(Battisti and Hirst,1989).This theory emphasizes equatorial wave processes(Rossby wave and its ref l ection along the low-latitude western boundaryinto a Kelvin wave).Another is the recharge/discharge mechanism(Jin,1997),which focuses on water exchange in the ocean on and off the Equator.As implied by these theories,the ENSO can be a cyclic oscillation between El Ni˜no and La Ni˜na conditions within the tropical Pacif i c climate system.

However,as observed,the ENSO also exhibits signif i cant variability from one event instance to another.For example,multi-year cooling events can be seen during ENSO cycles from historical SST data(e.g.,Hu et al.,2014).During 2010–12,the tropical Pacif i c had a persistent La Ni˜na condition,with a second-year sea surface cooling that occurred in the fall of 2011.Further,many coupled models have failed to predict the Ni˜no 3.4 sea surface temperature(SST)cooling when initialized from early-to mid-2011.Yet,one intermediate coupled model—an integrated climate model(ICM)operated at the Earth System Science Interdisciplinary Center(ESSIC),University of Maryland(UMD),the so-called ESSIC ICM(Zhang et al.,2003,2005)—gave a successful forecast of the 2011 negative SSTAs with a lead time of one year or so(Zhang et al.,2013,2014a).This presents a challenge to the ENSO prediction community and indicates an urgent need to understand processes leading to the secondyear cooling.

Previously,ICM-based experiments were carried out to examine the roles played by the temperature of subsurface water entrained into the mixed layer,and wind forcing(Zhang et al.,2013,2014①Zhang,R.-H,L.C.Feng,and Z.G.Wang,2014:Role of atmospheric wind forcing in the second-year cooling of the 2010–12 La Ni˜na event.Atmos.Sci.Lett.,submitted.).The reappearance of a negative SSTA in the central equatorial Pacif i c in early summer of 2011 was closely related to off-equatorial thermal anomalies in the South Pacif i c.However,the three-dimensional structure and evolution of these have not been illustrated,as theoceanicprocessesresponsibleforthesecond-yearcooling duringthe2010–12LaNi˜na eventare still poorlyunderstood.The causes of the occurrence of a multi-year La Ni˜na in general,and the 2011–12 La Ni˜na event in particular,are not fully understood(Hu et al.,2014).

In this paper,we examine the oceanic processes responsible for the second-year cooling of the 2010–12 La Ni˜na event using reanalysis data,with a focus on the roles played by off-equatorial subsurface anomalies in the South Pacif i c.To better represent pathways,isopycnal analyses were performed using three-dimensional temperature and salinity f i elds(Zhang and Rothstein,2000).Since subsurface temperature anomalies tend to propagate along density surfaces,an isopycnal analysis can better characterize the threedimensional structure and time evolution in a natural and physical way,therefore enabling us to trace pathways consistently throughout the basin.Our major f i nding was that a distinct pathway of off-equatorial temperature anomalies occurred along the South Equatorial Current(SEC),clearly associated with the onset of second-year cooling during the 2010–12 La Ni˜na event.Through examining the subsurface temperature evolution on isopycnals,connections were more clearly illustrated between thermal anomalies at the subsurface and surface,and off and on the Equator,leading to an improved understanding of ENSO variability.Additionally,re-evaluating the historical ENSO evolution showed that another multi-year cooling case occurred in the tropical Pacif i c in 2007–09.The similarities and differences of these two events were analyzed to describe the nature of these strikingly different ENSO evolutions associated with various forcings and feedbacks within the Pacif i c climate system.

The remainder of the paper is organized as follows.We introduce the data and methodologyused in this work in section 2.The results are presented in section 3,followed by a summary and discussion in section 4.

2.Data and methodology

Monthly-mean data for currents,sea surface height,temperature and salinity came from the Global Ocean Data Assimilation System(GODAS)(Behringer and Xue,2004),operational at the National Centers for Environmental Prediction(NCEP).GODAS has a horizontal resolution of 1°×1/3°in the zonal and meridional directions;it has 40 levels in the vertical,with a 10 m resolutionin the upper 200 m.We used the GODAS data coveringthe periodfromJanuary1980 throughDecember2012.Additionally,surface winds at 10 m height were from the NCEP–NCAR(National Center for Atmospheric Research)Reanalysis(Kalnay et al.,1996),with a longitudinal and latitudinal resolution of 1.904°×1.875°on a T62 Gaussian grid(192×94).

Long-term climatological f i elds were formed from the period 1980–2012,including monthly-mean current vectors.Interannualanomaliesfortemperature,windstress andothers werethencalculatedrelativetotheir climatologicalf i elds.Finally,isopycnal surfaces were estimated using monthly temperatureandsalinity data.Thetemperatureanomaliesat level depths were interpolated to constant density surfaces by using a cubic spline.Climatological current vectors on isopycnal surfaces were formed in the same way.In this study,interannual anomaly f i elds on isopycnal surfaces were used to investigate the roles played by anomalous temperature advection in the 2010–12 and 2007–09 La Ni˜na events.

3.Results

3.1.SST evolution

Figure 1 illustrates the horizontal distributions of SSTAs and surface wind anomalies for selected time intervals in 2011.In January,there was a La Ni˜na state over the tropical Pacif i c.Consequently,negative SSTAs prevailed in the central and eastern tropical Pacif i c with the maxima exceeding −2°C between 150°and 170°W along the Equator.Surface easterly winds were stronger than normal over the western central equatorial Pacif i c and southeasterly wind anomalies dominated off the Equator in the South Pacif i c(Fig.1a).Thereafter,the cold SSTA diminished and the SSTA became normal in the eastern tropical Pacif i c domain.Simultaneously,the wind stress anomalies weakened in most regions(Fig.1b).This warming process persisted during the following months and peaked in June,when a neutral SST state prevailed throughout the Equator except for a weak negative anomaly along 160°W.At this time,easterly wind anomalies weakened in the central tropical Pacif i c(Fig.1d).In August,negative SSTA strengthened in the central equatorial Pacif i c(Fig.1f),and this cooling tendency persisted during the following months(Figs.1g–h).

Themechanismofformationofthe coldSSTA in the central eastern equatorial Pacif i c during mid–late 2011 has not been fully explained.Some possible factors,such as wind forcing or a subsurface thermal anomaly,may play an important role.Note that southeasterly wind became stronger in the tropical South Pacif i c(Fig.1e),forcing the cold waters located in the South Pacif i c to move to the equatorial band(Fig.1e)and leadingto the negativeSSTA.However,the cur-rent driven by anomalous wind was not enough to produce strong and persistent negative SSTAs in the central equatorial Pacif i c,especially after the wind anomalies changed direction during September and October(Figs.1g–h).Other processes,such as subsurface effects,are required to fully understand the cause of the second-year cooling.

3.2.Subsurface temperature anomaly pathway

The climatological Bernoulli function(B)was calculated on the isopycnal surfaces to study the mean f l ow pattern.According to Cox and Bryan(1984),B can be written as

whereσ=ρ−1000 is an isopycnal surface,andρis density with units of kg m−3;ρ0is mean density;g is the acceleration due to gravity,andηis dynamic height.B represents geostrophic streamlines that measure the geostrophic fl ow away from the Equator;thus,it can be used to illustrate fl ow paths on isopycnal surfaces.

Figures 2a and 2c show the mean depth distributions of the 23.4 and 25.2 isopycnal surfaces.These two isopycnal surfaces had similar patterns in the tropical Paci fi c.On the Equator,the thermocline was deep in the west and shallow in the east.The deepest regions on the isopycnal surfaces were located around 15°N and 5°S,respectively,in the western central Paci fi c,with a relatively shallow band between 6°and 10°N.The isopycnal surfaces shoaled eastward along the Equator and reached minima in the far-eastern Paci fi c.The 23.4 isopycnal surface intersected with the sea surface(i.e.,outcropped)in the central and eastern basin on and south of the Equator(Fig.2a).

Pathways along which off-equatorial waters move onto the Equator have been examined in many studies(e.g.Zhang et al.,1999;Zhang and Busalacchi,1999;Wang et al.,2007).However,mostpreviousanalysesfocusedontheeffectsinthe North Pacif i c,with fewer studies in the South Pacif i c.Chang et al.(2001)pointed out the potential importance of south tropical Pacif i c variability in the decadal modulation of the ENSO.Luoetal.(2003)investigatedtheoriginofthedecadal ENSO-like variation.Luo et al.(2005)carried out 49-year simulations,and found that decadal variability of temperatureandsalinityalongtheEquatororiginatesfromsubsurface spiciness anomalies in the South Pacif i c.

FromFigs.2b andd,onecan see clear pathways originating from the southeastern tropical Pacif i c:water carried by the South Equator Current(SEC)extending northwestward to south of the equatorial band and then transported by the strong Equator Undercurrent(EUC)onto the Equator.The South Pacif i c water pathways intersect with the surface in the eastern equatorial and Southeast Pacif i c domain(Fig.2b).

Figure 3 gives subsurface temperature anomalies evaluated on the 25.2 isopycnal surface(see Fig.2c for its depth information)at some selected time periods in 2011;the vertical distribution of temperature anomalies in the upper ocean along the Equator is presented in Fig.4.During the 2010–11 La Ni˜na event,there was a buildup of warm waters in the western Pacif i c Ocean due to stronger than normal easterly winds in the central basin,characterizedby positive thermal anomalies in the upper ocean.For example,in January 2011,a large positive anomaly was observed in the western central tropical Pacif i c and a negative anomaly was located in the central eastern tropical Pacif i c regions.These two anomaly bands with opposite signs intersected along 160°W with a sharp temperature front(Figs.3a and 4a).Beginning in early 2011,accompanied by the seasonal strengthening of the EUC,warm waters in the western Pacif i c expanded eastward across the Equator;cold anomalies in the central eastern equatorial Pacif i c diminished and reversed to above normal(Figs.3b and 4b).This warming tendency peaked in April(Fig.3c),when positive temperature anomalies occupied almost the whole equatorial Pacif i c except for near 150°W.Temperature anomalies reached more than 2°C in the far-eastern equatorial Pacif i c.In the meantime,cold waters retreated to northeastern and southeastern regions off the Equator.As seen from the vertical section along the Equa-tor(Fig.4c),cold waters shrank back dramatically,and were conf i ned to a narrow region of the central Pacif i c.

In May,positive anomalies along the Equator were seen to have two separate western and eastern bands(Fig.3d),with below-normal temperature anomalies amplif i ed in the regions of 140°–160°W(Fig.4d).Subsequently,the neg-ative anomalies dominated over the central Pacif i c in June(Fig.3e),forming a horseshoe-like thermal anomaly pattern connecting large negative thermal anomalies on and off the Equator.Comparing Figs.3e and 3d,the EUC decelerated in the far-eastern equatorial Pacif i c in June(Yu et al.,1997),but the off-equatorialcold anomalies strengthenedin the central South Pacif i c.These changes were in favor of cold water advection to the equatorial regions through the well-def i ned SouthPacif i cwaterpathway(Fig.2d),andthenextendedinto the equatorial region to combine with the negative anomalies located north of the Equator.In July,the EUC weakened further,and was even replaced by the SEC in the eastern Pacif i c on the 25.2 isopycnal surface.At this time,cold anomalies were transported by SEC from the Southeastern Pacif i c,and amplif i ed on and off the central equatorial Pacif i c.This cooling tendency persisted in the following months.Positive anomalies along the Equator disappeared gradually,and cold anomalies dominated over the whole equatorial band(Figs.3g and h).The vertical sections along the Equator displayed the same behavior(Figs.4e–h).

3.3.Phase relationships between subsurface and surface temperature anomalies

As analyzed above,the subsurface thermal anomalies at the Equator exhibited similar evolution to the SSTAs,but with a 2 month phase lead time:negative sea temperature anomalies(Fig.3)re-strengthened at subsurface depths in June,while those in Fig.1 re-strengthened at the sea surface in August.This indicates the existence of close links between subsurface temperature anomalies and the SSTAs.During boreal spring,positive SSTAs in the far-eastern equatorial Pacif i c(Figs.1b and c)cannot be explained by surface temperature advection,and they are likely to originate from the outcrop of subsurface warm anomalies(Figs.4b and c).This process can be described as follows.During the previous La Ni˜na event,warm waters piled up in the western Pacif i c Ocean due to stronger than normal easterly winds in the central basin.As the EUC became seasonally strengthened,the subsurface warm water was transported from the western Pacif i c to the central and eastern Pacif i c across the Equator(Figs.3b–c).Since the thermocline shoaled eastward(Figs.2a and c),the warm water was exposed to the sea surface in the eastern Pacif i c,acting to generate positive SSTAs(Figs.1b and c;Figs.4b and c).

As for the sea surface coolingin the fall of 2011,it can be traced to the subsurface anomalies.Beginning in mid-2011,subsurfacecoldanomalieslocatedinthesoutheasterntropical Pacif i cwerecontinuallyadvectednorthwestwardbytheSEC,to the south of the equatorial band,and then transported by the EUC to the Equator,where they were accumulated(Figs.3e–h).But how did the subsurface cold water in the central Pacif i c affect the sea surface?Since there was no systematic surface wind stress curl(f i gures not shown),the related Ekman pumpingwas not a major factor inf l uencingthe outcropping of subsurface cold water,so the upwelling can only be driven by oceanic processes.Figure 5 presents the tempera-ture anomalies,and the horizontal and vertical velocity f i elds on the 23.4 and 25.2 isopycnals.The convergence pattern of the horizontal currents agreed reasonably well with the vertical velocity f i eld.For example,the convergence center was located on the Equator near 110°W,where the EUC met the SEC,giving rise to a strong upwelling(Fig.5b).

InJune,small coldanomalieswere accompaniedbyweak upwelling in the central equatorial Pacif i c(Fig.5a).With time,both cold anomalies and vertical velocity strengthened in the central equatorial Pacif i c on the 25.2 isopycnal surface(Figs.5b and c).For example,in June the cold anomalies were conf i ned between 130°W and 150°W along the Equator,but it dominated the eastern central Pacif i c in July.These changes were induced by the weakened EUC and strengthened SEC,which favored the accumulation of cold water at the Equator.Figures 5e–h indicate that the vertical current in the upperlayerwas strongerthanthat at the lowerlayer(Figs.5a–d),and the cold anomalies appeared later than that on the subsurface layer,which conf i rmed that the cold water originated from the subsurface.As discussed above,there was a clear pathway along which subsurface cold water was transported to the sea surface.Firstly,the subsurface cold water located in the southeastern tropical Pacif i c was advected by the SEC south of the Equator.Subsequently,the EUC transported it to the equatorial Pacif i c,where the EUC met the SEC and induced upwelling.Finally,under the effects of the EUC and SEC,the cold water spread upward and westward to the sea surface.

In the fall of 2011(Figs.1g and h),negative SSTAs dominated in the central and eastern equatorial Pacif i c basin.The negative SSTAs in the east affected winds to the west,which in turn affected the thermocline and SST in the east.This essentiallyinvolvedinteractionsamonganomaliesofSST,wind and the thermocline,forming a coupling loop and leading to the second-year cooling during 2010–12.

3.4.Evolution during the 2012 decay phase

Figure 6 gives the horizontal distributions of SSTAs and surface wind anomalies at some selected time intervals in 2012.From February onwards,the cold SSTA diminished and the SSTA became normal in the eastern tropical Pacif i c domain(Fig.6a).This warming process persisted during the following months,and the SSTAs in the central and eastern tropical Pacif i c rose above normal(Fig.6d),except in the far-eastern Pacif i c.Figure 7 illustrates the subsurface temperature anomalies evaluated on the 25.2 isopycnal surface at some selected time periods in 2012;the vertical distribution of temperature anomalies in the upper ocean along the Equator is presented in Fig.8.Beginning in early 2012,accompanied by the seasonal strengthening of the EUC,warm waters in the western Pacif i c expanded eastward across the Equator(Figs.7a and 8a).In May,with the seasonal maximum EUC,warm anomalies occupied the whole central eastern equatorial Pacif i c(Figs.7band8b).Negativeanomaliesre-emerged twice(Figs.7c and e;Figs.8c and e)in the central equatorial Pacif i c,since the EUC decelerated from June onwards.However,these coolingprocessesdidnotpersist anddevelop,perhaps because the cold anomalies in the South Pacif i c were too weak to provideenoughcold water(Figs.7d–f;Figs.8d–f).Finally,the SSTAs did not return to the La Ni˜na state,as happened during 2011.

3.5.Evolution during the 2008 La Ni˜na event

Figure 9 gives the horizontal distributions of the SSTAs and surface wind anomalies at some selected time intervals in 2008.In January,a La Ni˜na state occupied the tropical Pacif i c:negative SSTAs prevailed in the central and eastern tropical Pacif i c with the maxima exceeding −2.5°C,located at 170°W along the Equator(Fig.9a).Thereafter,the cold SSTA diminished and the SSTA increased above normal in the far-eastern tropical Pacif i c domain(Fig.9b).This warm-ingprocesspersistedduringthefollowingmonthsandpeaked in August(Fig.9d).In September,the negative SSTA restrengthened in the central equatorial Pacif i c(Fig.9e),and this cooling tendency persisted during the following months(Fig.9f).

Figure 10 illustrates the subsurface temperature anomalies evaluated on the 25.4 isopycnal surface at some selected time periods in 2008;the vertical distribution of temperature anomalies in the upper ocean along the Equator is presented in Fig.11.Beginning in early 2008,accompanied by the seasonal strengthening of the EUC,warm waters in the western Pacif i c expanded eastward across the Equator(Fig.10b).This warming tendency peaked in mid-2008(Figs.10d and 11d),when positive temperature anomalies occupied almost the whole equatorial Pacif i c.There was a 1–2 month lead time into the SSTAs.Compared with the warming process in 2011(Figs.3c and 4c),it lagged by about 2 months,possibly attributable to stronger negative anomalies in the eastern tropical Pacif i c.Beginning in August,the subsurface cold anomalies located in the southeastern tropical Pacif i c were continually advected northwestward by the SEC to the south of the equatorial band,and then transported by the EUC to the Equator,where they accumulated(Figs.10e–h).The cold anomalies were then transported by a vertical current to the sea surface and induced negative SSTAs.From September,cold water re-strengthened in the central-equatorial Pacif i c(Fig.9e),and this cooling tendency persisted and extendedeastwardduringthefollowingmonths(Figs.9h);consequently,the double-trough La Ni˜na developed.

4.Summary and discussion

The reanalysis products from GODAS were used to produce isopycnalsurfaces to better illustrate and understandthe processes leading to the second-year cooling of the 2010–12 La Ni˜na event.We found anomaly patterns originating at depth from the southeastern tropical Pacif i c that could be responsible for generating and sustaining negative SSTAs in the central equatorial Pacif i c.

A sequence of events leading to the La Ni˜na conditions in the fall of 2011 was described.During the 2010 La Ni˜na event,warm waters piled up at subsurface depths in the western tropical Pacif i c.Beginning in early 2011,and accompanied by a strongEUC,subsurfacewarm waters in the western Pacif i c transmitted eastward along the Equator.Positive temperature anomalies occupied the equatorial Pacif i c in April,and cold waters retreated to northeastern and southeastern off-equatorial Pacif i c regions.Since the thermocline shoaled along the Equator and was close to the surface in the eastern Pacif i c,subsurface warm waters were directly exposed to the sea surface in the eastern Pacif i c,and induced a warm SSTA.Normal SST conditions appeared in the central and eastern equatorial Pacif i c in mid-2011.

In August a negative SSTA reappeared in the central Pacif i c.We hypothesized that this anomaly came from the subsurface cold waters off the Equator through the Southern Pacif i c pathway.Based on the GODAS analyses,the processes were described as follows:Cold anomalies located in southeastern tropical Pacif i c region were advected continually by the SEC northwestward to the south of the equatorial band,and then by the EUC northeastward to the Equator.With time,the EUC weakened and the SEC strengthened in the eastern equatorial Pacif i c,inducing cold waters that accumulated in the central tropical Pacif i c and then tended to spread upwardwith theconvergenceofhorizontalcurrentsandeventually outcropped to the surface.These subsurface-induced SSTAs actedtoinitiatelocalcoupledair–seainteractionsgenerating atmospheric–oceanic anomalies that developed and evolved with the second-year cooling in the fall of 2011.

Further study of the 2012 processes indicated that the cooling tendency did not develop into another La Ni˜na event,since the cold anomalies in the South Pacif i c were not strong enough.An analysis around the 2007–09 La Ni˜na event revealed similar evolution processes with around a 2—month phase lag,compared to the 2010–12 La Ni˜na event.

These analyses provide an observational basis for an understanding of the processes involved.The results can be used to explain the ways in which coupled models predict the second-year cooling case,and offer guidance for historical analysesforothermulti-yearcoolingevents.Furthersupporting modeling studies are needed to quantify the role played by off-equatorial subsurface anomalies in triggering La Ni˜na events in the tropical Pacif i c.Here,we discussed the effect of interannual variability on the multi-year cooling.The effect of modulation of decadal to interdecadal timescale variability on the multi-year cooling,such as tropical Pacif i c decadal variability(Choi et al.,2013),requires further study.

Acknowledgements.This work has benef i ted a great deal from Prof.A.J.BUSALACCHI’s support.This research was jointly supported by National Natural Science Foundation of China(Grant No.40906014),the Ocean Public Welfare Scientif i c Research Project(Grant No.201205018-2),the National Key Basic Research Program of China(Grant No.2010CB950302),and the China Scholarship Council(CSC).ZHANG is supported partly by the National Science Foundation(NSF)(Grant No.ATM-0727668),NASA(Grant No.NNX08AI74G),and the National Oceanic and Atmospheric Administration(NOAA)(Grant No.NA08OAR4310885).

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