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 Xingrong

1National Marine Environmental Forecasting Center,State Oceanic Administration,Beijing100081

2Key Laboratory of Ocean Circulation and Waves,Institute of Oceanology, Chinese Academy of Sciences,Qingdao266071

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

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

FENG Licheng?1,ZHANG Rong-Hua2,3,WANG Zhanggui1,and CHEN Xingrong1

1National Marine Environmental Forecasting Center,State Oceanic Administration,Beijing100081

2Key Laboratory of Ocean Circulation and Waves,Institute of Oceanology, Chinese Academy of Sciences,Qingdao266071

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

Isopycnal analyses were performed on the Global Ocean Data Assimilation System(GODAS)to determine the oceanic processes leading to so-called second-year cooling of the La Ni?na event.In 2010–12,a horseshoe-like pattern was seen, connecting negative temperature anomalies off and on the Equator,with a dominant infuence from the South Pacifc.During the 2010 La Ni?na event,warm waters piled up at subsurface depths in the western tropical Pacifc.Beginning in early 2011, these warm subsurface anomalies propagated along the Equator toward the eastern basin,acting to reverse the sign of sea surface temperature(SST)anomalies(SSTAs)there and initiate a warm SSTA.However,throughout early 2011,pronounced negative anomalies persisted off the Equator in the subsurface depths of the South Pacifc.As isopycnal surfaces outcropped in the central equatorial Pacifc,negative anomalies from the subsurface spread upward along with mean circulation pathways,naturally initializing a cold SSTA.In the summer,a cold SSTA reappeared in the central basin,which subsequently strengthened due to the off-equatorial effects mostly in the South Pacifc.These SSTAs acted to initiate local coupled air–sea interactions,generating atmospheric–oceanic anomalies that developed and evolved with the second-year cooling in the fall of 2011.However,the cooling tendency in mid-2012 did not develop into another La Ni?na event,since the cold anomalies in the South Pacifc were not strong enough.An analysis of the 2007–09 La Ni?na event revealed similar processes to the 2010–12 La Ni?na event.

La Ni?na,second-year cooling,off-equatorial effects,isopycnal analyses,circulation pathways,GODAS

1. Introduction

The El Ni?no–Southern Oscillation(ENSO)is the leading mode of interannual variability in the tropical Pacifc climate system,signifcantly 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 Pacifc climate system(Battisti and Hirst,1989).This theory emphasizes equatorial wave processes(Rossby wave and its refection 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 Pacifc climate system.

However,as observed,the ENSO also exhibits signifcant 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 Pacifc 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 oneyear or so(Zhang et al.,2013,2014aZhang,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.).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.,2013,2014aZhang,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 Pacifc in early summer of 2011 was closely related to off-equatorial thermal anomalies in the South Pacifc.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 Pacifc.To better represent pathways,isopycnal analyses were performed using three-dimensional temperature and salinity felds(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 fnding 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 Pacifc 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 Pacifc 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 felds were formed from the period 1980–2012,including monthly-mean current vectors. Interannualanomaliesfortemperature,windstress andothers werethencalculatedrelativetotheir climatologicalfelds.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 felds 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 Pacifc.Consequently,negative SSTAs prevailed in the central and eastern tropical Pacifc 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 Pacifc and southeasterly wind anomalies dominated off the Equator in the South Pacifc(Fig.1a). Thereafter,the cold SSTA diminished and the SSTA became normal in the eastern tropical Pacifc 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 Pacifc(Fig.1d).In August, negative SSTA strengthened in the central equatorial Pacifc (Fig.1f),and this cooling tendency persisted during the following months(Figs.1g–h).

Themechanismofformationofthe coldSSTA in the central eastern equatorial Pacifc 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 Pacifc(Fig.1e),forcing the cold waters located in the South Pacifc 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 Pacifc,especially after the wind anomalies changed direction during September and October(Figs.1g–h).Otherprocesses,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 fow pattern.According to Cox and Bryan(1984),Bcan be written as

whereσ=ρ-1000 is an isopycnal surface,andρis density with units of kg m-3;ρ0is mean density;gis the acceleration due to gravity,andηis dynamic height.Brepresents geostrophic streamlines that measure the geostrophic fow away from the Equator;thus,it can be used to illustrate fow 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 Pacifc.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 Pacifc,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 Pacifc.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 Pacifc,with fewer studies in the South Pacifc.Chang et al.(2001)pointed out the potential importance of south tropical Pacifc 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 Pacifc.

FromFigs.2b andd,onecan see clear pathways originating from the southeastern tropical Pacifc: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 Pacifc water pathways intersect with the surface in the eastern equatorial and Southeast Pacifc 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 Pacifc 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 Pacifc and a negative anomaly was locatedin the central eastern tropical Pacifc 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 Pacifc expanded eastward across the Equator;cold anomalies in the central eastern equatorial Pacifc diminished and reversed to above normal(Figs.3b and 4b).This warming tendency peakedin April(Fig.3c),when positive temperature anomalies occupied almost the whole equatorial Pacifc except for near 150°W.Temperature anomalies reached more than 2°C in the far-eastern equatorial Pacifc.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 confned to a narrow region of the central Pacifc.

In May,positive anomalies along the Equator were seen to have two separate western and eastern bands(Fig.3d), with below-normal temperature anomalies amplifed in the regions of 140°–160°W(Fig.4d).Subsequently,the neg-ative anomalies dominated over the central Pacifc 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 Pacifc in June(Yu et al.,1997), but the off-equatorialcold anomalies strengthenedin the central South Pacifc.These changes were in favor of cold water advection to the equatorial regions through the well-defned SouthPacifcwaterpathway(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 Pacifc on the 25.2 isopycnal surface.At this time,cold anomalies were transported by SEC from the Southeastern Pacifc, and amplifed on and off the central equatorial Pacifc.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 temperatureanomalies(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 Pacifc(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 Pacifc 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 Pacifc to the central and eastern Pacifc 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 Pacifc,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 PacifcwerecontinuallyadvectednorthwestwardbytheSEC, 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 Pacifc affect the sea surface?Since there was no systematic surface wind stress curl(fgures not shown),the related Ekman pumpingwas not a major factor infuencingthe 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 felds on the 23.4 and 25.2 isopycnals.The convergence pattern of the horizontal currents agreed reasonably well with the vertical velocity feld.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 Pacifc(Fig.5a).With time,both cold anomalies and vertical velocity strengthened in the central equatorial Pacifc on the 25.2 isopycnal surface (Figs.5b and c).For example,in June the cold anomalies were confned between 130°W and 150°W along the Equator,but it dominated the eastern central Pacifc 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 confrmed 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 Pacifc was advected by the SEC south of the Equator.Subsequently,the EUC transported it to the equatorial Pacifc,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 Pacifc 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 Pacifc domain(Fig.6a).This warming process persisted during the following months,and the SSTAs in the central and eastern tropical Pacifc rose above normal(Fig.6d),except in the far-eastern Pacifc.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 Pacifc expanded eastward across the Equator (Figs.7a and 8a).In May,with the seasonal maximum EUC, warm anomalies occupied the whole central eastern equatorial Pacifc(Figs.7band8b).Negativeanomaliesre-emerged twice(Figs.7c and e;Figs.8c and e)in the central equatorial Pacifc,since the EUC decelerated from June onwards. However,these coolingprocessesdidnotpersist anddevelop, perhaps because the cold anomalies in the South Pacifc weretoo 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 intervalsin 2008.In January,a La Ni?na state occupied the tropical Pacifc:negative SSTAs prevailed in the central and eastern tropical Pacifc 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 Pacifc domain(Fig.9b).This warm-ingprocesspersistedduringthefollowingmonthsandpeaked in August(Fig.9d).In September,the negative SSTA restrengthened in the central equatorial Pacifc(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 Pacifc 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 Pacifc.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 Pacifc.Beginning in August,the subsurface cold anomalies located in the southeastern tropical Pacifc 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 Pacifc(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 Pacifc that could be responsible for generating and sustaining negative SSTAs in the central equatorial Pacifc.

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 Pacifc.Beginning in early 2011,and accompanied by a strongEUC,subsurfacewarm waters in the western Pacifc transmitted eastward along the Equator.Positive temperature anomalies occupied the equatorial Pacifc in April, and cold waters retreated to northeastern and southeastern off-equatorial Pacifc regions.Since the thermocline shoaled along the Equator and was close to the surface in the eastern Pacifc,subsurface warm waters were directly exposed to the sea surface in the eastern Pacifc,and induced a warm SSTA. Normal SST conditions appeared in the central and eastern equatorial Pacifc in mid-2011.

In August a negative SSTA reappeared in the central Pacifc.We hypothesized that this anomaly came from the subsurface cold waters off the Equator through the Southern Pacifc pathway.Based on the GODAS analyses,the processes were described as follows:Cold anomalies located in southeastern tropical Pacifc 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 Pacifc,inducing cold waters that accumulated in the central tropical Pacifc 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 Pacifc 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 Pacifc.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 Pacifc decadal variability(Choi et al.,2013),requires further study.

Acknowledgements.This work has benefted 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 Scientifc 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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(Received 7 February 2014;revised 12 July 2014;accepted 22 July 2014)

?Corresponding author:FENG Licheng

Email:fenglich@nmefc.gov.cn