Observed ENSO teleconnections to Southern Ocean SST anomaliesdiagnosed from a surface mixed layer heat budget

Laura M. Ciasto1 and Matthew H. England1

Received 30 January 2011; revised 17 March 2011; accepted 23 March 2011; published 3 May 2011.

[1] This study examines the mechanisms that govern theEl Niño Southern Oscillation (ENSO) teleconnections tovariability in extratropical Southern Hemisphere sea surfacetemperature (SST) anomalies using several observationaland reanalysis products. During the warm season, turbulentheat fluxes and heat advection by Ekman currents contributeequally to ENSO‐related SST variability throughout theSouthern Ocean, while shortwave radiation has a largercontribution in the subtropics. A comparison of the oceanmixed layer heat budget between multiple re‐analysis pro-ducts reveals that the relative contributions of the turbulent,radiative and Ekman heat fluxes to ENSO‐related SSTanomalies is consistent between the NCEP, ERA‐40 andOAFlux/ISCCP products, but discrepancies exist with regardto the structure and amplitude of the turbulent heat fluxesand, to a lesser extent, the shortwave radiation. However, acomparison between the tendencies in SST and flux‐derivedtemperatures reveals minimal residuals, suggesting that theENSO‐signal in the SST field can be resolved by turbulent,Ekman, and radiative heat fluxes, particularly in the ERA‐40products. Citation: Ciasto, L. M., and M. H. England (2011),Observed ENSO teleconnections to Southern Ocean SST anomaliesdiagnosed from a surface mixed layer heat budget, Geophys. Res.Lett., 38, L09701, doi:10.1029/2011GL046895.

1. Introduction

[2] The El Niño Southern Oscillation (ENSO) is the mostprominent mode of large‐scale variability in the global cli-mate system. The associated changes in tropical convectiveprecipitation give rise to changes in atmospheric circulationaround the globe via Rossby wave propagation [Hoskins andKaroly, 1981]. These global ENSO teleconnections act as anatmospheric bridge through which the remote atmosphericresponse to ENSO impacts the underlying sea surface tem-perature (SST) fields via fluxes of radiative (longwave+shortwave) and turbulent (latent+sensible) energy as wellas horizontal ocean heat transport due to Ekman currents[Alexander et al., 2002]. In the tropical Indian and AtlanticOceans, the SST anomalies associated with ENSO are pri-marily driven by surface turbulent heat fluxes as well as byshortwave radiation [Klein et al., 1999] while heat advectionby horizontal Ekman currents is thought to have a relativelyminor impact. In the extratropical North Pacific, the ENSO‐related turbulent heat fluxes are also largely responsible forgeneration of the associated SST anomalies, but the Ekman

heat fluxes become more dominant than shortwave radia-tion outside of the subtropics [Alexander et al., 2002].[3] In the extratropical Southern Hemisphere (SH), the

mechanisms that drive ENSO teleconnections to the oceanmixed layer are less understood. The relationships betweenENSO, SST, turbulent and Ekman heat flux anomalies havebeen presented in only a few studies [Verdy et al., 2006;Ciasto and Thompson, 2008], but the impact of radiative heatfluxes has not been assessed outside of the subtropics [Kleinet al., 1999]. Furthermore, the extent to which turbulent,radiative and Ekman fluxes account for fluctuations in SSTanomalies has not been quantified due, in part, to the lack ofreliable flux products over the SH mid and high latitudes. Asthe SH oceans remain the most poorly sampled regionsaround the globe, the most comprehensive flux productsavailable are reanalysis fields such as those provided bythe National Center for Environmental Prediction‐NationalCenter for Atmospheric Research (NCEP‐NCAR) and theEuropean Center for Medium‐Range Weather Forecasts(ECMWF), which have continuous global spatial and tem-poral data coverage. The reanalysis‐derived turbulent heatfluxes are calculated solely from model‐forced fields andoften exhibit large uncertainties [Josey, 2001; Sun et al.,2003], but have been shown to agree well with observedSST patterns associated with large‐scale modes of SHatmospheric variability [Screen et al., 2010]. In this study,the heat budget of the SH ocean mixed layer associated withENSO is examined, focusing on the relative contributionsof radiative, turbulent and Ekman heat fluxes to SST vari-ability. Each ENSO‐related flux component is comparedbetween several of the most commonly used flux productsto determine where the most robust features of the ENSOteleconnections are observed and also where the largestdiscrepancies exist. The total air‐sea heat and Ekman fluxfields are then compared to the tendency in SST anomalies todetermine the fraction of the ENSO‐related SST signal thatis captured by radiative, turbulent and Ekman heat fluxes.

2. Data and Methods

[4] In this study, we use ERA‐40 [Uppala et al., 2005] andNCEP‐2 [Kanamitsu et al., 2002] re‐analyses, which consistof in situ observations and satellite data that are qualitycontrolled using an assimilation system. The 500 hPa geo-potential height (Z500) and surface wind stress fields (thelatter are used to derive Ekman heat fluxes) are primarilyobtained from observations. The turbulent heat fluxes fromERA‐40 and NCEP‐2 are derived from bulk flux algorithmsusing model‐forced fields but employ different boundaryparameterizations. NCEP‐2 radiative heat fluxes rely onsatellite retrievals, which estimate vertical temperature and

1Climate Change Research Centre, University of New South Wales,Sydney, New South Wales, Australia.

Copyright 2011 by the American Geophysical Union.0094‐8276/11/2011GL046895

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humidity profiles through a series of empirical and statisticalrelationships, whereas ERA‐40 products assimilate the rawsatellite radiances. Turbulent heat fluxes are also obtainedfrom the Objectively Analyzed air‐sea Fluxes (OAFlux)project, which rather than using a single source, objectivelyblend data from NCEP and ECMWF re‐analyses as well assatellite and in situ data [Yu and Weller, 2007]. Surfaceradiative heat fluxes and cloud cover are obtained fromInternational Satellite Cloud Climatology Project (ISCCP)and are calculated from a radiative transfer model in theNASA GISS general circulation model [Zhang et al., 2004].[5] Monthly‐mean SST data were taken from the National

Oceanic and Atmospheric Administration/Optimum Inter-polation (OI) dataset [Reynolds et al., 2002], which arederived from a blended objective analysis of both in situ andsatellite observations. Mixed layer depths are taken from theOcean Mixed Layer Depth Climatology [de Boyer Montégutet al., 2004] and are defined as the upper‐most depth at whichtemperature differs from the temperature at 10 m by 0.2°C.ENSO is defined as monthly SST anomalies averaged overthe NINO 3.4 region. This study focuses on the austral warmseason (November–April) when ENSO teleconnections toSH SST anomalies are strongest. The analysis is also restrictedto 1979–2008 when re‐analysis products are best constrainedby both in situ and satellite data [Bromwich and Fogt, 2004].

3. Components of the ENSO Mixed Layer HeatBudget

[6] Maps formed by regressing anomalous warm seasonshortwave radiative heat fluxes (shading) and cloud cover(contours) onto standardized values of the NINO3.4 indexare shown in Figure 1 (top). The ERA‐40, NCEP‐2 andISCCP products each exhibit increases in shortwave radiation

in the western Pacific that are consistent with the anomalousreduction in cloud cover associated with the eastward shiftof the weakened Walker Circulation during the warm phaseof ENSO [Klein et al., 1999]. The positive ERA‐40 andNCEP‐2 anomalies are confined primarily along 30°–40°S,180°E–230°E, but the positive ISCCP anomalies extendfurther poleward and eastward into the central Pacific. Inthe eastern Pacific/Atlantic, negative anomalies are evidentin the ISCCP and ERA‐40 products but not in the NCEP‐2reanalysis. The magnitudes of the shortwave radiation anom-alies are similar amongst the flux products, but tend to bestronger in the NCEP‐2 reanalysis. In contrast, ENSO‐relatedlongwave radiation (Figure 1, bottom) exhibits relativelyweak variability, with only modest negative anomalies in thewestern Pacific and Indian Ocean, the latter only observedin the ISCCP data.[7] Given that the atmospheric circulation associated with

ENSO is equivalent barotropic, Z500 anomalies can be used toinfer the patterns of near‐surface atmospheric flow, revealinga broad consistency with the patterns of turbulent heat fluxanomalies derived from NCEP‐2, ERA‐40 and OAFlux(Figure 2). Positive turbulent heat flux anomalies coincidewith the anticyclonic circulation while negative heat fluxanomalies coincide with the cyclonic circulation. Howeverthe degree of spatial coherence between the Z500 and heatflux anomalies varies between flux products. The OAFluxand NCEP‐2 turbulent heat fluxes, which are nearly iden-tical to the NCEP‐1 turbulent heat fluxes shown by Ciastoand Thompson [2008], are more sparse and localized. TheERA‐40 turbulent heat fluxes provide the strongest agree-ment with the atmospheric circulation, encompassing largerregions under the Z500 anomalies. The amplitudes of theturbulent heat fluxes are strongest in NCEP‐2, particularlysouth of Africa where amplitudes are 2–3 times stronger than

Figure 1. Regressions of November–April (top) shortwave and (bottom) longwave radiative heat flux (shading) and cloudcover (Figure 1, top; contours) anomalies onto standardized values of the NINO3.4 Index for (left) ERA‐40, (middle) NCEP‐ 2and (right) ISCCP datasets. Positive heat fluxes are directed into the ocean and have units of Wm−2. Positive (solid) and neg-ative (dashed) cloud contours are drawn at intervals of 0.75%.

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those observed in the ERA‐40 fluxes. Stronger agreementexists between the re‐analysis products with regard to theENSO‐related Ekman heat fluxes, which are derived bycalculating the product of the curl of the horizontal windstress and the gradient in SSTs. Positive (negative) Ekmanheat fluxes can be attributed to warm (cold) water advectiondriven by poleward (equatorward) Ekman transport inducedby the anomalous easterlies (westerlies) in the lower atmo-sphere. The amplitudes of the Ekman heat fluxes are strongestin ERA‐40 and weakest in OAFlux, but are generally ofcomparable, if not stronger, magnitude relative to the tur-bulent heat fluxes.[8] The extent to which the radiative, turbulent and Ekman

heat fluxes contribute to ENSO‐related SST variability isassessed using a simplified thermodynamic relationship inwhich the governing equation for the surface temperaturetendency is expressed as:

dSST

dt¼ QRHF þ QTHF þ QEk

�cpHþ R ð1Þ

where QRHF, QTHF and, QEk are the radiative, turbulentand Ekman heat fluxes, respectively, cp is the heat capacityof seawater (4218 Jkg−1K−1), r is the density of seawater(1000 kgm−3) and H is the climatological mean warm seasonmixed layer depth. Eddy processes, Ekman pumping andhorizontal heat advection via geostrophic currents are effec-tively constrained in the residual term R. In the analysisbelow, the tendencies in OISST anomalies (Figure 3a), cal-culated as the difference between the regressions of SSTanomalies onto ENSO when SSTs lead and lag by 1 month,

are compared to the flux‐derived temperature tendencies(first term on right‐hand side of equation (1)) from each ofthe flux products (Figure 3b, top). The results in Figure 3aare not sensitive to the choice of the time interval used tocalculate the SST tendency term and are qualitatively similarto those obtained when SST data from the Hadley Centre MetOffice are used.[9] In the Pacific, the tendencies in ENSO‐related SST

anomalies are characterized by positive anomalies equator-ward of New Zealand and west of South America and nega-tive anomalies poleward of New Zealand and in the centralsubtropics. In the Atlantic and Indian oceans, negative (pos-itive) tendency anomalies are observed along 50°S (30°S). Asimilar pattern arises in the ERA‐40 flux‐derived tempera-tures (Figure 3b, top left) although the amplitudes are gen-erally larger than the SST tendency anomalies. Consequently,the associated residual fields (Figure 3b, bottom left) exhibitnegative anomalies, but are localized and lack a coherentlarge‐scale structure, suggesting that the ERA‐40 fluxanomalies are able to capture a significant fraction of theENSO teleconnections to SH SSTs. The patterns of NCEP‐2and OAFlux/ISCCP flux‐derived tendencies do not project asstrongly onto that of tendencies in SST anomalies. The largerresiduals (Figures 3b, bottom middle and 3b, bottom right)appear to arise from discrepancies in the turbulent andshortwave radiative heat fluxes. Relative to ERA‐40 fluxes,the shortwave radiation is stronger in the NCEP‐2 reanalysisand more widespread in the ISCCP data. The NCEP‐2 andOAFlux turbulent heat fluxes are weaker than the ERA‐40fluxes under the cyclonic Z500 anomalies but substantiallystronger around Australia and Africa. As a result, the signa-ture of the ENSO teleconnections to the SH surface mixed

Figure 2. Regressions of November–April Z500 (contours), (top) turbulent heat flux (shading) and (bottom) Ekman heat flux(shading) anomalies onto standardized values of the NINO3.4 Index for (left) ERA‐40, (middle) NCEP‐2 and (right) OAFluxdatasets. Z500 anomalies in Figure 2 (right) are also from the ERA‐40 reanalysis. Positive heat fluxes are directed into the oceanand have units ofWm−2. Positive (solid) and negative (dashed) Z500 contours are drawn at 10m intervals from −25m to +25m.

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layer is less discernible in the flux‐derived tendencies derivedfrom either the NCEP‐2 or OAFlux/ISCCP data.

4. Summary and Discussion

[10] This study utilizes several reanalysis products toexamine the role of turbulent, radiative and Ekman heatfluxes on SST variability associated with the extratropical SHatmospheric response to ENSO. Shortwave radiative heatfluxes exhibit a significant contribution to SST variabilityin the subtropical Pacific but are negligible at higher latitudes.In the extratropics, surface turbulent and Ekman heat fluxescontribute equally to ENSO‐related SST variability, a resultthat is unique to the SH; in the North Pacific, for example,Ekman heat fluxes are necessary to obtain the correct struc-ture and magnitude of the ENSO‐related flux patterns butthe magnitudes are half that of the turbulent heat fluxes.The overall contributions of turbulent, radiative and Ekmanheat fluxes to ENSO‐related SH SST variability are similarbetween the OAFlux/ISCCP, NCEP‐2 and ERA‐40 reanal-ysis products, but there is disagreement in terms of thestructures of turbulent heat and shortwave radiation, whichcould be due to inconsistencies in the boundary layerparameterizations in the flux algorithms [Renfrew et al.,2002] and the differing methods used to derive the radiativeheat fluxes.

[11] Comparisons between tendencies in SST and flux‐derived temperatures from each flux product reveal residualsthat could result from a number of factors. First, there areknown biases in the reanalysis flux products [Sun et al.,2003]. Second, the mixed layer depths used in equation (1)are based on a 6‐month climatological mean, encompassinga substantial seasonal cycle. However, similar results areobtained when the analysis in Figure 3 is repeated bycalculating flux‐derived temperatures for each month inNovember–April separately and then averaging over the6‐month period, suggesting that our choice of H is reason-able. Third, the ocean mixed layer budget in equation (1)neglects the effects of vertical entrainment and Ekmanpumping which can dampen SST [Frankignoul, 1985] thiscould thus explain why flux‐derived temperature tendenciesare observed along coastal regions (e.g., New Zealand, SouthAfrica and Australia) in the absence of SST variability. Thediscrepancy between SST and flux anomalies along coastalregions may also be due to the exclusion of eddy fluxesand/or mean advection by non‐Ekman ocean currents. Whilethese mechanisms not explicitly included in equation (1) mayimpact ENSO‐related SST variability, a comparison betweenthe temperature tendencies derived from each flux productand the tendencies in SST anomalies reveals mostly smallresiduals (particularly in the ERA‐40 data), suggesting that

Figure 3. (a) Tendency in November–April SST anomalies derived from the OI SST analysis and (b) (top) tendency influx‐derived temperature anomalies regressed onto the NINO3.4 index and (bottom) residual between the tendencies inSST and flux‐derived temperature anomalies for (left) ERA‐40, (middle) NCEP‐2 and (right) ISCCP/OAFlux datasets. Unitsin Figures 3a and 3b are in °C/2 month.

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the ENSO signal in the SH SST field can be effectivelycaptured by the turbulent, Ekman, and radiative heat fluxes.

[12] Acknowledgments. We thank the two anonymous reviewers fortheir constructive comments on this manuscript. This research was supportedby the Australian Research Council.[13] The Editor thanks Clara Deser and an anonymous reviewer for their

assistance in evaluating this paper.

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