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Changes in ENSO and Associated Overturning Circulations from Enhanced Greenhouse Gases by the End of.. Ye, Zhengqing; Hsieh, William W. 2008-11-30

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Changes in ENSO and Associated Overturning Circulations from EnhancedGreenhouse Gases by the End of the Twentieth CenturyZHENGQING YE AND WILLIAM W. HSIEHDepartment of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada(Manuscript received 11 July 2006, in final form 2 April 2008)ABSTRACTWith data from 12 coupled models in the Fourth Assessment Report (AR4) of the IntergovernmentalPanel on Climate Change (IPCC), climate under year 2000 greenhouse gas (GHG) H11001 aerosol forcing wascompared with climate under preindustrial conditions. In the tropical Pacific, the warming in the mean seasurface temperatures (SST) was found to have an El Niño–like pattern, while both the equatorial zonaloverturning circulation and the meridional overturning circulation weakened under increased GHG forcing.For the El Niño–Southern Oscillation (ENSO), the asymmetry in the SST anomalies between El Niñoand La Niña was found to be enhanced under increased GHG, for both the ensemble model data and theobserved data (1900–99). Enhanced asymmetry between El Niño and La Niña was also manifested in theanomalies of the zonal wind stress, the equatorial undercurrent, and the meridional overturning circulationin the increased GHG simulations. The enhanced asymmetry in the model SST anomalies was mainlycaused by the greatly intensified vertical nonlinear dynamic heating (NDH) anomaly (i.e., product of thevertical velocity anomaly and the negative vertical temperature gradient anomaly) during El Niño (but notduring La Niña). Under increased GHG, the enhanced positive NDH anomalies during El Niño, when timeaveraged over the whole record, would change the SST mean state by an El Niño–like pattern.1. IntroductionThe global atmospheric and tropical oceanic surfacetemperatures increased in the past century by H110220.5°C(Jones and Moberg 2003; Anthes et al. 2006). Thiswarming is, at least in part, a result of emissions ofgreenhouse gases (GHGs) from human activities(Houghton et al. 2001). The climate change caused bythe emissions of GHGs from human activities will alsochange the tropical Pacific climate state. ENSO, thestrongest interannual signal in the tropical Pacific,changes its characteristics with the change in the back-ground climatology (Fedorov and Philander 2001; Anand Wang 2000; Ye and Hsieh 2006). Our primary in-terest here is to study how changes in GHGs from thepreindustrial level to the year 2000 level would impactENSO.There is debate in the climate literature as to whetherthe low-frequency change of ENSO in the tropical Pa-cific is generated within the tropics by tropical internalinstability (Knutson and Manabe 1998), or uncoupledatmospheric noise (Thompson and Battisti 2001; Flügelet al. 2004), or whether it involves the interaction be-tween the extratropics and the tropics (Gu and Philan-der 1997; Kleeman et al. 1999; Barnett et al. 1999). Thezonal overturning circulation in the equatorial Pacificdirectly transports water mass and heat energy betweenthe western warm pool and the eastern cool upwellingarea, while the meridional overturning circulation con-veys water mass and heat energy between the tropicaland subtropical Pacific. McCreary and Lu (1994) andLiu (1994) investigated the fundamental dynamics ofthe meridional overturning circulation using simple lay-ered thermocline models. They postulated that the sub-tropical water from the northeast subtropics reachesthe equator via low-latitude western boundary currentsor via an interior path directly linking the northeastsubtropics to the central tropics.There is also debate on whether the climate is chang-ing into El Niño–like warming or La Niña–like coolingin the tropical Pacific. Cane et al. (1997) found that theobserved SST had a cooling trend in the eastern tropi-cal Pacific during 1900–91. However, Houghton et al.Corresponding author address: Zhengqing Ye, Jet PropulsionLaboratory, California Institute of Technology, 4800 Oak GroveDrive, Pasadena, CA 91109.E-mail: zye@pacific.jpl.nasa.govVOLUME 21 JOURNAL OF CLIMATE 15 NOVEMBER 2008DOI: 10.1175/2008JCLI1580.1© 2008 American Meteorological Society 5745JCLI1580(2001) found that the trend for SST was El Niño–like inthe tropical Pacific in many models. Meehl and Wash-ington (1996) also noted an El Niño–like climatechange in their coupled general circulation climatemodel (CGCM) with CO2doubling. This paper will usethe observed SST from 1900 to 1999, which was con-structed using the most recently available InternationalComprehensive Ocean–Atmosphere Data Set(ICOADS) SST data and improved statistical methodsthat allow stable reconstruction using sparse data, in-cluding a modified historical bias correction for the1939–41 period.The meridional overturning circulation has beenslowing down in the later decades during the period1950–99 (McPhaden and Zhang 2002), though it re-bounded during the short period 1998–2003 with re-spect to the period 1992–98 (Zhang and McPhaden2006). According to McPhaden and Zhang (2004),there appeared to be a strong decadal variability super-imposed on a linear weakening trend in the period1953–2001. Using a coupled model, Merryfield andBoer (2005) found that anthropogenic forcing may havecontributed to the observed slowdown of the meridio-nal overturning circulation. The zonal Walker circula-tion driven by convection in the western equatorial Pa-cific and subsidence in the east shows a weakeningtrend since the mid-nineteenth century due to anthro-pogenic forcing (Vecchi et al. 2006). Here we will in-vestigate the slowdown in the oceanic zonal and merid-ional circulations using the Intergovernmental Panel onClimate Change (IPCC) Fourth Assessment Report(AR4) multimodel ensemble based on 12 CGCMs.There is much uncertainty in how ENSO will changeits characteristics (e.g., the amplitude and frequency)under increased GHG in the CGCMs. Guilyardi (2006)assessed the ENSO mean state–seasonal cycle interac-tions in 23 coupled ocean–atmosphere models by com-paring the preindustrial control and the stabilized 2 H11003CO2and 4 H11003 CO2scenario runs. The ENSO amplitudewas found to be an inverse function of both the meantrade winds and the relative strength of the seasonalcycle, but the relation was less clear for the ENSO fre-quency. Van Oldenborgh et al. (2005) found very littleinfluence of global warming on ENSO from the resultsof 17 models. Merryfield (2006) compared the differ-ences in ENSO amplitude, period, and pattern underpreindustrial conditions and under CO2doubling: Theamplitude changes were not strongly related to themagnitude or pattern of surface warming. A narrow(wide) wind stress response was associated with ENSOamplitude decrease (increase). The models exhibited amean fractional decrease in the ENSO period by about5% and an increase in the amplitude of SST variationsin the central tropical Pacific. On the other hand, Meehlet al. (2006) found that ENSO events decreased in mag-nitude in the future warmer climate (2 H11003 CO2,4H11003 CO2,and other scenarios). Collins et al. (2005) showed thatthe most likely scenario is for no large amplitudechange toward mean El Niño– or La Niña–like condi-tion in the 20 models submitted to the Coupled ModelIntercomparison Project (CMIP) by comparing the 80-yr control simulation with fixed CO2level and the 80-yrsimulation in which CO2was increased from the controlvalue at a rate of 1% per year compounded. In thepresent paper, the changes in ENSO under the com-mitted CO2level (i.e., the year 2000 CO2level) areexamined.ENSO displays considerable asymmetry between itswarm phase (El Niño) and its cold phase (La Niña) (An2004; Rodgers et al. 2004). The asymmetry is due to thenonlinearity of the ENSO system, as the nonlinear dy-namic heating in the tropical Pacific ocean heat budgetis essential in producing decadal changes in ENSO non-linearity and asymmetry (Jin et al. 2003; An and Jin2004). Changes in the asymmetry and nonlinearity ofENSO and associated overturning circulations are alsoaddressed in our study.The observed SST and the ensemble model data aredescribed in section 2 and section 3, respectively. Bycomparing the ensemble model simulations under thepreindustrial GHG conditions and under the presentconditions, we identified climate change in the tropicalPacific (section 4), changes in the ENSO SST and zonalwind stress anomalies (section 5), and changes in theENSO ocean circulation (section 6). A diagnosticanalysis of the ocean surface temperature equationgave some explanation for the enhanced ENSO asym-metry (section 7).2. Observed SSTThe NOAA extended reconstructed SST data (Smithand Reynolds 2004) were used, with the data dividedinto the 1900–49 and 1950–99 periods. The climatologi-cal difference between the two periods suggests that theSST in the eastern-central tropical Pacific has increasedby at least 0.3°C (Fig. 1). The climatological differencepattern resembles the El Niño pattern, with the maxi-mum positive value in the eastern equatorial Pacific. InFig. 1, the significance test was performed using theStudent’s t test, where the equivalent sample size (i.e.,degrees of freedom) was estimated from the autocor-relation function (von Storch and Zwiers 1999, p. 115).Prior to calculating the composite maps for El Niñoand La Niña episodes, a 3-month running mean wasapplied after the climatological seasonal cycle and lin-5746 JOURNAL OF CLIMATE VOLUME 21ear trend were removed in each period. We examinedthe composite maps of SST anomalies for El Niño andLa Niña episodes during each period (Fig. 2). El Niñoand La Niña episodes were defined based on the Niño-3.4 index (i.e., the averaged SST anomalies over theregion 5°S–5°N, 120°–170°W exceeding H110061 standarddeviation). Compared to the 1900–49 period, a promi-nent eastward shift of positive SST anomalies appearsover the eastern equatorial Pacific during El Niñointhe 1950–99 period, where the maximum anomaly “H”has shifted from 120°W (Figs. 2a) to east of 90°W (Figs.2c). During La Niña, a westward shift of negative SSTanomalies is evident in the latter period; that is, theshift during La Niña is in opposite direction comparedto that during El Niño (Figs. 2b,d). These shifts obvi-ously enhanced the El Niño–La Niña asymmetry andFIG. 2. Composite maps of SST anomalies (°C) during (left) El Niño and (right) La Niña for (a), (b) the 1900–49 periodand (c), (d) the 1950–99 period: “H” and “L” mark the location of the highest and lowest values, respectively, and 5% and10% significance levels based on the t test are shown in gray.FIG. 1. (a) Observed mean SST (°C) during 1950–99, and (b) the mean SST of 1950–99 minus that of 1900–49, with5% and 10% significance levels based on the t test, shown in dark and light gray, respectively.15 NOVEMBER 2008 Y E A N D H S I E H 5747nonlinearity in the latter period. To derive an index forthe asymmetry, the SST composite (Fig. 2) for El Niñoand that for La Niña were added together at each gridpoint, then the root-mean-square (rms) deviation (av-eraged over the tropical Pacific) is a measure of theasymmetry, with a zero value indicating the La Niñapattern to be completely symmetrical to the El Niñopattern. The rms deviation was 0.11°C for the period1900–49 and 0.19°C for 1950–99, confirming the en-hanced asymmetry in the latter period. This change inthe asymmetry has a longer time scale (50 yr) comparedto the decadal change in ENSO asymmetry found afterthe late 1970s (Ye and Hsieh 2006; Wu and Hsieh 2003;An 2004). To find a possible cause for this longer timescale change in ENSO, we will examine the effects ofGHG forcing in the following sections.3. Model dataModel data from the IPCC AR4 database weredownloaded from the archive hosted by the Programfor Climate Model Diagnosis and Intercomparison(PCMDI). The 12 CGCMs used here are the CanadianCentre for Climate Modelling and Analysis (CCCMA)CGCM3.1; Centre National de RecherchesMétéorologiques Coupled Global Climate Model, ver-sion 3 (CNRM-CM3); Commonwealth Scientific andIndustrial Research Organisation Mark version 3.0(CSIRO MK3.0); Geophysical Fluid Dynamics Labo-ratory Climate Model version 2.0 (GFDL CM2.0);Goddard Institute for Space Studies Model E-R (GISS-ER); Institute of Atmospheric Physics Flexible GlobalOcean–Atmosphere–Land System Model gridpointversion 1.0 (IAP FGOALS-g1.0); Institute of Numeri-cal Mathematics Coupled Model, version 3.0 (INM-CM3.0); L’Institut Pierre-Simon Laplace CoupledModel, version 4 (IPSL-CM4); Model for Interdiscipli-nary Research on Climate 3.2, medium-resolution ver-sion [MIROC3.2(medres)]; Meteorological Institute ofthe University of Bonn (MIUB) ECHAM and the glob-al Hamburg Ocean Primitive Equation (ECHO-G);Meteorological Research Institute Coupled GeneralCirculation Model, version 2.3.2a (MRI CGCM2.3.2a);and the third climate configuration of the Met OfficeUnified Model (UKMO HADCM3). Documentationfor the models is available on the Web site http://www-pcmdi.llnl.gov/ipcc/model_documentation/ipcc_model_documentation.php. We consider the preindustrial con-trol runs (PIcntrl) and the Commit runs (Commit).PIcntrl is the preindustrial climate simulation withGHG-induced forcing fixed at the level of year 1850,whereas Commit simulates committed climate changeusing the forcing (GHG H11001 aerosols) at year 2000 levels.For each simulation run, 100 years of data were takenfrom each of the 12 models.For all models, the tropical Pacific SST and zonalwind stress (WS) were analyzed. The zonal currentwithin 2°S–2°N and the meridional current within 30°S–30°N from the sea surface to 450-m depth were alsoanalyzed.The meridional currents averaged over 100–300-mdepth were used to detect the meridional overturningcirculation in the region 30°S–30°N in the Pacific. Thetemperature and vertical velocity in the equatorial Pa-cific region (5°S–5°N, 0–50 m) were also used in thediagnostic analysis.For each model, the monthly mean SST, WS, oceanupper-level velocities were interpolated onto a 5° lon-gitude by 4° latitude regular grid (identical to that usedin the GISS-ER model) using bilinear interpolation.Anomalies were computed with respect to the meanseasonal cycle in each specific model simulation.4. Climate change simulatedThe climate change induced by the anthropogenicforcing was computed by subtracting the multimodelensemble mean in the PIcntrl experiment from that inthe Commit experiment. The warm pool in the westerntropical Pacific and the cool tongue in the eastern tropi-cal Pacific were both reproduced well in the mean cli-mate in the Commit experiment (Fig. 3a), althoughthere is a cool model bias (up to 1°C) in the east-centralequatorial Pacific ocean and an exaggerated westwardextent of the cold tongue (cf. Fig. 1a). These biases havebeen recognized as common problems in most of thestate-of-the-art coupled models manifesting a doubleintertropical convergence zone (ITCZ) (Zhang andWang 2006; Dai 2006). The difference between themeans of the two experiments shows nearly 1°C warm-ing over all of tropical Pacific (Fig. 3b), with the warm-ing pattern vaguely resembling the El Niño pattern,which has the maximum centered in the eastern-centralequatorial Pacific. While each individual model re-vealed an El Niño–like pattern of SST change (see thefigures in the appendix), there is considerable variabil-ity among the models (e.g., the maximum warming cen-ter ranged from the eastern to the western equatorialPacific), with the standard deviation of SST change inFig. 3c illustrating this intermodel variability.The difference between the means of the two experi-ments also showed a positive zonal WS pattern in thewestern tropical Pacific (Fig. 3e), which also resemblesthe zonal WS anomaly pattern found during El Niño(shown later). The enhanced GHG forcing decreasedthe mean easterlies (Fig. 3d) along the western equa-5748 JOURNAL OF CLIMATE VOLUME 21torial Pacific (Fig. 3e). This agrees with the finding thatthe Walker circulation has been slowing down in thetropical atmosphere since the mid-nineteenth century(Vecchi et al. 2006), as the trade wind blowing acrossthe tropical Pacific from east to west is associated withthe bottom branch of the zonal Walker circulation. Themechanism for enhanced GHG forcing to weaken theWalker circulation is as follows: Using a one-dimensional radiative convective model, Betts andRidgway (1989) found that the rate of moisture in-crease in the boundary layer, under the assumption ofconstant relative humidity, outpaced the rate of in-crease in evaporation, thereby decreasing the convec-tive mass circulation in the tropics. Held and Soden(2006) and Vecchi and Soden (2007) proposed that theweakening of the atmospheric overturning circulationin response to enhanced GHG forcing could be ex-plained by the differential response of global-mean pre-cipitation and atmospheric humidity to a warming cli-mate. Figure 3e also showed a strengthening of the east-erlies south of 10°S. The intermodel variability of thezonal WS change (Fig. 3f) revealed that the modelsFIG. 3. Multimodel ensemble mean for the Commit runs, its change from the mean of the PIcntrl runs, and the standarddeviation of this change: (a) mean SST (°C), (b) change in the mean SST, (c) standard deviation of the SST change, (d)mean zonal WS (0.01 Pa), (e) change in the mean zonal WS, and (f) standard deviation of the WS change. The 5% and10% significance levels from the t test are shown in gray in (b) and (e).15 NOVEMBER 2008 Y E A N D H S I E H 5749were most consistent with each other along the equa-torial belt.In the vertical section along the equator showing thezonal current, the equatorial undercurrent along thethermocline is seen in the subsurface layer (Fig. 4a)although the simulated undercurrent is not as strong asthe observed current (with a maximum of about 100cm sH110021). The overturning equatorial circulation hasweakened, as shown in the difference between themeans from the Commit and PIcntrl experiments (Fig.4b), while the undercurrent has shifted upward in theCommit runs, as Vecchi et al. (2006) have also noted anupward trend in the undercurrent position in the GFDLCM2.1 simulation data during 1861–2000. As the ob-served Walker circulation in the tropical atmospherehas been weakening in recent decades, Vecchi et al. feltthat global warming was a likely causative factor in theweakening of the wind pattern. Here the simulated re-sults confirm that the overturning oceanic circulation inthe equatorial Pacific has also been weakening betweenPIcntrl and Commit.Next, consider the meridional currents that are im-portant for interactions between the tropics and theextratropical Pacific, as studies have shown that extra-tropical effects influence the tropical ENSO (Gu andPhilander 1997; Kleeman et al. 1999). Observations in-dicated that the cool water in the subtropical Pacific,especially in the eastern area, is subducted down andmoves to the tropics by the meridional overturning cir-culation in the upper Pacific (McPhaden and Zhang2004). The Pacific overturning circulation can be pre-sented by the meridional oceanic velocity in the tropicaland subtropical Pacific. The difference in the mean sub-surface current (Fig. 4d) between the Commit andFIG. 4. Multimodel ensemble mean of velocity (cm sH110021) for the Commit runs and its change from that in the PIcntrl runs:(a) the mean zonal velocity (averaged over 2°S and 2°N), (b) the change in the mean zonal velocity, (c) the meanmeridional velocity averaged over 100–300-m depth, and (d) the change in the mean meridional velocity. The 5% and 10%significance levels from the t test are shown in gray in (b) and (d).5750 JOURNAL OF CLIMATE VOLUME 21PIcntrl runs shows the Pacific meridional overturningcirculation (Fig. 4c) tending to slow down, especially inthe western tropical Pacific. The recent observed evi-dence suggests that such a slowdown of the meridionaloverturning circulation does exist in the upper PacificOcean since the 1970s (McPhaden and Zhang 2002).Although McPhaden and Zhang (2004) found that themeridional overturning circulation rebounded during1998–2003, their latest result showed that there was aslowdown trend overlying the decadal variability in themeridional overturning circulation during 1953–2000(Zhang and McPhaden 2006). Figures 4c,d indicate thatthis slowdown in the meridional overturning circulationis at least partially due to the increased GHG forcing,with the associated weakening of the Walker circula-tion and equatorial upwelling. Figure 4d also shows thatthe slowdown of the overturning circulation occursalong the interior path.The difference in the mean between the Commit runsand PIcntrl runs also shows that the GHG-inducedchanges are El Niño–like patterns in the tropical PacificSST. Changes of the zonal WS, zonal overturning cir-culation, and meridional overturning circulation arealso El Niño–like patterns (El Niño patterns are shownin the next two sections). The observed El Niño–likepattern in SST (Fig. 1b) therefore appears, at least inpart, to be contributed by the GHG forcing.5. Changes in the ENSO SST and zonal WSThe data from GISS-ER and IAP FGOALS-g1.0 areexcluded in the analysis from now on owing to noENSO variability in the GISS-ER model (Guilyardi2006) and the very small seasonal cycle with the unre-alistic regular interannual cycle in the IAP FGOALS-g1.0 model (Guilyardi 2006; Van Oldenborgh et al.2005). To identify El Niño and La Niña episodes in themodel data, the simulated SST anomalies in the tropicalPacific (20°S–20°N, 150°E–80°W) were smoothed by a25-month running mean [as the period of the simulatedENSO was greater than 2 yr in all 10 coupled models(Guilyardi 2006)], then the leading principal compo-nent (PC: i.e., the time series) from principal compo-nent analysis (PCA) of the smoothed SST anomalieswas defined as a proxy ENSO index for each model run,with El Niño/La Niña defined based on the proxyENSO index exceeding H110061 standard deviation. Com-posite maps of El Niño and La Niña for various vari-ables (without the 25-month running mean smoothing)were computed for each individual model. The multi-model ensemble mean for each variable was then cal-culated from the individual composite maps.ENSO is an interactive thermodynamic system be-tween the atmosphere and ocean, where the atmo-sphere dynamically forces the ocean by the surface WS,while the ocean thermally drives the atmosphere byheating/cooling. Multimodel ensemble means of com-posite maps for SST and zonal WS anomalies over thetropical Pacific are shown in Fig. 5. A pronounced shiftin the zonal location of positive SST anomalies duringEl Niño occurred in the Commit runs relative to thePIcntrl runs (Figs. 5a,c). The shift is in the same direc-tion as observed (Figs. 2a,c), but there is model biassince the model SST anomalies are located farther westthan the observed during El Niño, as noted by Capo-tondi et al. (2006). During La Niña, the shift of thenegative SST anomalies is undistinguished between theCommit and PIcntrl runs (Figs. 5b,d), missing the west-ward shift of anomalies in the observational data (Figs.2b,d). The rms index of the asymmetry between ElNiño and La Niña was computed to be 0.045°C aver-aged for the PIcntrl runs and 0.057°C for the Commitruns. Although these values are considerably weakerthan the observed values given in section 2, they indi-cate the increase of ENSO asymmetry with increasedGHG. For each model, we computed the zonal locationof the “center of mass” of the SST anomalies aroundthe equator by H20858(SSTiLi)/H20858SSTi, where Liis the longi-tude in degrees and i is the spatial index in the domain2°S–2°N, 150°E–80°W. This center of mass for SST dur-ing El Niño has shifted eastward in the Commit runsrelative to the PIcntrl runs by H110021°,25°,6°,11°, H110022°,6°,5°, 105°,47°, and 3° for the 10 models, respectively.Only 2 models (CCCMA and INM-CM) have shiftedmarginally westward, as the zonal grid spacing is 5°.For the zonal WS during El Niño, there was eastwardshift and strengthening of the westerly anomaly centeralong the equator (as indicated by “H”) in the Commitruns relative to the PIcntrl runs (Figs. 5e,g), while dur-ing La Niña, the easterly WS anomalies along the equa-tor showed no obvious shift (Figs. 5f,h). The contrast inthe WS shift between El Niño and La Niña meant thatthe asymmetry and nonlinearity of ENSO were en-hanced in the Commit runs. The rms index of asymme-try for the zonal WS was 0.078 (in 0.01 Pa) for thePIcntrl runs and 0.100 (in 0.01 Pa) for the Commit runs.The change in the asymmetry and nonlinearity ofENSO in these experiments is similar to the observedchange after the late 1970s on the decadal time scale(Ye and Hsieh 2006; Wu and Hsieh 2003; An 2004).6. Changes in the ENSO ocean circulationIn the vertical section showing zonal currents alongthe equator, during El Niño the westward subsurface15 NOVEMBER 2008 Y E A N D H S I E H 5751FIG. 5. Multimodel ensemble mean of SST (°C) and zonal WS (0.01 Pa) composites for (left) El Niño and (right)La Niña for (a), (b), (e), (f) the PIcntrl runs; (c), (d), (g), (h) the Commit runs. “H” and “L” mark the location ofthe highest and lowest values, respectively, and the 5% and 10% significance levels from the t test are shown ingray.5752 JOURNAL OF CLIMATE VOLUME 21zonal current anomaly strengthened slightly and “L”shifted eastward and upward in the Commit runs rela-tive to the PIcntrl runs (Figs. 6a,c); in contrast, duringLa Niña, the eastward subsurface zonal currentanomaly weakened and “H” shifted slightly to the west(Figs. 6,d). Thus the asymmetry in the zonal currentanomalies between El Niño and La Niña strengthenedin the Commit runs relative to the PIcntrl runs, as therms index of asymmetry was 0.77 cm sH110021for the PIcntrlruns and 1.01 cm sH110021for the Commit runs.In both El Niño and La Niña composites (Fig. 7), thesubsurface meridional currents averaged between 100and 300 m depth clearly showed the interior path (Mc-Creary and Lu 1994, Liu 1994) running from the east-ern subtropical Pacific Ocean to the central tropicalPacific Ocean. In the equatorial area, the location of“L” (the strongest southward current anomaly) duringEl Niño was found to have shifted eastward by 10° inthe Commit runs relative to the PIcntrl runs (Figs. 7a,c),whereas during La Niña, “H” showed no obvious shiftbetween Figs. 7b,d. The rms index of asymmetry (com-puted between 5°S–5°N) was 0.20 cm sH110021for the PIcntrlruns and 0.32 cm sH110021for the Commit runs. Hence theasymmetry in the subsurface meridional currentanomalies between El Niño and La Niña was enhancedin the Commit runs relative to the PIcntrl runs.FIG. 6. Vertical section along the equator showing the multimodel ensemble mean of zonal velocity anomalies (cm sH110021)averaged between 2°S and 2°N for the composite (left) El Niño and (right) La Niña for (a), (b) the PIcntrl runs and (c),(d) the Commit runs: (e) the difference between (c) and (a); (f) the difference between (d) and (b). The 5% and 10%significance levels from the t test are shown in gray.15 NOVEMBER 2008 Y E A N D H S I E H 5753FIG. 7. Multimodel ensemble mean of subsurface meridional velocity anomalies (cm sH110021) averaged over 100–300-m depth for the composite (left) El Niño and (right) La Niña: (a), (b) for the PIcntrl runs; (c), (d) for theCommit runs. (e) The difference between (c) and (a); (f) the difference between (d) and (b). The 5% and 10%significance levels from the t test are shown in gray.5754 JOURNAL OF CLIMATE VOLUME 217. Diagnostic analysis of the surface temperatureequationThe ocean surface temperature equation can be writ-ten asH11128TH11128tH11005H11002H20873uH11128TH11128xH11001 H9271H11128TH11128yH11001 wH11128TH11128zH11001 uH11128TH11128xH11001 H9271H11128TH11128yH11001 wH11128TH11128zH20874H11002H20873uH11128TH11128xH11001 H9271H11128TH11128yH11001 wH11128TH11128zH20874H11001 residuals, H208491H20850where T, u, H9271, and w are, respectively, anomalies of theSST; zonal, meridional, and vertical ocean velocitieswith respect to the climatological mean variables (indi-cated by the overbar); and “residuals” include surfaceheat flux anomalies, SST diffusion, subgrid-scale ef-fects, etc. (An and Jin 2004). The first set of termswithin parentheses on the right-hand side of (1) repre-sents the linear dynamic heating (LDH) in the surfaceocean, and the second set, the nonlinear dynamic heat-ing (NDH).The temperature equation was applied to the surfaceocean layer, where the average of the SST and veloci-ties values within 0–50-m depth was used to representthe surface layer. The vertical temperature gradientwas computed from the difference between the tem-FIG. 8. Multimodel ensemble mean of (a), (b) vertical temperature gradient [°C (50 m)H110021]; (c), (d) vertical velocity (10H110026msH110021); (e),(f) vertical nonlinear dynamic heating anomalies (°C monthH110021); (g), (h) and total nonlinear dynamic heating anomalies (°C monthH110021)for (left) the Commit runs and (right) the change from the PIcntrl runs in the ocean surface layer. The 5% and 10% significance levelsfrom the t test are shown in gray.15 NOVEMBER 2008 Y E A N D H S I E H 5755perature at 0 m and that at 50 m. While this is a rathercrude usage of Eq. (1), a finer use of the temperatureequation is difficult to perform in practice since eachindividual model has its own vertical resolution andmixed layer depth, with the mixed layer depth changingfrom the eastern to the western Pacific.Since the vertical ocean velocities of the modelsMIUB ECHO-G and UKMO HadCM3 were not avail-able, these two models were excluded in the subsequentanalysis. Averaged over time (100 yr), the SST ten-dency contributed by the LDH terms is very small com-pared to the contribution from the NDH terms. Amongthe three NDH terms, the vertical term H11002wH11509T/H11509z isdominant (Figs. 8e,g and 8f,h). The NDH is about 30%stronger in the Commit runs than in the PIcntrl runs(Figs. 8f or 8h). The enhanced NDH in the Commitruns does not correspond to enhanced upwelling; in-stead the weakened upwelling (Figs. 8c,d) is consistentwith the finding in section 4 that the equatorial over-turning circulation weakened in the Commit runs.The multimodel ensemble composites during El Niñoand La Niña for the NDH in the ocean surface layer areshown in Fig. 9. During both El Niño and La Niña, theNDH anomalies are positive along the equatorial Pa-cific (Fig. 9). This feature of the NDH anomalies isknown to cause the asymmetry between El Niño and LaNiña, as the warm NDH anomalies enhance the warmEl Niño events and weaken the cool La Niña events(An and Jin 2004). There is very substantial strength-ening of the positive NDH anomaly in the equatorialbelt and an eastward shift by about 25° in the anomalycenter (marked by “H”) in the Commit runs during ElNiño (Figs. 9a,c). However, during La Niña, thechanges in the NDH anomalies are relatively small be-tween the Commit runs and PIcntrl runs (Figs. 9b,d).That the large changes in the magnitude and position ofthe NDH anomalies between Commit and PIcntrl runsonly occurred during El Niño reflects the nonlinear na-ture of the change in the NDH under enhanced GHG.With the increase of NDH during El Niño in the Com-mit runs, one would expect an overall increase in theENSO amplitude. However, no clear increase in theoverall ENSO amplitude was found between the Com-mit and PIcntrl runs (see appendix). Hence otherterms (e.g., the residuals and the LDH) in Eq.(1) must,over a broad area, essentially cancel the increase in theNDH.The NDH anomalies are mainly caused by the verti-cal NDH anomalies (H11002wH11509T/H11509z), as can be seen from thesimilarity between the total NDH anomaly (Figs. 9c,d)and the vertical NDH anomaly (Figs. 10a,c) during ElNiño and La Niña. The substantial change in the verti-cal NDH anomalies during El Niño in the Commit runsrelative to the PIcntrl runs is caused by the increasedvertical temperature gradient anomalies and the east-ward shift of downwelling anomalies in the centralequatorial Pacific (Figs. 10b,f,i,j). However, during LaNiña, the change in the vertical NDH anomalies is rela-tively minor (Figs. 10c,d).8. ConclusionsThe El Niño–like warming in the tropical Pacific seenin the observed SST record (1900–99) was confirmed byFIG. 9. Multimodel ensemble mean of surface layer nonlinear dynamic heating anomalies (°C monthH110021) for the composite (left) ElNiño and (right) La Niña: (a), (b) for the PIcntrl runs; (c), (d) for the Commit runs. The 5% and 10% significance levels from the t testare shown in gray.5756 JOURNAL OF CLIMATE VOLUME 21FIG. 10. Multimodel ensemble mean of the (a)–(d) surface vertical nonlinear dynamic heating (°C monthH110021), (e)–(h) verticaltemperature gradient anomalies [°C (50 m)H110021], and (i)–(l) vertical velocity anomalies (10H110026msH110021) during El Niño and La Niña for (left)the Commit runs and (right) the difference between the Commit and PIcntrl runs. The 5% and 10% significance levels from the t testare shown in gray.15 NOVEMBER 2008 Y E A N D H S I E H 5757FIG. A1. SST mean (°C) for (left) the Commit runs and (right) its difference from the mean in the Picntrl runs for12 individual models.5758 JOURNAL OF CLIMATE VOLUME 21FIG. A1. (Continued)15 NOVEMBER 2008 Y E A N D H S I E H 5759FIG. A2. SST standard deviation (°C) for (left) the Commit runs (contour interval 0.5°C) and (right) its differencefrom the standard deviation in the PIcntrl runs (contour interval 0.05°C) for 12 individual models.5760 JOURNAL OF CLIMATE VOLUME 21FIG. A2. (Continued)15 NOVEMBER 2008 Y E A N D H S I E H 5761the multimodel results comparing the climate from year2000 GHG(H11001aerosol) forcing (Commit) with the pre-industrial climate (PIcntrl). Both the equatorial zonaloverturning circulation and the meridional overturningcirculation weakened with increased GHG. The ideathat the GHGs might play a role on the slowdown ofthe zonal and meridional overturning circulations re-ceived strong support from the multimodel ensemblemeans. This conclusion also matches the finding thatthe Walker circulation in the atmosphere has beenweakening since the mid-nineteenth century owing tothe anthropogenic forcing (Vecchi et al. 2006).In the observed SST data, the positive anomalies dur-ing El Niño were located farther east in the 1950–99composite than in the 1900–49 composite, whereas thenegative anomalies during La Niña were shifted in theopposite direction. From the model data, the positiveSST anomalies during El Niño also shifted eastward inthe Commit runs relative to the PIcntrl runs, whereasthe negative SST anomalies during La Niña were notshifted zonally. Hence, both model and observed SSTresults were consistent with increasing asymmetry be-tween El Niño and La Niña as GHG increased. Theincrease in the asymmetry is associated with an increasein the nonlinearity of ENSO (Jin et al. 2003; An and Jin2004), with the nonlinear dynamical heating (NDH)terms producing the asymmetry between El Niño andLa Niña. A diagnostic analysis of the ocean surfacetemperature tendency equation revealed the verticalNDH term to be most dominant, with the positive ver-tical NDH anomalies in the equatorial Pacific enhancedsubstantially in the Commit runs during El Niño,caused by the increased vertical temperature gradientanomalies and the eastward shift of downwellinganomalies. Under increased GHG, the enhanced posi-tive NDH anomalies during El Niño when time aver-aged over the whole record would change the SSTmean state by an El Niño–like pattern, which couldoverride other mechanisms trying to induce a La Niña–like change in the mean state (Cane et al. 1997).For the simulated equatorial zonal circulation asso-ciated with ENSO, the zonal undercurrent anomaliesduring El Niño strengthened slightly and shifted east-ward in the Commit runs relative to the PIcntrl runs,whereas the undercurrent anomalies during La Niñaweakened and shifted slightly westward instead. Forthe simulated meridional overturning circulation in thePacific, the interior path from the eastern subtropicalPacific to the central equatorial region was clearly seenduring both El Niño and La Niña. With the NDH in-creasing nonlinearity and asymmetry between El Niñoand La Niña in the enhanced GHG experiments, simu-lations with enhanced GHG showed that the asymme-try was enhanced in the anomalies of the SST, the zonalwind stress, the equatorial undercurrent, and the me-ridional overturning circulation.Acknowledgments. We thank Dr. Aiming Wu forhelpful discussions. We acknowledge the modelinggroups for providing their data for analysis, the Pro-gram for Climate Model Diagnosis and Intercompari-son (PCMDI) for collecting and archiving the modeloutput, and the JSC/CLIVAR Working Group onCoupled Modelling (WGCM) for organizing the modeldata analysis activity. The multimodel data archive issupported by the Office of Science, U.S. Department ofEnergy. NOAA-ERSST-V2 data is provided by theNOAA/OAR/ESRL PSD, Boulder, Colorado, fromtheir Web site at http://www.cdc.noaa.gov/. This workwas supported by the Natural Sciences and EngineeringResearch Council of Canada.APPENDIXSST PatternsMaps of SST mean and standard deviation are givenfor individual models in Figs. A1 and A2.All 12 models exhibited positive patterns over theequatorial Pacific in the differences between Commitruns and PIcntrl runs (Fig. A1), despite the mean SSTpattern being quite different for each model. The stan-dard deviations for each model (Fig. A2) showed that,with increased GHG, the ENSO amplitude increased intwo models (GFDL CM2.0 and MRI CGCM2.3.2a),but decreased in two models (IPSL CM4 and MIUBECHO-G). For the other eight models, it is unclearwhether the ENSO amplitudes have changed in theCommit runs compared to the PIcntrl runs.REFERENCESAn, S.-I., 2004: Interdecadal changes in the El Niño–La Niñaasymmetry. Geophys. Res. Lett., 31, L23210, doi:10.1029/2004GL021699.——, and B. Wang, 2000: Interdecadal change of the structure ofthe ENSO mode and its impact on the ENSO frequency. 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