The ENSO Signal in the Stratosphere

TRENDS AND DIRECTIONS IN CLIMATE RESEARCH

The ENSO Signal in the StratosphereNatalia Calvo,a,b Ricardo Garcıa-Herrera,b

and Rolando R. Garciaa

aNational Center for Atmospheric Research, Atmospheric Chemistry Division,Boulder, Colorado, USA

bUniversidad Complutense de Madrid, Ciudad Universitaria Madrid, Madrid, Spain

Although the El Nino–Southern Oscillation (ENSO) is a tropospheric phenomenon, itseffects are also observed in the stratosphere. Traditionally, the study of ENSO abovethe troposphere has been difficult because of the lack of global observations at highaltitudes and also because of the presence of other sources of variability whose signalsare difficult to disentangle from ENSO effects. Recent work with general circulationmodels that isolate the ENSO signal have demonstrated its upward propagation into thestratosphere. Here we review the literature in this field and show results from the mostrecent version of the Whole Atmosphere Community Climate Model to illustrate thepropagation and the mechanisms whereby the signal manifests itself in the stratosphere.The ENSO signal propagates upward to about 40 km by means of large-scale Rossbywaves. The propagation is strongly influenced by the zonal mean zonal winds. Mostof the strong ENSO events tend to peak in the boreal winter and so the ENSO signalis observed mainly at high latitudes during the Northern Hemisphere winter wherethe winds are westerly and allow Rossby wave propagation. The ENSO signal is alsoidentified at polar latitudes in the Northern Hemisphere winter in the form of warmertemperatures and weaker winds during a strong El Nino event. This signal shows azonally homogeneous behavior from the intensification of the stratospheric meridionalcirculation (in which air rises in the tropics and moves toward the winter pole where itdescends) forced by anomalous propagation and dissipation of Rossby waves at middlelatitudes during strong ENSO events.

Key words: stratosphere dynamics; ENSO variability; climatology

Introduction

The El Nino–Southern Oscillation (ENSO)is a coupled phenomenon of the ocean and theatmosphere. Its warm phase, or El Nino, refersto the ocean component of the system and ischaracterized by an anomalous warming in theeastern tropical Pacific, usually in the northernwinter. The term Southern Oscillation refers tothe variation in sea level pressure between theeastern Pacific and the Indian Ocean. ENSOis known to be one of the main sources of vari-ability in the tropical troposphere and it has

Address for correspondence: Dr. Natalia Calvo, National Center forAtmospheric Research, Atmospheric Chemistry Division, P.O. Box 3000Boulder, CO 80307-3000. Voice: +303-497-1894. [email protected]

been related to changes in the jet stream, thestrong westerly wind current concentrated atmiddle latitudes in the upper troposphere1,2;to variations in the Monsoon system over Asiaand Australia3−5; and to changes in cyclonefrequency over Europe.6

In the last few decades, many authors haveanalyzed the effects of ENSO, both in the trop-ics and extratropics. Typically, these studieshave characterized the ENSO phenomenonby a pressure index, computed as the pres-sure difference between the eastern part andthe western part of the Pacific Ocean, or byusing a sea surface temperature (SST) indexobtained by averaging SST over a selected re-gion in the tropical eastern Pacific. Most au-thors have found a response in the tropical

Trends and Directions in Climate Research: Ann. N.Y. Acad. Sci. 1146: 16–31 (2008).doi: 10.1196/annals.1446.008 C© 2008 New York Academy of Sciences.

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Calvo et al.: The ENSO Signal in the Stratosphere 17

Figure 1. Microwave Sounding Unit (MSU T2lt) lower tropospheric temperatures forJanuary 1998. The contour interval is 1 K. Negative contours are dashed. Shadowed re-gions indicate temperature anomalies higher than 1 K and show the characteristic horseshoepattern.

troposphere that lags the ENSO index by 3–6 months, depending on the data set and in-dex used.3,7–11 The spatial distribution of theENSO signal has also been widely analyzed.During its warm phase, or El Nino, the ENSOsignal is characterized by an intense warmingover the central and eastern Pacific as well as inmost parts of the Indian Ocean and southeastof Africa.10,12,13 In the tropical western Pacificthe anomalies in temperature take the form ofa “horseshoe pattern” with the two extremespointing to the east. Observations of outgoinglong-wave radiation (OLR; the energy emittedby the Earth as infrared radiation) and of thetemperatures measured by the satellite-borneMicrowave Sounding Unit (MSU) radiometersshow that maximum warming occurs to theeast of the precipitation anomalies that are ob-served around the dateline.14 Figure 1 showsthe anomalous pattern in the tropical lowertroposphere for MSU temperatures in January1998 during one of the strongest El Nino ofthe last century. The horseshoe pattern is high-lighted in this figure in gray where the temper-ature anomalies are higher than 1 K.

The impacts of ENSO on temperature arealso noticeable at middle latitudes, although thethermal response of the troposphere in these re-gions is not as intense as in the tropics. Duringa warm El Nino episode, an anomalous warm-ing is observed in the northwest part of NorthAmerica and the western part of South Amer-ica, while an anomalous cooling is registered inthe southeast of North America and the cen-

tral part of the Andes. These anomalies thatoccur far away from the tropics are known as“teleconnections.”

Theoretical and model studies have shownthat the teleconnections are observed becauseconvective activity in the tropical region gen-erates Rossby waves (large-scale disturbancesfrom the variation of the Coriolis force withlatitude; see, e.g., Ref. 15), and, thus, anomaliesin the distribution of convective activity gener-ate changes in the atmospheric wave pattern1,16

that in turn produce anomalies in temperature.Horel and Wallace1 showed that the ENSOteleconnections in the Northern Hemisphereare observed only when the winds are westerlyat low latitudes (in the winter half of the yearwhen the Atlantic and Pacific troughs are dis-placed equatorward). The Pacific North Amer-ica Pattern (PNA) is among the most impor-tant teleconnections related to ENSO in theNorthern Hemisphere,1,16 which in its posi-tive phase produces above-average geopoten-tial heights in the vicinity of Hawaii and overthe intermountain region of North Americaand below-average geopotential height southof the Aleutian Islands and over the south-eastern United States. Its relationship withENSO is discussed in numerous papers.1,17–19

In the Southern Hemisphere, the studies showanomalies related to ENSO in high-latitude re-gions, such as Antarctica, and in lower latitudesin Australia and South America.19–23 In addi-tion, ENSO also has an influence on two modesof variability in the Southern Hemisphere: the

18 Annals of the New York Academy of Sciences

Pacific South American pattern (PSA) and theAntarctic Oscillation (AAO).24−27Some studieshave also shown that the response of atmo-spheric temperature to ENSO has a nonlinearcomponent, that is, the response is not symmet-rical for the warm (El Nino) and cold (La Nina)phases of the phenomenon.28,29

The behavior of ENSO appears to have beenaffected by the interdecadal change detectedin several climatic variables that occurred atthe end of the 1970s. On one hand, thischange modified the development and evolu-tion of the SST in the tropical Pacific.29–32 After1976/1977, the surface temperature warmingdeveloped in the western and central Pacific,extending later toward the eastern part of thebasin,10 while before that date the positive tem-perature anomalies were first observed alongthe west coast of South America and movedwestward afterward, as shown in Rasmussonand Carpenter’s4 analysis of the 1951–1972 pe-riod. Furthermore, changes in the large-scalecirculation patterns, such as an intensificationof the PNA pattern during strong ENSO events,have occurred after 1979.33,34 The origin ofthese changes is not well understood, althoughsome works point to a major influence of theannual cycle during the ENSO events after the1970s,34 while others suggest a modulation ofthe tropical teleconnections from the PacificDecadal Oscillation related to the SSTs in theNorth Pacific.35

The temporal evolution of the ENSO signalhas also been widely studied. Kelly and Jones36

identified two patterns of variability in the SSTrelated to ENSO, with a lag of 3 months be-tween them. In tropospheric temperature, Yu-laeva and Wallace14 also identified two differentpatterns lagged by 5 months. More recently,Calvo et al.11 obtained three different spatialpatterns in tropospheric temperatures relatedto ENSO and studied their temporal evolu-tion. ENSO-related temperature anomalies oc-cur initially as two different wave patterns; thefirst one precedes the full development of tropi-cal SST anomalies and the second one developssimultaneously with it. These patterns appear

to represent the equatorial wave response toanomalous latent heat release.11,14 The zon-ally symmetric atmospheric warming that lagsthe development of the SST anomalies is inter-preted as a diabatic response to changes in thesurface energy balance during ENSO.10,14

Besides the ENSO signal in the troposphere,several recent studies have presented evidencethat ENSO propagates upward and also has aneffect in the stratosphere. This layer, where thetemperature increases with height mainly fromthe absorption of ultraviolet radiation by ozone,spans the range of altitude from about 15 kmin the tropics, or 8–10 km in the extratropics,to about 50 km. It thus lies between the tropo-sphere (below) and the mesosphere (above). Inthis paper, we present an overview of the ENSOsignal in the middle atmosphere, together withresults from a new general circulation modelthat illustrate the propagation of the ENSOsignal and the mechanisms by which the sig-nal reaches the stratosphere. Section 2 reviewsthe literature on the stratospheric ENSO sig-nal; Section 3 introduces the numerical modeland methodology used in this paper; and Sec-tion 4 presents the main results. Conclusionsare summarized in Section 5.

The ENSO Signal in theStratosphere

The first works that studied the influence ofENSO in the stratosphere, by means of obser-vations, were those of Wallace and Chang, vanLoon and Labitzke, Hamilton, Baldwin andO’Sullivan, and Kodera et al.37−41 However,the results of these studies are not uniformlyconsistent. Some of the studies show a rela-tionship between ENSO and the large-scalecyclonic circulation observed over the polar re-gion in the Northern Hemisphere stratosphere,usually known as the polar vortex. During astrong El Nino event, some authors found aweaker polar vortex together with an intensifi-cation of the high-pressure center located overthe Aleutian region. Other authors do not findany statistically significant relationship between


ENSO and the stratosphere and they pointout the difficulty of isolating the ENSO signalfrom other sources of variability that affect theNorthern Hemisphere polar stratosphere in theshort observational records available (between20 and 30 years).

The effects of ENSO on temperature werestudied in particular by Reid et al.,42 whoused several radiosonde stations in the PacificOcean, and Yulaeva and Wallace14 and Calvoet al.,11 who used MSU satellite temperaturesover the tropical region. They all showed anENSO signal in the tropical lower stratosphereof opposite sign to that in the troposphere.Some authors suggested that an intensificationof the meridional circulation in the tropicaltroposphere, known as the Hadley cell, withair rising in the tropics and descending in thesubtropics, might explain the opposite behav-ior in the troposphere and stratosphere.42,43

However, Calvo et al.11 explained the tropicalstratospheric cooling over the eastern Pacific asthe upper air manifestation of internal equa-torial waves forced by anomalous convectionin the troposphere from changes in SSTs. Aswe will show later, both explanations were rea-sonable as they refer to different behaviors ofthe ENSO manifestation in the atmosphere: itszonal-mean behavior and the wave-like ENSOsignal, respectively.

Several problems have been found in theanalysis of the ENSO signal in the stratosphere.First, ENSO is not one of the main sources ofvariability in this atmospheric layer as it is inthe troposphere, and this makes it more diffi-cult to identify its signal among those generatedby other phenomena. Among the latter is theQuasi Biennial Oscillation (QBO), an oscilla-tion of the stratospheric winds in the tropicalregion wherein the equatorial winds oscillatefrom westerlies to easterlies with a period ofabout 27 months. The QBO is known to shiftthe region where large-scale Rossby waves dis-sipate in the stratosphere either poleward orequatorward, depending on whether the windsin the tropics are easterly or westerly, and thiscan affect the interaction between the waves

and the mean flow and, ultimately, the stateof the stratosphere in the polar region.38,44,45

The short records available make it difficultto stratify the observations according to QBOphases versus ENSO phases and, thus, to an-alyze both phenomena independently. Otheradditional sources of variability that might in-terfere with the detection of the ENSO signal inthe stratosphere are volcanic eruptions, whichinject aerosols into the lower stratosphere andcan change the stratospheric composition andglobal circulation46; climate change related tothe greenhouse gases, which affects the radia-tive balance near the tropopause and influencesthe local thermal structure and circulation47;and solar variability, which can generate chem-ical and dynamical changes in the stratosphereand mesosphere.48,49

An additional problem in analyzing theENSO phenomenon in the stratosphere arisesfrom the lack of global observations. The coarsehorizontal resolution in observations and thefew vertical levels typically available have madethis analysis difficult. This is why in the last fewyears general circulation models (GCMs), witha global and homogeneous grid and the abilityto isolate different sources of variability, havebecome one of the more important tools avail-able to investigate the ENSO phenomenon inthe middle atmosphere (the part of the atmo-sphere that includes the stratosphere and spansup to about 100 km).

Some of the early modeling studies simplycompared model integrations with and with-out imposed SST anomalies in the tropical Pa-cific. They used models that included only afew levels in the lower stratosphere.49–55 Even-tually, other works dealt with the influence ofSST variability on the polar stratosphere in theNorthern Hemisphere, using simplified simula-tions with more complete models that includeda more ambitious treatment of the middleatmosphere. Hamilton56 reproduced ENSOvariability in the stratosphere with the SKYHImodel from the Geophysical Fluid Dynam-ics Laboratory in Princeton, New Jersey, usingidealized SST perturbations typical of ENSO


events during winter months. The results werein very good agreement with the observationalworks discussed before and showed that ENSOevents are associated with stationary large-scaleRossby wave perturbations of zonal wave num-bers 1 and 2. One manifestation of these waveanomalies is the intensification of the Aleu-tian High in the middle stratosphere. Lahoz57

studied the possible influence of the SST vari-ability on the trends in temperature using atropospheric–stratospheric version of the UKMeteorological Office Unified Model. An en-semble of nine simulations for winter conditionsin the Northern Hemisphere from the 1980s tothe 1990s showed that the UK Met Office Uni-fied Model reproduced at least part of the ob-served temperature trend (cooling in the North-ern Hemisphere lower stratosphere) with SSTvariations but did not find conclusive results interms of the response to ENSO.

In the last few years, several multidecadalintegrations have been run with GCMs. Usu-ally several realizations of each model are ob-tained and the average of those realizationsis computed to produce an ensemble average,or ensemble mean. Braesicke and Pyle58 usedthe UK Met Office Unified Model to com-pare different ensembles with and without SSTvariability and obtained a weaker polar vortexwith SST variability that does not appear to berelated to extreme ENSO events. Some otherworks specifically focusing on ENSO effects arethose of Sassi et al.,59 with the Whole Atmo-sphere Community Climate Model, version 1(WACCM1), developed at the National Centerfor Atmospheric Research (NCAR); Manziniet al.,60 using the Middle Atmosphere Euro-pean Center Hamburg Model MAECHAM5;and Garcia-Herrera et al.,61 who compared indetail results from these two models and theEuropean Center for Medium-range WeatherForecasts (ECMWF) reanalysis of atmosphericobservations, ERA-40. None of the modelsused in these studies produced a QBO. Al-though this is a shortcoming of the models, itdoes have the advantage that it allows analysisof model results without this additional source

of variability. These studies did find changes inthe stratospheric circulation related to ENSOand showed the wave-like ENSO signal in thestratosphere and the way it propagates upwardfrom the troposphere. In addition, they also re-vealed a new aspect not documented before:ENSO has an effect on stratospheric zonalmean temperature related to changes in thestratospheric mean meridional circulation.

Sassi et al.59 showed anomalies that have thestructure of planetary Rossby waves propagat-ing up to the mesosphere and studied the mo-mentum deposition by resolved and parame-terized waves during strong ENSO events inthe Northern Hemisphere. Manzini et al.60 ob-tained a significant response in the zonal meanflow that had not been shown previously, witha weaker polar vortex during warm ENSOevents but no significant signal from cold events(La Nina). In addition, an enhancement ofthe downward propagation of wave–mean-flowinteraction in the middle atmosphere clearlyappears in this model. The work of Garcia-Herrera et al.61 shows good agreement betweenmodels and reanalysis, which highlights the ro-bustness of the results. While most previousworks focused on the Northern Hemisphere,this work also analyzes the ENSO signalin the tropical and Southern Hemispherestratosphere comparing different latitudes. Thedirect influence of ENSO on the stratosphericcirculation through changes in the upwardpropagation of Rossby waves and the relation-ship between these two is explicitly shown inthis paper for the first time by means of waveactivity and circulation velocities anomalies.

Model and Methodology

The WACCM3 is a GCM with interactivechemistry based on the US National Centerfor Atmospheric Research’s Community Atmo-sphere Model (CAM3). It includes most of thephysical and chemical processes that are im-portant for describing the dynamics and chem-istry of the atmosphere above the troposphere


and up to about 140 km. It has 66 vertical lev-els from surface up to about 140 km, and itsvertical coordinate is isobaric above 100 hPa(approximately 16 km) but hybrid below thatlevel. The vertical resolution is variable, rang-ing from 1.1 km in the troposphere, above theplanetary boundary layer, to about 3.5 km inthe upper atmosphere.

The model is used to simulate the period1950–2004. As boundary conditions, SSTs areprescribed from the global Met Office HadleyCenter, Exeter, U.K., data set prior to 1981 andfrom the Smith/Reynolds data set after 1981.62

There is no QBO in the model, either internallygenerated or externally prescribed. As notedpreviously, this may be considered an advan-tage for our purposes as it allows study of theENSO signal in isolation from this other impor-tant source of stratospheric variability. Chemi-cal effects of volcanic aerosols are included butnot their radiative effects. The 11-year solarcycle irradiance variability is parameterized interms of the observed f10.7 radio flux. Detailedinformation about the model, processes, andparameterizations can be found in both Garciaet al.63 and Richter et al.64

The main differences between WACCM3and WACCM1 (which has also been used tostudy ENSO effects in the stratosphere, asnoted above) include the use of Lin’s finite vol-ume formulation65 for advection in WACCM3as opposed to the semi-Lagrangian methodused in WACCM1. The finite volume methodis a mass-conserving approach to the solution ofthe governing equations in the model and is es-pecially appropriate for modeling the advectionof chemical species. In addition, WACCM3includes interactive chemistry and solar cy-cle variability that were not incorporated inWACCM1.

The results presented here are based uponthree realizations of WACCM3 run at horizon-tal resolution of 4◦ × 5◦ (latitude × longitude).The ensemble average of the three realizationswas computed for the period 1979–2000 in or-der to better compare with previous results thatused the same period of analysis. Composites

TABLE 1. Central Month and Its CorrespondingN3.4 Value for the Warmest and Coldest ENSOEvents Considered in the Study

Warm ENSO Events Cold ENSO Events

January 1983 2.74 December 1984 −1.21January 1992 1.82 November 1988 −1.91December 1994 1.32 December 1998 −1.51November 1997 2.76

for the strongest El Nino and La Nina eventswere constructed and then the differences (ElNino minus La Nina averages) were analyzed.

As noted in the Introduction, different in-dices can be used to characterize the ENSOphenomenon. We have chosen the Nino3.4 in-dex (N3.4), which is the SST averaged over theregion between long 120◦W and 170◦W andlat 5◦S to 5◦N. Warm and cold ENSO eventshave been chosen whenever this index exceeds1.2 standard deviations (Table 1), as in Garcia-Herrera et al.61 All the ENSO events peak in latefall–early winter except the strong warm ENSOevent in August 1988, which has not been con-sidered in this study to avoid misleading inter-pretation of the mechanisms involved since thestratospheric dynamics is strongly modulatedby the seasonal cycle. In the composite analy-sis, month 0 indicates the month in which allthe ENSO events reach their maximum N3.4index value.

The significance of the anomalies with re-spect to internal natural variability has beencomputed by a Monte Carlo test following themethodology of Garcia-Herrera et al.61 In thefigures, the shadowed areas denote anomaliessignificant at the 95% confidence level.

Results

Wave-like ENSO Signal

Figure 2 displays the composite difference (ElNino minus La Nina) for monthly mean tem-perature anomalies at lat 40oN from month 0to month 5. Shadowed regions denote the


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Figure 2. Composite differences (El Nino minus La Nina) of the temperature anomaliesat lat 40oN from the WACCM3 ensemble simulation for months 0 to 5 after an El Nino event.Solid (dashed lines) denote positive (negative) anomalies. Light (dark) shadowed regionsindicate positive (negative) statistically significant anomalies at the 95% confidence level.Contours are drawn every 1 K (zero line has not been displayed).

significant anomalies according to the MonteCarlo test mentioned in Section 3. The temper-ature anomalies show the upward propagationof the ENSO signal from the Pacific Oceantoward the middle atmosphere by means oflarge-scale Rossby waves, with negative anoma-lies in the troposphere and positive anomaliesabove the tropopause that tilt westward with

height, indicating upward propagation. Rossbywaves are generated in the troposphere mainlyby large-scale orography, large-scale convectiveheating, and land–sea contrasts. From there,they propagate into the stratosphere primarilyduring winter because vertical propagation re-quires that the zonal wind be westerly (fromwest to east) and lower than a certain critical


value. This threshold depends on the spatialscale of the waves such that only ultralongRossby waves (zonal wave numbers 1 to 3) areable to propagate into the stratosphere. Theseconditions are known as the Charney–Drazincriterion.66 Significant anomalies are observedin WACCM3 during the first 5 months after themaximum of N3.4 index up to about 40 km al-titude, in very good agreement with the resultsfrom the WACCM1 and MAECHAM5 mod-els shown in Garcia-Herrera et al.61 but withslightly smaller anomalies reaching higher alti-tudes than for the ERA-40 observational dataset (see Figure 2 in Garcia-Herrera et al.61).

In the middle latitudes of the SouthernHemisphere, the ENSO propagation is not aseffective as in the Northern Hemisphere (figurenot shown), mainly because of the timing of theENSO maxima with respect to the seasonal cy-cle. ENSO events tend to occur in the borealwinter, when stratospheric winds are westerly inthe Northern Hemisphere and allow the verti-cal propagation of Rossby waves, in accordancewith the Charney–Drazin criterion. However,in the Southern Hemisphere the winds are east-erly in the boreal winter and this prevents up-ward wave propagation. Significant anomaliesare observed in this case only in the troposphereand lower stratosphere below 20 km.

At tropical latitudes, a wave-like ENSO sig-nal confined to the troposphere and lowerstratosphere that shows composite differences(El Nino minus La Nina) for monthly meantemperature anomalies at lat 2oN is clearly ob-served in Figure 3. The wave-like ENSO signalhas opposite signs in the troposphere and lowerstratosphere as previous works have shown.11,14

This is from the small vertical group veloci-ties of tropical Rossby waves and the easterlywinds that inhibit the propagation in this re-gion. Beyond the lower stratosphere, the sig-nal acquires a zonal symmetric behavior withthe largest negative significant anomalies inmonth 3, which is known to be related tothe mean meridional circulation as will be ex-plained next.

Zonal Mean ENSO Signal

In addition to the wave-like signal, ENSOalso generates anomalies in zonal mean fields.Figure 4 shows the composite differences (ElNino minus La Nina) for zonal mean tempera-ture. WACCM3 shows a significant warmer po-lar stratosphere during a strong El Nino event,in good agreement with previous works (e.g.,Refs. 59–61), up to 3 months after the maxi-mum of the N3.4 index, accompanied by ananomalous cooling in the tropical stratosphere.The combination of warm and cold anoma-lies forms a dipole structure that propagatesdownward as time progresses, in good agree-ment with results shown in MAECHAM5 byManzini et al.60 The largest significant anoma-lies in WACCM3, both in the tropical and polarregions, are observed in month 3, with valuesup to 7 K at high latitudes. In the tropical re-gion, the anomalous warming observed in thetroposphere corresponds to the zonally sym-metric anomalies that are observed to developduring the mature phase of a warm ENSOevent.11,14 Weaker zonal mean zonal winds arealso observed (figure not shown) at high lati-tudes in the winter hemisphere during strong ElNino events accompanied by the warm anoma-lies, as expected from geostrophic balance.

Wave–Mean-flow Interaction

The generally accepted mechanism that ex-plains the ENSO signal in the polar winterstratosphere involves the propagation and dis-sipation of Rossby waves at middle latitudesand their interaction with the zonal mean flow.In order to analyze in depth this mechanism,we have studied the propagation and dissipa-tion of the Rossby waves in the stratospherein WACCM3 and their impact on the strato-spheric branch of the mean meridional circula-tion, also known as the Brewer Dobson circu-lation, which moves air from the tropical lowerstratosphere to the polar regions in the winterhemisphere. These anomalies in the meridional


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circulation are known to generate the anoma-lies in the zonal mean zonal wind and tem-perature in the polar region as we will explainnext.

The propagation of the Rossby waves canbe visualized using the Eliassen Palm flux (EPflux), which can be considered as a measure ofthe propagation of wave activity.67 The diver-gence of the EP flux is a direct measure of theforcing of the mean flow by the wave pertur-bations15,67 so that negative values of the EPflux divergence indicate regions where Rossbywaves are dissipating and transfer momentum

to the zonal mean flow. Figure 5 shows the com-posite difference (El Nino minus La Nina) of theEP flux anomalies (arrows) and its divergenceanomalies (contours) from month 0 to month 5.The upward Rossby wave propagation is in-tensified (upward anomalies) from month 1 tomonth 5 in the Northern Hemisphere. As ex-pected, the winter climatology [December, Jan-uary, February (DJF)] in WACCM3 (Fig. 6) forthe period 1979–2000 shows westerly winds inthe Northern Hemisphere and easterly windsin the Southern Hemisphere during the borealwinter. Thus, following the Charney–Drazin


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Figure 4. As Figure 1 for zonal mean temperature anomalies.

criterion explained above, westerly winds fa-vor the upward wave propagation observed inFigure 5 during strong ENSO events in theNorthern Hemisphere. Together with the in-creased EP flux, negative EP flux divergence isobserved at middle latitudes in the stratospherewith the largest values during months 2 and3 indicating strong wave dissipation in thoseregions.

These anomalies in the EP flux divergenceare accompanied by an intensification of themean meridional circulation. Figure 7 displays

the composite difference of the anomalies ofthe mean meridional circulation (V∗, W∗) to-gether with the anomalies in the EP flux diver-gence also shown in Figure 5. From month 1to month 5, a clear intensification of the strato-spheric circulation is observed, correspondingto the anomalous wave dissipation indicated bythe EP flux divergence anomalies. This acceler-ated circulation moves air from the lower tropi-cal stratosphere to the polar stratosphere in theNorthern Hemisphere. The rising air in thetropics generates adiabatic cooling as observed


Figure 5. Composite differences (El Nino minus La Nina) for the EP flux anomalies (ar-rows) and its divergence (contours) anomalies for months 0 to 5 after an El Nino event. EPflux values drawn are Fy × 10−5 kg s−2 and Fz × 10−3 kg s−2. Contours of the EP fluxdivergence are drawn at ±0.1, 0.5, 1, 2, 4 m s−1 day−1. Shadow regions for EP flux diver-gence anomalies larger than ±0.1 m s−1 day−1: light (dark) shadowed for positive (negative)values.

there (see Fig. 4), while the descending air inthe polar region produces the polar adiabaticwarming.

In short, WACCM3 results show that, dur-ing the warm phase of ENSO, a more in-tense Rossby wave propagation and dissipa-tion occurs at middle and high latitudes in theNorthern Hemisphere. As they dissipate, theplanetary waves deposit easterly momentum,

which decelerates the mean westerly flow in theNorthern Hemisphere stratosphere. The wavedissipation also alters the extratropical angularmomentum balance and forces a stronger meanmeridional circulation that in turn gives rise totropical cooling and high-latitude warming athigh latitudes, as has been shown in Figure 5.Figure 8 summarizes all the processes and in-teractions described above.


Figure 6. Winter climatology (December, January, February) of the zonal mean zonal winds forthe period 1979–2000. Contours are drawn every 10 m s−1; solid (dashed) contours for westerly(easterly) winds.

Summary

This paper has reviewed the literature onENSO effects in the stratosphere. The maindifficulties traditionally found in the study ofthe ENSO signal in the stratosphere have beendiscussed here; namely the lack of global obser-vations and the fact that ENSO is not alwaysthe major source of variability in the strato-sphere, which makes it difficult to disentan-gle its signal from other sources of variabil-ity. For these reasons, GCMs able to isolatethe ENSO signal have become widely used inthe study of the ENSO effects in the strato-sphere. Results from the most recent version ofthe Whole Atmosphere Community Climate

Model (WACCM3) have been presented hereto illustrate the current theoretical understand-ing of the propagation of ENSO effects from thetroposphere into the stratosphere. The ENSOsignal propagates from the troposphere to thestratosphere, up to about 40 km height, in theform of Rossby waves. This is mainly observedat middle latitudes in the Northern Hemisphereduring winter months because ENSO tends topeak in the northern winter when stratosphericwinds are westerly in the Northern Hemisphereand allow vertical propagation of Rossby waves.In addition, the dissipation of Rossby waves atmiddle latitudes, which deposits easterly mo-mentum on the background flow, weakens thepolar vortex and intensifies the stratospheric


Figure 7. As Figure 4 for the meridional mean circulation velocities anomalies (ar-rows) and the EP flux divergence (contours) anomalies. Values of the meridional circula-tion are shown in 0.1 m s−1. Contours of the EP flux divergence are drawn at ±0.1, 0.5,1, 2, 4 m s−1 day−1. Shadow regions are for EP flux divergence anomalies larger than±0.1 m s−1 day−1.

mean meridional circulation. This circulation,which moves air from the tropical lower strato-sphere to the winter polar regions with air risingin the tropics and descending at high latitudes,gives rise to an anomalous polar warming (andtropical cooling) a few months after the maxi-mum of the N3.4 index in the Northern Hemi-sphere.

There are still some aspects of the strato-spheric ENSO signal that remain uncertain,

since the results from the different data setsanalyzed in different studies do not agree com-pletely as regards the timing, location, and mag-nitude of the significant anomalies. Because dif-ferences among model simulations can arisefrom the internal variability of each model,additional experiments with different GCMsare required to increase the significance androbustness of the results and it is hoped to re-duce the disparities among the models. Further


Figure 8. Main mechanisms involved in the propagation of the ENSO signal in thestratosphere. The thin arrows indicate the anomalous wave propagation and point to thewave dissipation region. The gray arrow indicates the air movement by the intensification ofthe stratospheric mean meridional circulation from anomalous wave dissipation. As a result,the air cools down in the tropical stratosphere and warms up at polar latitudes in the NorthernHemisphere as is indicated in the figure.

improvements in middle atmospheric modelingand also a better understanding of the relation-ships involved in wave–mean-flow interactionare still required to assess the remaining uncer-tainties.

Acknowledgments

This work has been funded by the Span-ish Ministry of Education and Science andthe Fulbright Commission in Spain. TheWACCM3 simulations discussed here werecarried out at the Barcelona SupercomputingCenter, Barcelona, Spain, and at the NationalCenter for Atmospheric Research (NCAR),Boulder, CO, USA. NCAR is sponsored by theUS National Science Foundation.

Conflicts of Interest

The authors declare no conflicts of interest.

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Documents

The ENSO Signal in the Stratosphere