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Environ. Res. Lett. 10 (2015) 094003 doi:10.1088/1748-9326/10/9/094003

LETTER

On the surface impact of Arctic stratospheric ozone extremes

NCalvo1, LMPolvani2 and S Solomon3

1 Departamento de Fisica de la Tierra II, Facultad de Ciencias Fisicas, UniversidadComplutense deMadrid,Madrid, Spain2 Department of Applied Physics andAppliedMathematics, Department of Earth and Environmental Sciences, and LamontDoherty Earth

Observatory, ColumbiaUniversity, NewYork, NY,USA3 Department of Earth, Atmospheric, and Planetary Sciences,Massachusetts Institute of Technology, Cambridge,MA,USA

Keywords: ozone variability, stratosphere–troposphere coupling, chemistry–dynamical coupling

Supplementarymaterial for this article is available online

AbstractAcomprehensive stratosphere-resolving atmosphericmodel, with interactive stratospheric ozonechemistry, coupled to ocean, sea ice and land components is used to explore the tropospheric andsurface impacts of large springtime ozone anomalies in the Arctic stratosphere. Coupling between theAntarctic ozone hole and SouthernHemisphere climate has been identified in numerous studies, butconnections of Arctic ozone loss to surface climate have beenmore difficult to elucidate. Analyzing anensemble of historical integrations with all knownnatural and anthropogenic forcings specified overthe period 1955–2005, we find that extremely low stratospheric ozone changes are able to producelarge and robust anomalies in tropospheric wind, temperature and precipitation inApril andMayover large portions of theNorthernHemisphere (most notably over theNorthAtlantic and Eurasia).Further, these ozone-induced surface anomalies are obtained only in the last two decades of the 20thcentury, when high concentrations of ozone depleting substances generate sufficiently strongstratospheric temperature anomalies to impact the surface climate. Our findings suggest that couplingbetween chemistry and dynamics is essential for a complete representation of surface climatevariability and climate change not only in Antarctica but also in theArctic.

1. Introduction

The profound andmanifold climate impacts caused bythe depletion of stratospheric ozone over the SouthPole in the last decades of the 20th century have beenwidely reported (Thompson et al 2011, Previdi andPolvani 2014). A large literature, comprising bothobservational and modeling studies, suggests that theformation of the ozone hole has affected nearly everycomponent of the Southern Hemisphere climatesystem, from tropospheric winds and clouds, to sur-face temperatures and precipitation, and the circula-tion and ventilation of the Southern Ocean. In theNorthern Hemisphere, however, connections havebeen less evident (e.g., Thompson and Solomon 2005).

The observed multi-decadal trends in ozone aremuch smaller over the Arctic than over the Antarctic(WMO2014). Extensive Antarctic ozone depletion hasoccurred in all austral springs (with the possible excep-tion of 2002) since the mid-1980s (WMO 2014). Incontrast, the interannual variability in ozone is much

larger in the Northern than in the Southern Hemi-sphere, owing to the abundance of planetary scalewaves that propagate into the Arctic stratosphere andperturb the circulation there, leading to frequent ‘sud-den warming’ events (Charlton and Polvani 2007).However, in years when the Arctic stratosphere is rela-tively unperturbed, anomalously weak transport bythe Brewer–Dobson circulation (BDC) together withthe concurrent anomalously low temperatures doconspire to yield extremely low ozone values in thespring. Such ‘extreme ozone’ events occur every fewyears: the latest one in 2011—with extremely coldtemperatures from December to March and excep-tionally large springtime Arctic ozone losses (Manneyet al 2011)—has been extensively studied (see Solo-mon et al 2014, and references therein).

In contrast to the upper stratosphere where carbondioxide increases have dominated temperature trendsof recent decades, ozone depleting substances (ODS)are the main driver for lower stratospheric coolingtrends that have been observed in the Arctic over the

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last couple of decades (Rieder et al 2014). Years withgreater depletion can be expected to display greatercooling, and vice versa: ozone and temperature arecoupled. Here we examine whether Arctic strato-spheric ozone minima are able to impact surface con-ditions, given what has been learned about thepervasive impact on the ozone hole on the SouthernHemisphere surface climate. Three previous studieshave addressed that question.

Cheung et al (2014), using the UK Met Officeoperational weather forecasting system, explored thepossible surface impacts associated with the 2011extreme event (the largest on record). After substitut-ing the standard ozone climatology in the model withthe observed 2011 ozone from the Earth ObservingSystem Microwave Limb Sounder, they found no sig-nificant reduction in tropospheric forecast errors inthe month of March. Karpechko et al (2014) alsoinvestigated the possible surface response to the extre-mely low ozone anomalies in the spring of 2011, butwith transient experiments performed with the atmo-spheric circulation model ECHAM5 (which has a wellresolved stratosphere). They reported significantimpacts on tropospheric climate only when strato-spheric ozone anomalies were specified together withsea surface temperature for that year, but concludedthat stratospheric ozone changes alone did not appearto have an effect on surface conditions. Finally, Smithand Polvani (2014) performed long, time-slice inte-grations with the Community Atmosphere Model,version 3 (CAM3)—to generate a large signal to noiseratio—and contrasted pairs of runs with high and lowvalues of synthetic stratospheric ozone. They alsofound no significant tropospheric response to ozonedifferences for extreme amplitudes within theobserved range of variability, but reported a statisti-cally significant surface response when the amplitudewas made somewhat larger than the observed1979–2011 range.

From this one might be tempted to conclude thatstratospheric ozone anomalies are simply unable toaffect surface conditions. However, we note that thesestudies all suffer from several unphysical features thatpreclude such a hasty conclusion. First, all of these stu-dies prescribed zonal-mean ozone fields in their mod-els: there is now evidence that this considerablyweakens the tropospheric response (Crook et al 2008,Gillet et al 2009, Waugh et al 2009). Second, they allused monthly-mean ozone fields, with daily ozoneobtained via simple linear interpolation: Neely et al(2014) have recently shown that specifying daily ozoneyields a substantial difference in years with large ozonelosses, which amplifies the tropospheric and surfaceresponse to large ozone anomalies. Thirdly, all of thesestudies treat ozone as a fixed external forcing, and thusignore any chemical–dynamical coupling: the severingof the relation between ozone concentrations and thestratospheric circulation implies that a potentially

important feedback mechanism is absent from allthose studies.

Recall that year-to-year variations in the amountof stratospheric ozone result from both dynamical andchemical processes (e.g. Tegtmeier et al 2008). Ozoneis transported into the polar region by the BDC, andlost via heterogeneous chemical reactions on polarstratospheric clouds (PSC), which require cold tem-peratures below about 192 K in order to effectivelydeplete ozone. Winters with an anomalously coldpolar stratosphere—associated with a strong polarvortex and a weak BDC—are more prone to the for-mation of PSCs, which destroy more ozone. Thisanomalous chemical ozone loss adds to the low dyna-mical ozone values associated with the weaker BDCtransport from the tropics. As ozone further dimin-ishes, the polar stratosphere cools further, yielding aneven stronger polar vortex and a weaker BDC: hencethe positive chemical–dynamical feedback. Con-versely, in winters with an anomalously warm polarstratosphere, PSCs formation are obviously inhibited,thus reducing the loss of ozone. As more ozone buildsup in the polar region, the temperatures there increasefurther which, in turn, prevents more heterogeneouschemistry and destruction of ozone, again providing apositive coupling between chemistry and dynamics.

In this paper we avoid the limitations present inearlier studies by using stratosphere-resolving atmo-spheric model with interactive stratospheric ozonechemistry, coupled to dynamical ocean, sea ice andland components. The ozone concentrations in ourmodel, therefore, are fully consistent with the atmo-spheric circulation at all times, without any degrada-tion from spatial or temporal averaging. Furthermore,the ozone chemistry is allowed to both respond to andfeedback on the atmospheric circulation. Using anensemble of model integrations from 1955 to 2005with all known historical forcings, we find that theincrease in ODS in the latter decades of the 20th cen-tury results in surface signals of Arctic ozone extremesthat are robust and statistically significant, affectingsurface winds, temperature and precipitation overlarge portions of theNorthernHemisphere.

2.Methods

In this study, we analyze output from the WholeAtmosphere Community Climate Model, Version 4(WACCM4), one of the atmospheric components ofthe Community Earth System Model (CESM1).WACCM4 is a chemistry-climatemodel, with 66 levelsin the vertical, and the model lid at 140 km. Thevertical resolution is about 1.25 km in the troposphereand lower stratosphere, and up to 1.75 in the upperstratosphere; the horizontal resolution is 1.9° inlatitude and 2.5° longitude. WACCM4 is run coupledto interactive ocean, sea ice and land components. Theclimate simulated by WACCM4 has been evaluated

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Environ. Res. Lett. 10 (2015) 094003 NCalvo et al

and documented in Marsh et al (2013), which thereader may consult for other details about the modelsetup.

A total of six historical WACCM4 simulations,from 1955 to 2005, similar to those analyzed in Marshet al (2013), are analyzed here. These simulationsincorporate all historical forcings as specified by theClimate Model Intercomparison Project, Phase 5(CMIP5), which include observed greenhouse gasesand halogen concentrations, total spectral irradiance,and volcanic aerosol. The six simulations only differ intheir initial conditions, and were conducted with animposed quasi-biennial oscillation, which greatlyimproves the representation of ozone variability in thetropical stratosphere. Note that using six 50-year longruns, we have 300 winters at our disposal: this is amuch larger sample that is available in the observa-tions, and it provides a large signal-to-noise ratioallowing us to extract surface signatures of large strato-spheric ozone anomalies that may not be mirrored inthe single realization available in observations; wereturn to this point below. We have checked that themembers of the ensemble are statistically independentby computing the correlation between the daily timeseries of Arctic polar cap temperature at 50 hPa: thevalue of the correlation never exceeds 0.1, as onemightexpect from the large internal dynamical variabilitywhich largely controls the occurrence of extremeozone events.

3. Results

We start by illustrating the large interannual variabilityin the seasonal cycle of Arctic stratospheric ozone.Each curve in figure 1 shows the simulated timeevolution of polar cap ozone (averaged between 65 and90N) at 70 hPa, from October of one year to

September of the following year. The largest year-to-year differences occur in the spring, notably in April,when values range from 1.3 ppmv to 3.1 ppmv. Asdiscussed above, these large interannual differencesappear as a consequence of the coupling betweendynamics and chemistry. As shown in supplementaryfigure S1, the range of WACCM4 ozone in Arcticcompares favorably with ozonesonde observations.

Next we focus on the crucial role of ODS in produ-cing extremely low-ozone years. We do this by con-sidering separately the first and last two decades of oursimulations, which we illustrate in figure 1 by color-coding years from1955 to 1975 in blue, and years from1985 to 2005 in red (the winters in the middle decade,1975–1985, are not shown). The thick blue and redcurves show the average of the corresponding 120curves (20 years x 6 simulations). Notice that there is avery clear separation between the early and late dec-ades. In the early period (blue curves), i.e. before thelarge increase in ODS, the interannual variability isrelatively modest, and it is largely due to dynamics. Inthe late period (red curves) the year-to-year spreadbecomes much larger, as a consequence of the higherconcentrations of ODS. Furthermore, notice thatwhile high ozone winters occur in both periods, thevery low ozone winters are seen exclusively in the lasttwo decades, i.e. after 1985, as ODS are needed to pro-duce extreme ozone depletion. This is consistent withobservations (see, for instance, Dameris et al 2014),and the extreme low values of about 1.5 ppmv in Aprilat 70 hPa agree with satellite data for 2011 reported inManney et al (2011).

One might wonder how the ranges between highand low ozone years in our WACCM4 simulationscompare with those explored in Smith and Polvani(2014; see their figure 1), who used highly idealized‘synthetic’ ozone concentrations in order to controlthe amplitude of the high/low range (they focused on

Figure 1. Seasonal evolution of polar cap ozone (averaged 65–90N) at 70 hPa. Thin lines: ozone fromOctober of one year toSeptember of the following. Blue: years from the period 1955 to 1975, using six simulations (for a total of 120 curves). Red: same asblue, but for the period 1985–2005. Thick lines are themeans of the corresponding thin lines.

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±15% and ±25% of the March total column ozone).With reference to our own figure 1, the range in theearly period—computed from the 70 hPa polar capozone difference in April between the 30 highest andthe 30 lowest blue curves—is approximately ±10%,whereas for the late period the range is approximately±20%. These numbers are consistent with the find-ings Smith and Polvani (2014), although one ought tokeep inmind that the ozone used in that studywas spe-cified from zonally symmetric and monthly meanvalues and, furthermore, that the ozonewas decoupledfrom the dynamics in that study.

To examine whether the dynamics is fundamen-tally different between the earlier and later periods, wehave computed the statistics of stratospheric suddenwarmings (SSW) events for the two periods, followingthe simple method of Charlton and Polvani (2007).We find that the frequency of occurrence of SSWs,approximately 4.5 events per decade in WACCM (asalready reported in Marsh et al 2013), is identical—interms of statistical significance—between the 1955−1975 and the 1985–2005 periods. Hence the pre-sence of elevated levels of ODS, and the accompanyingozone depletion, do not seem to affect the frequencyof SSWs.

We now turn to the main question of interest,namely whether one can detect a signature of theextreme ozone anomalies in the troposphere and atthe surface. We do this following the methodology ofSmith and Polvani (2014), which consists of taking thedifference between the extremes of high and low ozoneyears. To bring out the key role of ODS, we do thisseparately for the early (1955–1975) and the late(1985–2005) periods. Since we have 120 years availablefor each period, we bring out the ozone signal by sim-ply subtracting in each period the 30 highest from the30 lowest winters at 70 hPa in April, when the differ-ences are the largest (see figure 1). The results areshown in figure 2, with the differences for the earlyperiod shown in the left column, and for the late per-iod on the right.

The vertical structure of the polar cap ozone differ-ences, as a function of month, is shown in the top rowof figure 2: they are, of course, much stronger for thelate period (panel b), as already shown, owing to thehigh levels of ODS. Accompanying the ozone differ-ences, one finds clear polar cap temperature differ-ences, shown in the middle row. Again, they are muchstronger in the late period (panel d), as one mightexpect.More surprisingly, perhaps, is the fact that theyare only barely significant in the early period (panel c;significance is indicated by stippling). This is because,in the absence of ozone depletion, the interannualozone variability is not sufficiently large to produce aclear temperature difference between the high and lowozone years (note the relatively small spread of theblue curves infigure 1).

The novel result of our study becomes apparentonce we move from temperature to wind differences,

shown in the bottom row of figure 2. For the late per-iod (panel f), the ozone and temperature signals(which peak inMarch andApril) are accompanied by avery strong signal in the winds (here averaged60–75N), which extends all the way to the surface inApril and May. This agrees well with the findings ofSmith and Polvani (2014), who also found that thelarge signals due to stratospheric ozone differencesappear below the tropopause in April andMay. Recall,however, that the ozone values had to be artificiallyinflated in that study and the model they used lacked awell-resolved stratosphere and interactive strato-spheric chemistry. In contrast, for the simulationsanalyzed here, all forcings are consistent with observa-tions and have realistic values: the large ozone anoma-lies that produce a clear surface signal in WACCM4are a natural consequence of the positive feedbackbetween chemistry and dynamics. Needless to say, inthe early period, whenODS concentrations are low, nostatistically significant wind signal is found, even in thestratosphere (panel e).

It is interesting to document the role of SSWs inproducing the differences between high- and low-ozone composites in figure 2. Although, as mentionedabove, the frequency of SSWs in our model (about 4.5events per decade) is the same in the early and late per-iods, the role of wintertime SSWs in determining thespring ozone anomalies appears to be fundamentallydifferent between the two periods. In the early period,we find little difference in SSW frequency between the30 high-ozone and the 30 low-ozone years (12/30 forthe high-ozone years, and 10/30 for the low-ozoneyears have one or more SSWs in that winter). In con-trast, for the later period, the frequency of SSWs is verydifferent between the high and low ozone years (19/30and 0/30, respectively). This clearly show that, in thepresence of elevated ODS concentrations, the occur-rence of SSWs becomes crucial for determining theApril ozone anomalies, whereas SSWs play a relativelyminor role in the early period.

The zonally-averaged picture presented in figure 2is severely limiting, as the tropospheric and surfacesignature of ozone anomalies possesses considerableregional structure. To illustrate this, focusing now onApril–May when the signal is largest at the surface, wenow turn to latitude–longitude plots, which reveal thefull complexity of the tropospheric and surface sig-natures. In figure 3 (top row) we show the differencesin zonal mean zonal wind at 500 hPa, between the lowand high ozone years. For the early period (panel a)there is little to see, but for the late period (panel b) avery robust signal can be seen over the North Atlanticand Eurasia, corresponding to a poleward shifted jet,as one might expect to accompany a strong vortex(which is typically associatedwith a low-ozone year).

Going from the mid-troposphere to the surface,one can see the extreme ozone signature in the April–May differences in sea level pressure (SLP, figure 3,middle row). The structure is reminiscent of a positive

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Northern Annual Mode (NAM) pattern, in agreementwith the strengthening of the zonal winds comingfrom the stratosphere. Negative SLP values appear inthe polar cap region and positive values at middle lati-tudes, mainly in the Atlantic and the Pacific coast ofAsia. Thus, two dipoles are clearly simulated duringApril–May, one in the Atlantic suggestive of the posi-tive phase of the North Atlantic Oscillation, and theother in eastern Asia. We note, however, the absenceof positive anomalies over theNorth Pacific, which aretypical in a positive NAM phase. Together with thepositive anomaly over eastern Asia, the ozone signal inSLP is closer to the pattern of the seasonal varying

NAM, which is somewhat different from the typicalwinterNAM (e.g. Ogi et al 2004).

The April–May surface temperature differencesarising from stratospheric ozone anomalies are shownin the bottom row: again, large significant areas areonly found for the 1985−2005 period (right column,panel f). Two robust features stand out. First, a sig-nificant warming in Northern Europe and Siberia,with the largest values over this region in excess of 5 K;note also the accompanying cooling that appears overChina, consistent with a positive NAM phase. Second,we note significant cooling in Greenland and thenortheastern part of Canada, again with an amplitude

Figure 2.Time-height evolution of the composite differences (LOW-HIGHozone) between the 30 years with the lowest polar capozone values and the 30 years with the highest polar cap ozone values at 70 hPa in April (see figure 1). (a), (b)Polar cap (65–90N)ozone. (c), (d)Polar cap (65–90N) temperature. (e), (f) 65–75N zonalmean zonal wind. Left: composite differences for the 1955–1975period; right: for the 1985–2005 period. Units are ppmv for ozone, K for temperature andm s−1 forwind. Stippling indicatessignificant differences at the 95%confidence level (using a Student t-test).

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of several degrees. Interestingly, both of these featuresare identical (and of opposite sign, as one mightexpect), to those observed on a seasonal basis follow-ing SSW events (see, e.g., figure 2 of Butler et al 2014),when they are associated with an anomalously weakpolar vortex and anomalously high levels of strato-spheric ozone.

Finally, recalling that changes in the location of thejet stream are usually accompanied by changes in pre-cipitation, we conclude by showing differences in thelatter that arise from the ozone anomalies. Since thetropospheric signal of such anomalies is far from zon-ally symmetric, as we have just shown, a signal in thezonal mean precipitation field is not expected. Keep-ing inmind the patterns infigure 3(b), we focus on tworegions: the Atlantic and Eurasia, where the ozone sig-nals appear at distinct latitudes. In figure 4 we plot theApril–May difference, separately, over these two areas(70W to 20E for the North Atlantic Ocean, panel a;

and 50E to 120E for the Siberian region, panel b). Inboth regions, a significant enhancement in precipita-tion at high latitudes can be seen for the period1985–2005 (red curves), associated with extremely lowpolar stratospheric ozone. This response is largelyabsent in the earlier period (blue curves), as the ozoneanomalies prior to the large anthropogenic ODSincreases are not sufficiently strong to be significant atthe surface.

4. Conclusion

We have shown that the year-to-year variability oflower stratospheric ozone over the Arctic, which ariseswhen the internal variability of the stratosphere-troposphere system occurs in the presence of highlevels of anthropogenic ODS, can have a significantimpact on surface climate. Specifically, extreme ozone

Figure 3.As infigure 2, but for the April andMay average of the LOW-HIGHozone differences in (a), (b) zonalmean zonal wind at500 hPa, (c), (d) sea-level pressure and (e), (f) surface temperature. Units arem s−1 for wind, hPa for sea level pressure andK forsurface temperature. Stippling indicates significant differences at the 95% confidence level.

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loss events over the North Pole—as simulated in aensemble of simulations by a state-of-the-art chemis-try-climate model with standard CMIP5 forcingstypical of the late decades of the 20th century—are ableto affect surface pressure, temperature and precipita-tion over much of the Northern Hemisphere. Ensem-ble simulations are key to our findings, allowingsufficient sampling to clearly identify the effects ofozone changes despite themany other factors that leadto high interannual variability in tropospheric climate.Thus the use of such model simulations provides abasis for analysis that is a far more sensitive test thanthe real world’s single realization. Compounding whathas been learned over the Southern Hemisphere in thelast decade, our results further highlight the impor-tance of stratospheric processes for a complete under-standing of surface climate variability and climatechange in both hemispheres.

Our study also shows that when the levels of ODSare relatively low (as in the period 1955–1975) thedynamical variability alone is unable to produce ozoneextremes of sufficient magnitude to be significant atthe surface. This suggests that the coupling betweenchemistry and dynamics, therefore, might be essentialfor the amplification of the stratospheric signal and itspropagation into the troposphere. The importance ofchemistry–dynamics coupling has been noted in priorstudies (e.g. Milewski and Bourqui 2011). In order toproperly quantify the magnitude of this coupling, adirect comparison of coupled and uncoupled modelintegrations is needed. We do not have those at ourdisposal, but plan to explore this issue in future papers.

Our results have clear implications for weatherforecasting. The recent study of Cheung et al (2014),which focused onMarch of 2011 when the largest Arc-tic ozone minimum in the record has been observed,concluded that merely specifying observed (and verylow) ozone concentrations is not sufficient to see anyreduction in forecast errors in the UK Met Office

operational forecast model, although that study did notpresent results for later times in the spring. Our resultssuggest that the absence of coupling between chemistryand dynamics in that model might explain the negativeresult. Whether the inclusion of stratospheric chem-istry, perhaps in some simplified form with a reducednumber of species, is practically feasible in any existingoperational forecast model remains to be examined.The findings of our study offer clear evidence that thispossibility shouldbe explored.

Acknowledgments

The WACCM4 simulations analyzed in this studywhere performed thanks to high-performance com-puting support from Yellowstone (ark:/85065/d7wd3xhc), provided by the Computational andInformation Systems Laboratory at the NationalCenter for Atmospheric Research (NCAR), which issponsored by the US National Science Foundation.The authors thank Dr D R Marsh and Dr R Neely forkindly providing the model simulations and Dr D Ivyfor providing the plot of ozone data presented in thesupplement. NC acknowledges partial support fromthe Spanish Ministry of Economy and Competitive-ness through the MATRES (CGL2012-34221) projectand the European Project 603557-STRATOCLIMunder program FP7-ENV.2013.6.1-2. The work ofLMP is funded in part by a grant of the US NationalScience Foundation to Columbia University. Thework of SS is funded in part by grant 1419667 of theUSNational Science Foundation toMIT.

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