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The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring

Citation for published version: Lee, AM, Roscoe, HK, Haynes, PH, Shuckburgh, EF, Morrey, MW & Pumphrey, HC 2001, 'The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring', Journal of Geophysical Research, vol. 106, no. D3, pp. 3203-3211. https://doi.org/10.1029/2000JD900398

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Download date: 24. Sep. 2021 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D3, PAGES 3203-3211,FEBRUARY 16, 2001

The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring Adrian M. Lee,1 HowardK. Roscoe,2 Anna E. Jones9. Peter H. Haynes, Emily F. Shuckburgh,• Martin W. Morrey,4 and HughC. Pumphrey4

Abstract. Calculations of equivalent length from an artificial advected tracer provide new insight into the isentropic transport processesoccurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly mixed vortex core and a broad ring of we.akly mixed air extendingout to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and midspring. Satellite measurementsof stratospheric H20 confirm that the isolation lasts until at least mid-October. A three-dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly mixed broad ring is not complete. The reasonsare twofold. First, warmer temperatures in the broad ring preventcontinuous polar stratosphericcloud (PSC) formation and the associatedchemical processing (i.e., the conversionof unreactivechlorine into reactiveforms). Second,the isolationprevents ozone-rich air from the broad ring mixing with chemically processedair from the vortex core. If the stratosphere continuesto cool, this will lead to increased PSC formation and more complete chemicalprocessing in the broad ring. Despite the expected decline in halocarbons, sensitivity studies suggestthat this mechanismwill lead to enhanced ozone loss in the weakly mixed region, delaying the future recoveryof the ozone hole.

1. Introduction alyzeozone loss in [ WorldMeteorological Orga- nization,1989]. In the vortexcore, PSCs are ubiquitous [McCormick,1983; Caccianiet al., 1997],so that ozone The Antarcticozone hole [Farman et al., 1985]is now lossis determined by the supply of unreactive chlorine. a well-establishedfeature of the Antarctic stratosphere We expect ozoneloss in the vortex core to diminish dur- in spring. It is confined within a vortex of strato- ing the next 50 years, as amountsof halocarbonsdecline spheric winds and remains largely isolated from the due to the provisions made in the Montreal Protocol middle latitudes. The persistent mean meridional cir- and subsequentrevisions [Pyle et al., 1996]. culation brings unreactive chlorine compounds,which With the rapid polar ozoneloss processes initiated by originate from man-made halocarbons, from the up- sunlight, the ozone hole developsfrom the vortex-edge per stratosphere into the polar vortex and to altitudes region inward to the core. The poleward propagation where polar stratosphericclouds (PSCs) form. Reac- of this ozone lossin the presenceof reactive chlorine is tions on these cloud particles convert the unreactive limited by the polewardretreat of the terminator (the chlorinecompounds into reactiveforms which then cat- boundary which delineatespolar ), by planetary waves exposing deeper vortex air to sunlight and by the strength of mixing within the vortex region. In a modelstudy of the 1994winter and spring,Lee [1996] XCentre for AtmosphericScience, Department of Chemistry, University of Cambridge,Cambridge, England. showedthat during late winter, ozoneloss did not prop- agate polewardsignificantly faster than the terminator. 2BritishAntarctic Survey, Cambridge, England. This result suggestedthat the mixing of air betweenthe 3Centrefor AtmosphericScience, Department of Applied vortex-edgeregion and the vortex core was weak, but Mathematicsand Theoretical Physics,Cambridge, England. the study did not assessthe accuracyof this aspect of 4Departmentof Meteorology,University of Edinburgh,Edin- the model. burgh, Scotland. Similar results were found in a model study of the Copyright2001 by theAmerican Geophysical Union. 1996winter and spring[Lee et al., 2000],using the same modelas in this work (seesection 4). Measurementsof Papernumber 2000JD900398. ozone were also assessedand showedvery good agree- 0148-0227/01/2000JD900398 $09.00 ment with all aspectsof the model calculations.

32O3 3204 LEE ET AL.: OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX

However, these studies could not differentiate whether gion representsthe "vortex boundary," delineating the this mixing property was a result of either the whole vortex from the surf zone. vortex, or just the region near the vortex edge,being a However, few studieshave examined mixing between regionof weak mixing. Nor couldthey tell if the region the vortex-edgeregion and the vortex core. Paparella near the vortex edge remained isolated from the vortex et al. [1997]found significantapparent mixing when core after late winter, as this cannot easily be deter- the vortex-edge region was defined by PV gradients, mined from models or measurements of ozone because but when they used a new vortex definition based on of the nature of the latitudinal gradientsin ozone. kinetic energy, a broad isolated region emergedin the In spring,there are frequentinstances of large plan- vortexedge (their Figure 8a). etary waveswhich distort the vortex suchthat its edge Recently, diagnosticsof transport and mixing behav- region is over populated areas of southern South Amer- ior calculatedfrom tracer fields have been developedby ica [Joneset al., 1998]. During one suchepisode at Nakamura[1996] and appliedto satelliteobservations of Punta Arenas (53øS) in 1995, the ozonecolumn fell chemicalspecies [Nakamura and Ma, 1997]. The diag- from morethan 310 Dobsonunits (DU) on October8 to nosticcalculated is the equivalentlength (Leq),which 200 DU on October13 [Kirchhoffet al., 1997].If such is a measure of the geometric structure of the tracer low-ozoneevents happen late enough in spring when and hence of the mixing strength. Regions of strong the midday Sun is at higher elevation,the lack of ozone mixing have large values of equivalent length and re- can cause significant increasesin UVB radiation with gionsof weak mixing (thoseassociated with barriersto the potentialfor biologicaldamage [Lubin and Jensen, transport)have small values. 1995]. In ourstudy, following Haynes and Shuckburgh [2000], In this paper, we diagnose the transport structure the equivalent length is calculated from an artificial of the lowerstratosphere, enabling us to investigatethe tracer field advected on isentropic surfaces. The tracer isolationof the vortex-edgeregion from the vortex core. field was initialized on May 1, 1996, with the potential The impact this isolation has on the developmentof vorticity distribution and advected using U. K. meteo- the Antarcticozone hole is investigatedusing a three- rological analysesat 1ø latitude x 1ø resolu- dimensionalchemical transport model. tion. In our results,we avoid zonal averagesat fixed lati- For presentation purposesthe square of the equiva- tudes becausereversible dynamical effectssuch as fluc- lent length is normalized by the squareof the circum- tuations in the shape of the vortex can be wrongly ferenceof the zonal circle at the equivalent latitude of attributed to mixing. Instead, we use a tracer-based the tracer contourto givethe quantity Aeqdefined by equivalent-latitudecoordinate (the equivalentlatitude is the latitude at which a tracer contour would lie if the dl

tracer distributionwas deformed into a zonallysymmet- ' -- dl ' ric atmospherewhile conservingthe area within tracer t) cos).f Ivl contours,a techniquefirst used by Butchart and Rems- (1) berg[1986]). Calculationis basedon an advectedar- Here •beis the equivalent latitude, r is the radius of the tificial tracer initialized with potential vorticity (PV , and the integrals are around a tracer contour [Hoskinset al., 1986])before the periodof study,rather c(x, y, t) = C. For further details and discussion,see than the noisier PV calculateddirectly from the mete- Shuckburghet al. [2000]. orological analyses. This artificial tracer is used as the Figure I showsthe evolution of the lognormalized basis for all the equivalent latitude calculations in this equivalent length as a function of equivalent latitude study. It should also be noted that this artificial tracer on the 480 K isentropicsurface during the 1996 winter is alsoused for diagnosticstudies of transport and mix- and spring. The regionof strongestmixing is associated ing in section 2. with the surf zonein the middlelatitudes [Juckes and Mcintyre, 1987]. During the winter a broad regionof weak mixing between equivalentlatitudes of 58øS and 2. Diagnosing the Flow Structure in the 68øS(the vortex-edgeregion) separates regions of com- Lower Stratosphere paratively strong mixing in the middle latitudes and the vortex core. Air masseswithin this vortex-edgere- The isolation of the polar vortex from the surf zone gion of weak mixing will remain isolated from either in the middle latitudes is due in part to the Rossby- the middle latitudes or the vortex core. It is appropri- restoringmechanism in the large scale,and in part to ate to identify the equatorwardlimit of this region of horizontalwind shearacting at smallerscales [Juckes weakmixing with the vortexboundary. Thus within the and Mcintyre, 1987]. The importanceof the vortex- polar vortex there are two distinct regionsof approxi- edge region as a barrier to transport has motivated mately equal area identifiedby their mixing character- many studiesof atmospherictransport [e.g., Norton, istics. As spring progresses,the region of weak mix- 1994; Waughet al., 1994; Chen, 1994]. It is usefulto ing persists,but its width and positionchange over the note that the equatorwardlimit of the vortex-edgere- .After mid-Octoberthe regionstarts to narrow LEE ET AL.- OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX 3205

-4O

-5O

= -60

ß1• _

• -

_ > -70

_

• - ILl -

-80 -

_

_

_

-90 1 i Jul Aug Sep Oct Nov (1996)

Figure 1. The evolution of lognormalizedequivalent length as a function of equivalent latitude on the 480 K isentropicsurface during the 1996 winter and spring. The broad region of weak mixing centered at around 63øS equivalent latitude will act as a significant transport barrier preventing the mixing of air in the vortex core with air in the middle latitudes.

and in late November there is an increasein equivalent length in the region, indicating that in late November the ability of this region to act as a barrier to transport has been reduced. Note that Haynes and Shuckburgh [2000]only foundevidence of mixingin the vortexcore in the midstratosphere. This is believed to be a conse- quenceof the lower resolutionused in the tracer calcu- lations of that study. Equivalent length is a function of equivalent lati- tude and hence of tracer concentration. It may there- fore be regarded, just as a tracer concentration may be regarded, as a function of latitude and longitude. Thus isentropic distributions of equivalent length can be achievedby mapping the equivalentlength onto the isentropicdistribution of equivalentlatitude. Figure 2 showsthe lognormalizedequivalent length for Septem- ber 1, 1996, on the 480 K isentropic surface. In this representation it is strikingly clear how a broad ring of weak mixing (the vortex-edgeregion) separatestwo regions of comparatively strong mixing in the middle latitudes and in the vortex core. 0.5 i 1.5 2 2.5 3 3.5 4 4.5 5 3. Weak Mixing Inferred From Measurements of Water Vapor Figure 2. Lognormalizedequivalent length for September1, 1996 mapped onto the 480 K isentropic in the Vortex-Edge Region surfaceusing the equivalentlatitude distribution. The region of weak mixing identified in Figure I is dis- The calculationsof section2 relied on the accuracyof tributed as a broad ring separatingregions of compara- meteorologicalanalyses in a data-poor area, so we ex- tively strongermixing in the vortexcore and the middle amined measurementsof tracers which might help di- latitudes. 3206 LEE ET AL- OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX

195o surementsin Figure 3 show this behavior very clearly. The low temperatures in the vortex core form PSC ice 1700 particles which are large enough to sediment, leading •45o to dehydration,as can also be inferred from Figure 3. 1260 In the lower stratosphere, it leads to a dry vortex core 1100 surroundedby a ring of more humid air in the vortex- edge region, with drier air outside the ring at middle 960 latitudes. Persistenceof this pattern after dehydration 840 has ceasedin Septemberwould suggestisolation of the 740 vortex core from the vortex-edgeregion. 655 Water vapor from the MLS instrument aboard the 585 Upper AtmosphereResearch Satellite (UARS) is now 530 retrieved with a nonlinear algorithm which extends the 0 -20 -40 -60 -80 retrieval into optically thicker regionsat lower altitudes Equivalent latitude [Pumphrey,1999]. Figure 4 showsthe evolutionof MLS water vapor as a function of equivalentlatitude on Figure 3. Equivalent latitude-potential temperature the 480 K isentropicsurface during the 1992 spring. The cross section of water vapor measured by MLS from expected pattern is seen throughout September. Dur- August19 to 23, 1992 (takenfrom Morrey[1997]). Ox- ing October the MLS instrument is pointing northward. idation of CH4 creates moist air aloft, which descends during poleward transport in winter. The low temper- When in November the instrument is again pointing atures in the lower stratospheric vortex core form PSC southward a drier vortex core is still visible, although ice particleswhich are large enoughto sediment,leading some moistening has occurred. More complete mixing to dehydration. Following dehydration, a water vapor betweenvortex core and edgeoccurs during November, maximum appears in the vortex-edgeregion. before the final vortex breakdown. agnosemixing. In Antarctica, water vapor is a par- 4. Chemical Model Calculations for 1996 ticularly useful tracer. Descent throughout the vor- tex duringwinter bringsair from aloft, whichfollowing In this section we present calculationsfrom a three- methane oxidation is rich in water vapor. Microwave dimensionalchemical transport model SLIMCAT. The Limb Sounder(MLS [Waters,1993]) water vapor mea- model, describedby Chipperfieldet al. [1996],is for-

-4O

- -60 - --

--

- _ -70 -

_

- -

_ -80 -

_

' ' I Jul Aug Sep Oct Nov Month (1992) Figure 4. The evolutionof MLS water vapor as a functionof equivalentlatitude on the 480 K isentropicsurface during the 1992 spring. The water vapor was averagedwithin 4ø equivalent latitude bands. The MLS measurementsshow the contrastin water vapor betweenthe vortex- edgeregion and the drier vortex coreduring spring. This supportsour analysisof weak mixing diagnosedby equivalentlength (Figure 1). LEE ET AL.: OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX 3207 mulated in an isentropic coordinate. The upstream ClO+ClO+M > C1202 +M advection scheme is based on conservation of second- C1202 +h• > Cl+ClO0 order momentsof tracer distribution during advection ClO0+M ,> C1+02 +M [Prather,1986]. The circulationand temperaturesare 2x(Cl+O• ,• ClO+O•) specifiedfrom the 24-hourly analysesof the U.K. Meteo- net: 03 + 03 '• 02 +02 + 02. rologicalOffice data assimilationsystem [Swinbank and O'Neill, 1994]. Verticalmotion is derivedfrom heating In the middle latitudes, where C10 concentrationsare ratescalculated by the radiationscheme of Shine[1987]. a small fraction of those found in the wintertime polar The transport code is coupled to a detailed strato- vortex, the rate of the C10 dimer cycle is insignificant. spheric chemistry scheme,including heterogeneousre- Thus air masseswhich have experiencedozone loss due actions. to this cycle are air massesof polar vortex origin. Fig- The model was initialized from a low-resolution mul- ure 5 showsthe evolutionof ozoneloss (ppmv) due to tiannual simulation, details of which have been reported the C10 dimer cycleas a function of equivalentlatitude by Chipperfield[1999]. The modelsimulation was started onthe 480 K isentropicsurface: loss begins in midwinter on May 9, 1996, and continueduntil November 15, 1996. at the vortex edgewhere it protrudesout of , The resolution was 2.5ø latitude and 5.6ø longitude. as confirmedby observations[Roscoe et al., 1997];loss There were 12 isentropic levels in the vertical, at inter- remains confined to the sunlit side of the terminator vals of between 1.5 and 2.0 km in the lower stratosphere. until the end of July; subsequently,ozone loss rapidly General model accuracy was confirmed by comparing penetratesthe regionpoleward of the terminator[Lee the modelozone field with numerousozonesondes [Lee et al., 2000]. et al., 2000],showing very goodagreement. However, a maximum in ozone loss persists in the The ozone tendenciesassociated with catalytic ozone vortex-edgeregion until mid-September. This is readily loss cycles are easily diagnosedwithin the model. The interpreted in terms of the equivalentlength. Ozoneloss dominant ozoneloss cycle in the polar region is the C10 which occurswithin the region of weak mixing remains dimer cycle[Molina and Molina, 1987]: largely confined to that region, and ozone loss which

-90 Jul Aug Sep Oct Nov Month (1996)

Figure 5. The evolution of accumulated model ozone loss due to the C10 dimer cycle as a functionof equivalentlatitude on the 480 K isentropicsurface (reproduced from Leeet al. [2000]). The ozoneloss has been averagedwithin 2.5ø equivalentlatitude bands. Contoursof ozoneloss are given in mixing ratios (ppmv). The bold line showsthe approximatelatitudinal positionof the terminator on the 480 K isentropicsurface. During winter and early spring, significantozone lossis strongly confinedto the sunlit region at the edge of the vortex. 3208 LEE ET AL.- OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX occursin the region of strong mixing within the vortex Figure 6 showsthe evolutionof ozoneloss which oc- core is rapidly mixed throughout the vortex core. In curs in the equivalent latitude range 60ø to 65øS: the the vortex-edge region, stratospheric temperatures are loss remains confined to the vortex edge region until generallywarmer than in the core [e.g., Tao • Tuck, mid-October. To determine how this compares to the 1994], so that PSCs are lessfrequent and so there is evolution of ozone loss which occurs within the vortex less reactive chlorine. However, the edge region sees core, Figure 7 showsthe evolutionof ozoneloss which more sunlight earlier in the year. This'combination re- occursin the equivalentlatitude range 70ø to 75øS:the sults in greater ozone lossin the edge region until mid- lossis rapidly mixed throughout the core, rather than September. It should be noted that all the ozone loss being confinedto the equivalent-latitudeband at which shown in Figure 5 occurs and remains confinedto the the lossoriginally occurred. polar vortex. Further, we can determine the confinement of ozone 5. Discussion lossoccurring within the vortex-edgeregion. Although the edge region occupiesa broad band of width 10ø in From our modeling studies we now have a new pic- equivalent latitude, one problem is that its mean equiv- ture of the Antarctic ozonehole. There are two regions alent latitude changes during the winter and spring. which remain distinct until at least mid-October: a core However, Figure I showsthat the 60ø to 65øSequivalent- which is well mixed and a broad edge region in which latitude range remains a region of weak mixing through- mixing is weak. The properties of each region differ out the winter and spring. Hence we can explore the considerably. In the edge region, stratospheric tem- confinementby examining ozone losswhich occursonly peratures are warmer and so PSCs are less frequent, within this narrower range and by followingthat ozone but it seesmore sunlight earlier in the year. This re- loss as it is transported to other equivalent latitudes. sults in greater ozone lossin the edge region until mid- This was done during the model run by calculating a September(Figure 5). tracer which is the integral only of ozone loss that oc- Although the area coveredby the two regionsis sim- curs in a particular equivalent latitude band. The tracer ilar, the fact that isentropic surfaces bend upward to is then advected by the analyzed winds in the same way lower pressuresat the pole means that the edge region as other trace gasesin the model: no specifictrajecto- is more massivethan the core. As a result, by the end of ries need be calculated. September,approximately 20% more ozoneloss occurs

-9O Jul Aug Sep Oct Nov Month (1996) Figure 6. The evolution of accumulatedmodel ozone lossthat occurred within the 60ø to 65øS equivalentlatitude band as a function of equivalentlatitude on the 480 K isentropicsurface. The ozone loss has been averagedwithin 2.5ø equivalent latitude bands. Contours of ozone loss are labeledas mixing ratios (ppmv). The bold line showsthe approximatelatitudinal positionof the terminator on the 480 K isentropicsurface. The ozoneloss remains largely confinedto the region of weak mixing until at least mid-spring. LEE ET AL.: OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX 3209

-40

= -60-

> -70

-80

-90 Jul Aug Sep Oct Nov Month (1996) Figure 7. As Figure 6, but for ozoneloss that occurredwithin the 70ø to 75øSequivalent latitude band. Transport of ozoneloss southward soon after it occursat these latitudes in early spring (August)is clearlyevident, unlike Figure 6. This illustratesthe comparativelywell-mixed nature of the vortex core.

within the 60ø to 70øS equivalent-latitude band than supply of unreactive chlorine, unlike ozone loss in the occursfurther poleward: the edge region is not the mi- vortex core where PSCs are ubiquitous. nor part of the ozonehole that might be thought from PSC amounts are determined by the temperature of linear latitudinal plots. the lower and middle stratosphere, whose global aver- Becauseof the lack of mixing, the properties of the agehas cooled by about i K since1980 [e.g., Pyle et al., edge region contribute separatelyto the ozone loss po- 1999].Calculations by Forsterand Shine [1999] indicate tential averaged over the Antarctic as a whole. To- that this coolinghas arisen from observedincreases in gether with the larger mass of the edge region, this CO• and H•O and reductions in ozone. We do not un- clearly has implications for the ozone deficit averaged derstandthe causesof the increasesin H•O [Pyle et al., over the whole Southern Hemisphereafter vortex break- 1999],and coolingby reducedozone acts as a positive down. These implications are beyond the scopeof this feedbackwhich causessignificant uncertainty about its paper. future size. However, CO• is expected to continue to This new picture of the ozone hole also has implica- increaseduring the next 50 years. tions for future ozoneloss. Ozoneloss in the edgeregion If current trends continue, a likely result will be an is limited by a function both of PSC amount and of the increasedfrequency of PSCs in the Antarctic vortex-

Table 1. Ratios to That in the Control Run of Ozone Loss Calculated by Model Runs With Different Amounts of Total Chlorine and Different Temperatures.

Total C1, ppbv C10-Dimer Cycle All Cycles AT, K 3.6 3.0 2.5 2.0 3.6 3.0 2.5 2.0

0 1.00 0.79 0.61 0.44 1.00 0.92 0.83 0.72 -2 1.12 0.91 0.72 0.53 1.09 1.02 0.93 0.82 -4 1.21 1.00 0.80 0.60 1.15 1.09 1.00 0.89 -6 1.28 1.07 0.87 0.66 1.18 1.13 1.05 0.94 -8 1.34 1.13 0.93 0.71 1.20 1.15 1.08 0.96

The ozoneloss was averagedover the equivalentlatitude band from 60ø to 65øSon October 16, 1996, on the 480 K isentropicsurface. Lossdue to the catalytic cycle involving the C10 dimer and net lossdue to all cycleswere independently diagnosed. 3210 LEE ET AL.: OZONE LOSS AND MIXING WITHIN ANTARCTIC VORTEX

edgeregion during the mid-21stcentury, similar to that Chen, P., The permeability of the Antarctic vortex edge, J. predictedin the Arcticvortex as a whole[Shindell et al., Geophys. Res., 99, 20,563-20,571, 1994. 1998].In the edgeregion of the Antarcticvortex, the Chipperfield, M.P., Multiannual simulations with a three- result will be a more completeconversion of unreactive dimensionalchemical transport model, J. Geophys.Res., 10•, 1781-1805, 1999. chlorine into reactive forms and thus more ozone loss Chipperfield, M.P., M. L. Santee, L. Froidevaux, G. L. Man- than otherwise. ney, W. G. Read, J. W. Waters, A. E. Roche, and J. M. To test this theory, the model was run at different Russell, Analysis of UARS data in the southernpolar vor- temperatures and different amounts of total chlorine tex in September 1992 using a chemicaltransport model, J. Geophys.Res., 101, 18,861-18,881, 1996. loading(i.e., both inorganic and organic chlorine). Val- Dunkerton, T. J., C.-P. Hsu, and M. E. Mcintyre, Some ues of net ozoneloss in springin the isolatedvortex- Eulerian and Lagrangian diagnosticsfor a model strato- edgeregion, both from the C10 dimer cycleand from spheric warming, J. Atmos. Sci., 38, 819-843, 1981. all cycles, are listed in Table 1. The table showsthat Farman, J. C., B. G. Gardiner, and J. D. Shanklin, Large if temperatureswere 4 K colder, ozoneloss similar to lossesof total ozonein Antarctica revealseasonal C1Ox/ NOx interaction, Nature, 315, 207-210, 1985. or greater than today's values would occur at all total Forster, P.M. de F., and K. P. Shine, Stratospheric water chlorineloadings greater than 2.5 ppbv. The chlorine vapor changesas a possiblecontributor to observedstrato- loadingof the atmosphereis forecastto remaingreater sphericcooling, Geophys.Res. Left., 26, 3309-3312, 1999. than 2.5 ppbvuntil after2030 [Pyle et al., 1999].These Haynes, P. H., and E. F. Shuckburgh, Effective diffusivity results indicate that the horizontal extent of the ozone as a diagnosticof atmospherictransport, 1, Stratosphere, J. Geophys. Res., 105, 22,777-22,794, 2000. hole, which dependson the ozoneloss in the weakly Hoskins, B. J., M. E. Mcintyre, and A. W. Robertson, On mixedvortex-edge region will not recedeuntil after 2030 the use and significanceof isentropic potential vorticity if this regioncools by 4 K or more. maps, Q. J. R. Meteorol. Soc., 111, 877-946, 1986. (Cor- Hence as the amounts of halocarbons decline accord- rigendure, Q. J. R. Meteorol. Soc., 113, 402-404, 1986.) ing to the provisionsmade in the MontrealProtocol, we Jones, A. E., T. Bowden, and J. Turner, Predicting total ozone based on GTS data: Applications for South Amer- wouldnot expectozone depletion in the vortex-edgere- ican high-latitude populations, J. Appl. Meteorol., 37, gion to recoveras rapidly as anticipated for the vortex 477-485, 1998. core. It is ozone-depleted air in the isolated vortex- Juckes,M. N., and M. E. Mcintyre, A high-resolution,one- edgeregion that frequentlypasses over populated areas layer model of breaking planetary waves in the strato- of southernSouth America, where in mid-Octoberthe sphere, Nature, 328, 590-596, 1987. Kirchhoff, V. W. J. H., C. A. R. Casiccia, and F. Zamorano, middaySun is at sufficientlyhigh elevationfor signif- The ozone hole over Punta Arenas, Chile, J. Geophys. icant UV damage to occur during low column ozone Res., 102, 8945-8953, 1997. episodes. Lee, A.M., Numerical modelling of stratospheric ozone, Nor wouldwe expectthe lossaveraged over the whole Ph.D. thesis, Univ. of Cambridge, Cambridge, England, ozonehole to recoveras rapidly as anticipated. This 1996. has obviousimplications for future ozone values in late Lee, A.M., H. K. Roscoe,and S. Oltmans, Model and mea- surementsshow Antarctic ozoneloss follows edge of polar summerin the SouthernHemisphere as a whole. night, Geophys.Res. Left., 27, 2000. Lubin, D., and E. H. Jensen, Effects of clouds and strato- Acknowledgments. We thank M.P. Chipperfieldand spheric ozone depletion on ultra-violet radiation trends, J. A. Pyle for SLIMCAT code, and S. J. Oltmans and Nature, 377, 710-713, 1995. R. S. Harwood for useful discussions. A.M. Lee thanks McCormick, M.P., Aerosol measurementsfrom Earth orbit- UGAMP and DETR EPG 1/1/83 for funding.P. H. Haynes ing spacecraft, Adv. SpaceRes., 2, 73-86, 1983. thanksUGAMP and EC (TOASTE-C). E. F. Shuckburgh Molina, L. T., and M. J. Molina, Production of C1202 from thanksNERC, EC throughthe Polar Vortex Changeproject the self-reaction of the C10 radical, J. Phys. Chem., 91, (ENV4-CT97-0540)and Trinity College,Cambridge. M.W. 433-439, 1987. Morrey and H. C. Pumphreythank NERC for funding.The Morrey, M. 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