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The Impact of the Mixing Properties Within the Antarctic Stratospheric Vortex on Ozone Loss in Spring

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The Impact of the Mixing Properties Within the Antarctic Stratospheric Vortex on Ozone Loss in Spring Edinburgh Research Explorer 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 Digital Object Identifier (DOI): 10.1029/2000JD900398 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Journal of Geophysical Research Publisher Rights Statement: Published in Journal of Geophysical Research: Atmospheres by the American Geophysical Union (2001) General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. 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 sunlight[ 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 night), 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 frequent instancesof 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 longitude 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 Earth, 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].
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