RESEARCH GEOPHYSICAL LITTERS,VOL.20, NO. 12,PAGES 18, 1235-1238,/UNE 1993 WATER SPRINGTIMESTRATOSPHERIC IN VAPOUR THE SOUTHERN HEMISPHEREAS MEASUREDBY MLS Harwood Cart L. Froidevaux Jarnot R.S. •, E.S. •, •, I•.F. •, W.A. •, s, LahozC.L.Lau G.E.Peckham •, P.D. •, s, s,W.G.P•ead RicaudR.A.Suttie J.W.Waters • Abstract. The effectsof the break-upof the antarc- AlthoughHaO in the antarctic vortex has been stud- tic vortex on the water vapour distribution are studied from of systems, iedpreviously a variety observing MLS MLS measuren•ents using made of watervapour during allowsthe evolution of broad-scalefeatures of the ff20 dis- 1991 September and November 1991. In earlyNovember tribution to be observed daily overan unprecedented areal at 22 hPa a moistareais foundwithin the polarvortex, and vertical extent. Here we make use of the southward- consistentwith an observed descent of order 10 km and in with potentialvortic- lookingobservations conjunction strong cooling. the vortex radiative As erodes(beginning ity (PV) based windandtemperature on analysesfromthe of November1991), parcelsof moist air become detached UK Meteorological Office and (Swinbank O'Neill, 1992)to fromthe edgeof the vortex and mix rapidly (within 2-3 discuss of someaspects the circulationduringthe break- days)with driermid-latitudeair. Whenthe vortexbreaks up of the antarctic vortex in spring, showingevidenceof largerparcels moist fromboth up(mid-November), of air in at descent the vortex,erosion the vortexedgeandtrans- the edgeand the inner vortex migrate to mid-latitudes. port of vortexair to low latitudes. These parcels have a. longerlifetime than thoseproduced by vortexerosion, probablybecause they are correlated The vertical descent withhigherpotentialvorticitygradients. The break-up of in In the antarcticvortexin the lowerstratosphere win- by the vortex is accompanied a mean adiabaticequator- wardtransportresultingin a significant in increase mid- of ter, a process dehydration and denitrificationis believed stratospheric water vapour valuesat mid-latitudesin late to take placeas a resultof the formationof polar strato- spring. spheric to clouds.This allowsactive chlorine be released which results in the "ozone hole". By early November Introduction 1991, however, whenMLS began an extendedperiodof lookingsouth,the main area of the vortex was found to The Microwave Limb Sounder (MLS) carriedby the be moistat 22 hPa (the highest pressures which at UpperAtmosphere ResearchSatellite(UARS)launched the profileshavesignificant informationin the currentre- on 12 September1991 measuresconcentrations severa! of trieval software). Figure 1 showsH20 at 22 hPa on 8th species importance the middleatmosphere, of in princi- Novemberi991 (hereafterdates shownas e.g. 911108). pallyClO, OaandH20 (Waters, 1989).The satellite yaws The moist area is found to be well correlated with the aroundat intervalsof approximately one month. This highestvaluesof-PV as discussed further below. The leads a periodin whichthe region to sampled MLS by a highmixingratiosin the vortexare presumably result is from • 80øS to ~ 34øN followedby one in which it is bringingmoist air from above. Severalstud- of descent from ~ 34 øS to • 80 oN. iesof zonalmeancirculationand heatingrates havefound we on from in thispaper concentrate measurements the cooling in in and descent polar regions spring(e.g. Gille MLS H2¸ channelat 183 GHz. The MLS measurements and Lyjak, 1986). It is alsowell known(e.g. Schoeberl of watervapourhavea horizontal verticalresolution et al., 1992),that verticaldescent and (downward isen- across at at of With the present tropes) ~ 50 hPaoccurs the edge the polarvortex of • 400 km and ,.• 4 km respectively. version theretrieval of software (V0003),theprecisionand in winter and early spring. Sincein November1991 the accuracy individual for profiles 46hPaare0.5ppmv at and vortex is shifted from the pole by at least 10ø, evidence 15% while respectively, at 4.6hPathey 0.3ppmv are and that vertical descent has taken place is clearly visiblein The data for each24 hourperiod 1,5%, centred !200 on vs (r/ a longitude r/ cross-section = /,•.(0),0 = potential UT havebeenlinearly onto interpolated a fixedlatitude- as 2a temperature) in figure in whichMLS H20 values for longitude Ascending descending grid. and portions the of 911108havebeenlinearly interpolatedin the verticalonto orbitweretreated and separate!y thenaveraged. and isentropes averaged within a 70øS-60øS latitudebin. are of Alsoshown estimates the Erte! PV (figure2b) mid the net diabaticheatingrates(figure2c) averaged within the same latitude bin. University, ....•'Edinbt[rgh UK by The heatingratesare calculated the methodof Laboratory, JetPropulsion USA a Herlot-Watt UK University, Haigh(1984) using of MLS retrievals tiaO, O• andtem- as on the perature input.Asa check thesensitivity input by fieldswereperturbed typicalMLS uncertainties. The 1993 Copyright bytheAmerican Geophysical Union. to rate perturbation theheating was_<10% thestrato- in sphere. Paper 93GL00832 number isentrope Ona given through of most thestratosphere 0094-8534/93/93GL-00832503.00 2, is of with in figure there a region highH•O correlated 1235 1236 ter Ilarwoodet a,l.:MLS Wa, Vapour of or •wa.11' the vortex can be definedby the valueof its steepestgradientand labeled(Tucket al., 1992)a.s'con- (a.nyparcelof air insidea givenPV contour servative' is witlfin the vortex) or 'liberal' (a.nyparcel of air outsidea PV is the tn 3, given contour outside vortex). figure lightly shadedareasrepresentthe 'wall' of the vortex betweenthe 'liberal'contourvalueo|'-10x10-SKm-2kg-•s-• andthe tive' 'conserva. contour x -'5 valueof -12.,5 10 Km-2kg-•s-•. The heavily areas the siva,deal tel)resent moistregion where I120 valuesexceed,5.1 l)pmv. correla,tion The spatial an(1teinl)ora.1 betweenthe moist and r ca.• seen areat thepola.vortex beclearly 3. fromfigure stereogra.pl•ic o1' Fig. 1. Pola.r at map H20 (ppmv) 22l•Pa Initially l,]•evortex is coherentaa•doffset somewhatfrom on 911104.The ma.pextendsto 30øS. it tes, the pole. As tingeproceeds, elo•ga. splitsinto two into pieces.Oul. andfinally fi'a.gments several in si(!etl•e ma, high-PV. It is seenthat the radiativecooling(negative ir vortex, there are sotne•l•oist a. parcelswhich are corre- net heating 71 rate) above = 6.6 is greater withi• thevortex 1,tl, with PV a.reas ecl havingt]•esigna. of the interior ture or of (rate of change temperature 3 K.day a.t71 6.8) ,-0 -• = of •wa,11' tire vortex. On 91110,1 one sucl• ex-vortex moist than outsideit (,-0 1.5 K.da,y-' a.t r/ = 6.8) and 11•a.tit l,ed air parcelis 1oca, i• the region30-3.5øSand 10-90øW increases with 'q. The inferredverticaldisplacement the o1' (over South America). Th(, move•nentof this feature west- water isopleths la.rge, is beingfrom r] = 6.9 to 6.5, which ward along the 35øSlatitude circle to southern Australia to corresponds ,-- 10 kin. In comparison et Schoeberl al. with the observed is consistent winds. Despitea reduction ,-• with (1992)founda.valueof 2-3 k•n at 71 6, consiste•lt our in thesizeof thevortex in andthedilIiculty establishing calculationof a smallernet heating rate at that l•eigltt. the lifetimeof such (Tucket al., 1992),its lifetime features is estimatedto be muchgreater than two weeks. In view Movement of the moist area we of thesecorrelatioits, use the measured11.20 fieldsin an attempt to qum•tify the impact of vortex erosionand To understand better the movement of tim moist area. break-up at mid-latitudes. and the pola.rvortex we have interpolatedthe lX•LS H•O a.t of lnea.surements a series daysin November1991and su- k-up of the vortex Erosionand brea. perimposedt, ted hem on PV maps calcula. fi'om tire [IKMO assimilateddata set (see figtire 3). We obtain essentia.lly It is widely believedthat stronggradientsot' PV a.t similar PV distributions based on NMC data. Note that the edge of the polar vorticesinhibit tra.nsfer into tl•e vor- on whereas the PV is 1)a•se½l analyses for 1200 tIT, the tex (Mcintyre, 1989). However,the srna.ll-scale I•orizontal II20 fields are effectivelya 24 hour average. The 'edge' at mixingoccuring tl•e vortexedgeisstrongenougl• peelto of air. out fila.•nents tile vortexto tl•c surrounding Thisef- by and fecthasbeenn•odeled .luckes Mclntyre(1987)and 7.2 42 reported by Kelly el al. (1989) for the sout]•en• vortex (ø) 7.0 and by Tuck el, a.1.(1992) for the northern vortex. During the period 911104 to 911108 a filament of moist and ,-, 20øE) from the main wet air is extruded(,-, ,10øS 6.6 region. The PV analysisstronglysuggests tl•a.t this is a 27•. 6.4 22 pieceof the vortexedgewhichis beingpulledoff. There- after, trace of this moisl filament is lost. This could be (b) 7.0 due to a lack of resolutionand/or rapid horizontalmix- ing which dilutes any wet area.soutside the vortex into • •.8 the surroundingdrier air. "l'helifetime of these southern 6.6 27•. henrispherespringfeaturesis estimatedto be 2-3 daysin 6.4 22 for contrastto a lifetimeof 2 weeks the air parcelmoving along the 3.5øSlatitude mentioned above. Tills is consis- 7.0 at tent with PV gradients the vortex 'xvall'beingweaker in late springthan in early spring. Murphy et al. (1989) showthat erosionof 11•evortex 6.6 27 & titude represents surface by 3-4ø in la. a loss ,-, ,50 area. of %. 22 ify In order to qt•a.•l. the effect of the erosionand break-up 180W90W GM 90E 180E ()• processes l,l•exortex during the period 911IO1-91112,5, Longitude ca. fron• 90øSto the equatortim stirface we l•a.ve lc•la.l.e(1 vs for Fig. 2. Longitude In(O) cross-sections the lat- wil, area lossa.ssociatt,d l• the moistregionwl•ere 1:I20is for of itudebin 70øS-60øS 911108 a) fI:•0 mixingratio nd greatel' titan ,5.1 ppmv, a. the surfacearea lossesa.ssoci- b) -s -•) ting (ppmv), PV/(10 K.m'.kg-•.s andc),,et hea, a.te(l with tl•e 'liln'ra.l' and 'conservative' definitions of the rate, J/(K.da.y-•). vortex. etal' Vapour Harxvood •[LSWaler 1237 4 Nov 1991 6 Nov1991 B Nov 1991 10 Nov 1991 12 Nov 1991 14 Nov 1991 16 Nov 1991 18 Nov 1991 .,•"...•?":'" .,..-' 4 ¾"W .... 19 Nov 1991 21 Nov 1991 23 Nov 1991 25 Nov 1991 .. ?" • .... / .... • ... ..... ..... .... .... ofPV and K 1991. Fig.3. Timesequence (UKMO-derived)H20at 650 forNovember Themaps onaare polar projection 30øS 90øS. lightly stereographic from to The area a representation shaded provides ofthe of by regions 'edge' thevortex marking where PV/(10 K.m2.kg-l.s) lies -s -1 -12.5and-10.The between shaded marks heavily area where regions 5.1 I-[.•0exceeds ppmv. From 911104 to 911108 we find • decrease in stirface 911104 and 911125. The distributions are broadly con- areaof 4.1 x 106km=(• 10 %) for the 'liberal'vortex, sistent with the co•cept that air is dried as it enters the • vortexand 2.1 x 10 km• (• 10 %) t•r the •conservative' at of stratosphere the tropical tropopausebecause the low • 4..5x 10 km2 (• 20 %) for the moistregion. The in- temperatures,and that it progressively moistensthrough termedlate region encompa.ssi•g mixing ratios ,1.8 to 5.1 methaneoxidation(e.g. Joneset al., 1986) as it is carried ppmvexpands 9.4 x 106km=(•,, 70 %). The loss-rate by tion and subjected to lateral round in the diabatic circula. for the period 911118-911125(post break-up)in the area mixing by planetary-scale waves. of the moist region a. lmost doubles,decreasing area the The stronggradie•ts around 60øSin the zonally-avera- the by 5.7 x 106km• (,--,35 %), whereas intermedia.re- te to ged H20 fieldson 910923, which correspond the 'edge' gion by expands 4.9 x 106km (• 15 %). Thusthe to- = vortex (see figure 1,), fi'om r/ = of the zonally-a.vera.ged tal area is conservedto within 15 % in the later period; however, this is not the casein the earlierperiod. This 7.2 suggeststhat horizontalmixing is dominant duringvor- but tex break-up that diabatic are effects of moresignifi- caace earlier. The discrepancy the between PVa.nd moist o.. % 6.8 may to are• changes be attributable thedifferent observ- ingtechniques conservation and/orthedifferent properties .,-) 6.6 •nd and of tracers PV (Haynes Mcintyre, 1987).Further- 6.4 the in more, change areas couldindicatethat t hemotionis F• (b) 7.0 divergent and (Butchart Remsberg, 1986).It is notpossi- for ble to extend estima.tes area-lossbasedon ti•e previous > O 'E' 6.8 _ PV valuesinto the laterperioda.s define theyno longer z thevortex A edges. consideration the ofwhether dramatic risein the rate of lossof moist are• is due to an accelera.ted to scales, ormovement mixing unresolved convergence off is the of thisisentrop• beyond scope thisletter. > 32 o, Effect on zonal mean fields 7 - ø u• 6.6 27 & t"q " of The movement wet air from polarto lniddlela.t.i- 6.4 • _ , .-" 22 caused theequatorward tudes by transport i•(luced the by 90S 60S 3OS Eq 30N LetRude break-up vortex detectable produces in changesthezonal means, to which we now turn. of Fig. 4. Latitudevs in(O) cross-sectionszonal-mean 4 Figure shows meridional the cross-sections oftiao and H•O mixingratio (ppmv)for a)910923, b)9] 110.1 c) 911125. ratio mixing zonally along for910923, averaged isentropes 1238 Harwoodet al.' M LS Water Vapour 6.5 to 71= 6.8 ma, provideevidenceof strong descentat y References the vortex edgeand limited mixing across it. Butchart, N., and E.E. Remsberg, The area of the strato- for The cross-section 911104 corresponds the firstto spheric for polar vortexas a diagnostic tracer trans- in map in the sequence figure3 and thus shows situa- the surfa, port on an isentropic ce, J. A•mos. Sei., .•$, tion prior to the transport of vortex air to lowerlatitudes. The mid-latitudedry air (<_4.7,5ppmv) at r/ = 6.5 is in J.C.,andL.V. Lyjak,Radiative Oille• and heating cooling stark contrastto the moisthigh latitude air (> 5 ppmv). ratesin the middle J. atmosphere, A•mos.Sci., By 9111215 (figure4c), after the break-upo[ the vortex, 2215-2229, 1986. there is no longera.n/-/20 gra.dient polewardfi'om30øS J.D., Radiative Ha.igh, of heating the lowerstratosphere dry and the previously mid-la,titude regionat t?= 6.5 has and the distribution of ozone in a two-dimensional _> beenfilled by wet high latitude a.irto reachva.lues 4.7,5 model,•uart. J. Roy. Met. Soc,110, 167-185,1984. ppmv. This rapid equatorwardtransport inducedby the Haynes,P.H., and M.E. Mcintyre, On the evolutionof of is direction break-up thevortex •husin theopposite potentia.1 vorticity thepresencealia.ha.tic in of hea.ting the prevailingmeandiabaticdrculation. J. and frictionalor otherforces, Atmos. 828-841, 1987. Conclusions R..L.,et al., The watervapour Jones, of budget the strato- sphere studied usingLIMS and SAMS satellitedata, MLS measurements H20 in spring1991in the south- of Quart. J. Roy. Met. Soc., 112, 1127-1143. 1986. interpolatedonto isentropes ern hemisphere, have shown Juckes,M.N., and M.E. Mcintyre,A high-resolution one- tha• i) significant occursat the centreand edge descent layermodelof brea.king planetary in waves the stra.to- of the southern vortex which moistened its interior be- sphere, Nat'ure,328, 590-596, 1987. tween 650 and 900 K, 5) the tempora.1 spatial evo- and Kelly, K.K., et a.l., Dehydration in the lower Antarc- lutions of the moist areas detected a,t 650 K in Novem- during tic stratosphere lt•tewinterandearlyspring, of ber 1991 correlatewell with differentphases the vortex Res., 9•, 11,317-11,357, 1987, J. Geophys. 1989. erosion and break-upand iii) the break-upof the vortex Mcintyre, M.E., On the Antarctic ozone hole, j. Atmos. at 650 K genera,ted mean equa,torwa, transport,in the a rd Terrest. Phys., 51, 29-43, 1989. opposite directionto the prevailingmeandiaba,tic circula- of a.nd Murphy,D.M., et a,l.,Indicators tra.nsport vertical tion. This moistens the mid-la.titudes in the zonal mean. motion from correlations between in situ measure- Moist air parcels detected outsidethe vortexcorrela.tewith mentsin the AirborneAntarcticOzoneExperiment, air parcels havingthe signature the 'edge'a,ndinterior of d. Geophys.Res., 9d, 11,669-11,685,1989. of the vortex. Someex-vortexair parcelsgenerA,ted by Schoeberl, of M.R., et al., The structure the polarvortex, the erosionof the vortex in November 1991 dilute very J. Geophys. Res., 97, 7859-7882,1992. rapidly (2-3 da,ys) with the surrounding drier air and any R., nd Swinbank, a. A. O'Neill, A stratosphere-troposphere impact on the mid-la. titude zonal-mean 1'120 distribution data simulation system, 28 prej>rint, August1992. is difficult to detect. Others, however,mea.sured around Tuck,A.F., et al., Polarstratosphericcloudprocessed air 35øS,have a muchlongerlifetime (>_ 10 days). These and potentialvorticity in the Northern hemisphere of canbe attributedto an erosion the vortexoccurringin lower at stratosphere mid-latitudes duringwinter,j. earlyspringwherethe PV gradienta.tthe vortexedgeis Geophys. Res., 97, 7883-7904,1992. mixing may greaterthan in late spring,thus horizonta.1 Waters, J.W., Microwave of limb-sounding earth's up- be expectedto be lessefficient. A further effect of the per atmosphere, Atmospheric 23, Research, 391-410, and break-upof the vortex a.s erosion by detected MLS is 1989. a movementof air at 0 -- 650 K which is both H20-rich E.S. Cart, R.S. Harwood, W.A. Lahoz, P.D. Ricaud, and Os-poor(not shown here), overinhabitedareas:over South America,in early November 1991 a. over South nd EdinburghUniversity,Scot- Department of Meteorology, land UK EH9 3JZ. Australia in late November 1991. L. Froidevaux,R.F. Jarnot, W.G. Read, J.W. Waters, Laboratory, Jet Propulsion Institute Tech- California of We Acknowledgements. thank ma.ny who colleagues nology,Pasadena.,CaliforniaUSA 91109. to havecontributed the MLS projectand in particularto C.L. Lau, G.E. Peckham, of R.A. Suttie, Department the H20 measurements'NASA, the UARS projectoffice; Physics, Scotland Heriot-Watt University, UK EH14 4AS. at colleagues JPL, EU, H-WU and RAL; A. O'Neill for ana. meteorological lyses.The work in the UK was funded ReceivedFebruary 1, 1993; by SERCand NER.C,and in the US by NASA. acceptedMarch 1, 1993.
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