RESEARCH BRIEF #2016-01, August 18, 2016

Southern Polar Ozone in MERRA-2 KrzysztofWargan

MERRA-2 provides a good representation of the year-to-yearvariationsandthelong-termchangesintotalozonecolumnoverAntarcticafortheentiredatarecord,beginningin1980.WhenMLSdata are introduced into MERRA-2 in 2004, agreement withindependentdata improvescompared to earlier yearswhen theSBUVobservationswereassimilated.

StratosphericozoneisakeycomponentoftheEarth’ssysteminthatitshieldsthebiospherefromharmfulultravioletradiationandlargely determines the thermal structure of the middleatmosphere.InagreementwiththeresultsofMolinaandRowland(1974),anthropogenicemissionsofchlorinecompoundsinto theatmosphere have led to a decline in global stratospheric ozoneobservedsincethe1980s.Thepresenceofthesesubstancesinthestratosphereisalmostentirelyaresultofpastindustrialemissionsofchlorofluorocarbons(CFCs)beforetheinternationalagreementbanning their production (MontrealProtocolof1986)went intoeffect.Thestratospheric ozone layer is expected to return to itspre-1980valueswithinafewdecades(WMO2014).

BACKGROUND

SUMMARY

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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Acorrectrepresentationofthesouthernmosttotalozone inatmosphericreanalyses isimportantforclimatestudies. Inparticular, ithelpstomonitor long-termtrendsinthestratospheric ozone layer. It is also challenging, as most of historical ozone data arederivedfromsatellitemeasurementsofscatteredsolarradiation,whicharenotavailableduring polar night. NASA’s Modern Era Retrospective Analysis for Research andApplications – 2 (MERRA-2) is the GMAO’s latest atmospheric reanalysis covering theperiodof1980topresent(Bosilovichetal.2015).Thereanalysiscombinesobservationsfromsatellites and conventionaldata, suchas radiosondeandaircraftmeasurements,withsimulationsperformedbyanatmosphericgeneralcirculationmodel.BoththemodelandtheobservingsystemusedinMERRA-2havebeensubstantiallyupgradedsincetheprevious reanalysis,MERRA. Since 1979, the ozonedata inMERRAwere observationsfrom the Solar Backscatter Ultra Violet Radiometers (SBUV) flown on various NOAAsatellites.MERRA-2usesSBUVonlyuntilOctober2004andthenswitchestoobservationsfromtheOzoneMonitoringInstrument(OMI)andstratosphericozoneprofilesfromtheMicrowaveLimbSounder(MLS).Unliketheotherinstruments,MLShasnear-globaldayandnightcoverage.BothOMIandMLSareflownonNASA’sEarthObservingSystemAurasatellite.

AntarcticOzoneHolesinMERRA-2

Themost dramaticmanifestation of the 20th century ozone depletion is the seasonaloccurrence of ‘ozone holes’ over Antarctica. An ozone hole is a large region of theatmosphere,highlydepletedofstratosphericozone.Theamountofatmosphericozonevaries both seasonally and spatially, but the typical values of vertically integratedoverhead concentrations (the total column) oscillate around 290 Dobson units (DU),correspondingto8×1018ozonemoleculespersquarecentimeter.Undertheozoneholeconditions,thetotalcolumnfallstounder220DU,sometimesreachingvaluescloseto100DUwith concentrations in the lower stratospheredroppingdown toalmost zero.Ozoneholesformoverthesouthernhighlatitudesduringtheaustralspringeveryyearsincethe1980s,duetoacomplexsequenceofchemicalreactionsthattakeplaceonthesurfaces of small particles and droplets of polar stratospheric clouds (PSCs), wherebyactivechlorineisreleasedfromitschemicallyinertreservoirspecies.Thechlorineatomsthencatalyticallydestroyozone.Threekeyconditionsfortheozoneholeformationare1)stratospherictemperatureslowenoughforPSCstoform,2)stronglyisolatedairmassover the Antarctic, and 3) the presence of chlorine (and bromine) compounds in thestratosphere.Thefirsttwoconditionsaremetduringsouthernwinterandspringoverthe

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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SouthPole,where strong circumpolar circulation (thepolar vortex) isolates extremelycoldairfromthemiddlelatitudes.Antarcticozoneholesdidnotexistpriorto1980andareexpectedtodisappearwithinafewdecadesoncetheremainingchlorinecompoundshavebeenremovedfromthestratospherebylarge-scalecirculation.ThetopportionofFigure1showsamapofthetotalozonecolumn(TOC)inthesouthernhemisphereaveragedfromMERRA-2ozonefieldsbetween20Septemberand10Octoberof2015withthe220DUozoneholeboundarydepictedasablackcontour.Thebottomportionshowstheozoneholeareaasafunctionoftime.Readilyseenisanupwardtrendinthe1980s,consistentwiththeincreaseofanthropogenicchlorineinthestratosphere.Theaverageareareachesaplateauinthe1990smodulatedbyyear-to-yearvariations.

Figure1.

Topofimage:AmapoftotalozonecolumninthesouthernhemisphereaveragedfromMERRA-2ozonefieldsbetween20Septemberand10October2015.The220DUmark(theboundaryoftheozonehole)isshownasathickblackline.

Bottomofimage:Thetimeseriesoftheozoneholeareabetween1980and2015.Theyear1994isomittedduetoinsufficientdatacoverageinthesouthernhemisphere.

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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A series of total ozone maps showing the MERRA-2 total ozone averaged over thesouthernhemispherebetween20Septemberand10Octoberinyearsbetween1980and2015hasbeencombinedintoananimation:gmao.gsfc.nasa.gov/animations/MERRA-2_SP_ozone.phpFigure2showstheMERRA-2ozoneholesin1980and2000.Duringtheinterveningyears,theaverage sizeofozoneholes increased fromabout1,900,000km2(zero inpreviousdecades) to 20,000,000 km2 and more. Interesting features seen in Figure 1 are thecrescent-shapedpatchesofhighozonevaluesovertheSouthernOcean.Theserepresent

Figure2.

Topofimage:AmapoftotalozonecolumninthesouthernhemisphereaveragedfromMERRA-2ozonefieldsbetween20Septemberand10October1980.The220DUmark(theboundaryoftheozonehole)isshownasathickblackline.Thelowestvaluesareshowninyellow.

Bottomofimage:Thesameasabovebutforthe2000ozonehole.

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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whatwouldbetheseasonalozonemaximainthesouthernhemispherehadtheynotbeenerodedbytheextremedepletionwithintheozoneholes.It isevidentfromthetimeseriesshowninFigure1,thatthesizeofozoneholesvariessignificantly from year to year. These variations, which are superimposed on decadaltrends, are caused by interannual variability of stratospheric dynamics: cold (warm)stratosphericconditionsleadtostronger(weaker)ozonedepletion.Forexample,theverycold, austral spring of 2015 with a strong, relatively undisturbed stratospheric polarvortexproducedanozoneholeexceptionallylargeinspatialextent(Figure1).Bycontrast,lessozonedepletionisobservedinyearswhenthepolarvortexisweak.Asasidenote,thisisthereasonwhyozoneholesgenerallydonotformovertheArcticwherethespringtimestratosphereiswarmeranddynamicallymorevariablecomparedtotheAntarctic.Anextremeexample isaveryunusual, suddenstratosphericwarmingeventthat occurred in late September 2002, leading to an early disintegration of the polarvortex and a breakup of the ozone hole. The average ozone hole area between 20Septemberand10October2002wasonlyabout2,600,000km2(Figure3)despitelargeconcentrationsofchlorinecompoundsinthestratosphere.

Figure3.

AsinFigure2butforthe2002ozonehole(20September–10Octoberaverage).Theextremelysmallozoneholeinthatyearwasaconsequenceofamajorsuddenstratosphericwarmingevent–theonlyoneeverobservedinthesouthernhemisphere.

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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The representation of ozone holes and their interannual variability in MERRA-2 is inagreementwithindependentanalyses,e.g.ozonewatch.gsfc.nasa.gov.

AgreementwithSouthPoleOzonesondes

Figure4comparestheTOCovertheSouthPolefromMERRAandMERRA-2withdataderivedfromozonesondeobservations.Sinceozonesondemeasurementsarenotusedineitherreanalysis,theyprovideanindependentdatasetforassessingthequalityoftheGMAOanalysis.BothMERRAandMERRA-2reproducearealisticannualcycleoftheSouthPoleozoneconsistentwiththeozonesondedata.

Theozonesonde-reanalysisstatistics(Table1)showthatMERRA-2isincloseragreementwiththeozonesondesthanMERRAthroughouttheperiodofcomparison(1991-2014)andthatitperformssignificantlybetterfromtheyear2004onwards,whenitassimilatesstratosphericozoneobservationsfromthe

Figure4.Toppanel:TimeseriesofSouthPoleozonederivedfromozonesondes(black),MERRA(red)andMERRA-2(green).Bottompanel:thedifferences:MERRAminusozonesondes(red)andMERRA-2minusozonesondes(green).TheSouthPoleozonesondedatawereobtainedthroughtheNOAAEarthSystemResearchLaboratoryGlobalMonitoringDivision(www.esrl.noaa.gov/gmd/ozwv/ozsondes/spo.html)

GMAO RESEARCH BRIEF Southern Polar Ozone in MERRA-2

Global Modeling and Assimilation Office NASA Goddard Space Flight Center

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MicrowaveLimbSounderwithnear-globalcoverageduringbothdayandnight.ThisisalsoseeninthestatisticsaccumulatedinTable1.Inparticular,thestandarddeviationofthedifferencesbetweenMERRA-2andozonesondesdropfrom28.23DUto11.1DUbetweentheperiodsofSBUVandMLSassimilation.However,weemphasizethatalsointheSBUVassimilationperiod(before2004)thecorrelationbetweenMERRA-2andthetotalozonederivedfromthesondesishigh(0.88),indicativeoftheabilityofthereanalysistocapturethevariabilityofthetotalozonecolumnovertheSouthPole.

1991-2004 2005-2014

Analysis-sondedifference 14.03DU-6.72DU

26.56DU-6.77

Analysis-sondeCorrelation 0.870.88

0.800.98

StandarddeviationoftheAnalysis-sondedifferences

30.19DU28.23DU

36.0011.10

Table1.StatisticsoftheSouthPoleOzonesonde–ReanalysisComparisonsforMERRA(red)andMERRA-2(green)

Insummary,MERRA-2providesarealisticrepresentationofthetotalozonecolumninthesouthern polar region throughout the entire period of reanalysis, including theinterannual variability of the ozone holes in agreement with established science.Agreementwithindependentdataimprovesduringtheperiodof2004topresentwhenozoneobservationsfromtheMLSinstrumentareassimilatedinthereanalysis.

References

BosilovichM.andco-authors(2015):MERRA-2:InitialEvaluationoftheClimate.TechnicalReportSeriesonGlobalModelingandDataAssimilation,NASA/TM–2015-104606/Vol.43.Molina,M.J.,&Rowland,F.S.(1974):StratosphericSinkforChlorofluoromethanes:ChlorineAtom-CatalysedDestructionofOzone.Nature,249(28),810-812.WorldMeteorologicalOrganization(WMO2014):ScientificAssessmentofOzoneDepletion:2014.GlobalOzoneResearchandMonitoringProject–Reportno.55.