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Chapman Conference on the NAO, Spain, 28� �

November - 1 ��December

Variability of TotalOzonedueto theNAO asRepresentedin two DifferentModelSimulations

P. Braesicke,P. Hadjinicolaou,A. JrrarandJ.PyleCentre for Atmospheric Science, Cambridge University, UK

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ModelDescriptionsandExperimentalDesignAbstract:In this study two model systemsare assessedon how they representtotalozonevariability dueto theinfluenceof theNorthAtlantic Oscillation(NAO).Themain focuswill beon thenorthernhemispherewinter andespeciallyonJanuary. A brief comparisonwith TOMS datawill alsobeincluded.One model is the chemistry-transportmodel (CTM) SLIMCAT, which isdrivenby meteorologicalobservationsandthereforeincludesmeteorologicaltrendsin the atmosphericcirculation. The otheris the Unified Model (UM)of theUK Met. Office, which is a comprehensive generalcirculationmodelforcedby prescribedsea-surfacetemperatures(SSTs)only. In bothmodelsasimplifiedozonechemistry(CariolleandDéqúe,1986)is usedwith a param-eterizationof polarozoneloss,but with no allowancefor changingchlorineor aerosollevelson thechemistry.Suitableindicesof the NAO will be usedin the diagnosisof the two modelsystemsandthecorrelationbetweentheseindicesandthe total ozoneasde-rived by the modelswill be compared.Compositeozonemapswill be con-structedfor highandlow index phasesandwill bealsocomparedwith TOMSobservations.

Unified Model:horizontalresolution96 � 73 grid points (3.75� in longitude

and2.5� in latitude)verticalresolution 58 levels (L58); z=1.3km, modeltop

in 0.1hPa (65 km)timestep 15minutes(900s)tracertransport Roe flux redistribution method with

choiceof limiters;Superbeeor VanLeer(CullenandBarnes,1997)

gravity wavedrag basedonPalmeretal. 1986radiation 2-streamradiationcode;6 bandsSW, 8

bandsLW; every 3 hours(EdwardsandSlingo,1996)

ozone Cariolle parameterizationwith an addi-tional ”cold tracer”(passive/interactive)

ocean climatologicalSSTs(AMIP II, monthlymeandata)

SLIMCA T:horizontalresolutionT21, 64 � 32 grid points (5.6� in longi-

tudeand5.6� in latitude)verticalresolution 11 isentropiclevels (L11); z=2.5 km,

model top in 1030 K (8 hPa, 32 km),modelbottomin 345K (250hPa,10km)

timestep 1 hour(3600s)tracertransport Second-ordermomentsadvectiondriven

by ECMWF analysis (Prather, 1986;Gibson,1997)

radiation calculationof heatingratesfor theverti-cal transport;2 dummylevelsabove andbelow modeldomain;usesclimatologi-cal ozone(Shine,1987;ShineandRick-aby, 1989)

ozone Cariolle parameterizationwith an addi-tional ”cold tracer”

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TheNAO Indicesin ComparisonThe plots in this box show theNAO indices for December, Jan-uary and February for the UM(left), SLIMCAT (middle) andfrom stationdata(right, afterHur-rell, 1995). The UM, only drivenby prescribedAMIP II SSTs,dis-playsmany differencescomparedto theothertwo time series.How-ever, thereis goodagreementforselectedyears(e.g.1984).

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1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

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Time [Years]

NAO Indices for the Unified Model

DecemberJanuary

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1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

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Time [Years]

NAO Indices for SLIMCAT (ECMWF)

DecemberJanuary

February -4

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1978 1980 1982 1984 1986 1988 1990 1992 1994 1996

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Time [Years]

NAO Indices Derived from Observations

DecemberJanuary

February

Thegridpointdatausedfor SLIM-CAT (ECMWF, L31) shows goodagreement to the station data,but does not capturethe largestamplitudes. This is expected,becausethe nearestgridpoint isnotonly determinedby thenearestmeteorologicalstationandis alsoinfluencedby the model systemusedfor theassimilation.

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CorrelationsbetweenNAO IndicesandTotalOzoneThis box shows a comparisonbetweentheUM(left) andSLIMCAT (rght) correlationsfor theNAO index with total ozoneasderived by themodelsfor JanuaryandFebruary. Focussingonthe European/Atlanticsectionin January, bothmodels show similar patterns: High positivecorrelationsoverGreenlandandabandof nega-tivecorrelationsoverEurope,with positivecor-relationstowards the southeast. The positivecorrelationsover Greenlandarestrongerin theUM.

In Februarythe correlationover Greenlandisstronger in SLIMCAT comparedto the UMresultsbut thepatternsareagain similar in bothmodel systems. On the hemisphericscalethecomparisonis not that straightforward - awayfrom theEuropean/Atlanticsectormany differ-encescan be seen,e.g. the smallereastwardextend of the band with positive correlationsfrom Africa to Asia in Januaryin SLIMCAT.

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CorrelationsbetweenNAO IndicesandGeopotentialHeights

CorrelationsbetweengeopotentialheightsandtheNAO index in Januarywithin theUM arediscussedfor threedifferentpressurelevelsin thisbox: In 850hPaalargeareaof negativecorrelationsis foundin polar latitudes,centredover IcelandandGreenland.Themaximumpositive correlationsareasexpectedover SpainandNorth Africa - in accordancewith thedefinitionof theNAO index. In 200hPathepatternof thecorrelationresemblesthepatternseenin theozone(with oppositesign). This implies that theamountof total ozoneover Europeis mainly controlledby themodulationof tropopauseheightsdueto theNAO. In 30 hPa thecorrelationis not ashigh asin lower levels,anda dipolestructureemerges.This maynot only imply a strengthening/weakeningof thevortex, but alsoa twisting oftheAleutianHigh/Vortex system.Similar resultscanbeobtainedfor theSLIMCAT/ECMWF modelsystem(exceptfor 30hPa,wereamorezonalsymmetricpatternappears).

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Compositesof OzoneAnomaliesBy stratifying the monthly meantotal ozonedataaccordingto theNAO indices and taking differ-encesbetweenlow and high in-dex cases,the compositeson theright were derived. The generalshapeof thepatternis quitesimilarbetweenmodels(UM, SLIMCAT)andobservations(TOMS). SLIM-CAT producesthe largest ampli-tudes and the strongestgradientaround55� N.

The UM has a weaker gradientin this region, which is closer toobservations. It seemsto un-derestimatethe negative anomalyover Europe. Also a small dis-placementto thenorth is obvious.The elongated positive anomalyover the Atlantic in the SLIM-CAT runis muchlargerthanin theUM experiment. It seemslikelythat SLIMCAT overestimatesthisanomaly.

Usingonly a simplestratificationmaystill leaveothersignals(e.g.a lineartrend)in thedata(SLIMCAT andTOMS) - furtherwork hasto bedone!

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SummaryandConclusions� Bothmodelsshow asimilar responsein totalozonedueto their

internalNAO.

� Theresponseis mainlycontrolledin bothmodelsystemsby thetropopauseheights/topography of the200hPasurface.

� Firstcomparisonswith TOMStotalozonedatashow agoodagree-mentbetweenmodels(UM andSLIMCAT) andobservations.

Furtherwork: How doestheNAO variability affect thetrendsin total ozone?