Long-lived tracer transport in the Antarctic stratosphere

Recent observationsmade by the CryogenicLimb Array Etalon Spectrometer (CLAES) on the Upper AtmosphereResearchSatellite(UARS) indicatethat duringthe australfall, CH 4 zonal mean isoplethsin the Antarctic vortex appear to descendmore rapidly than thoseof N20. How is this possiblein an isolatedregion suchas the vortex when photochemicalsinksare insignificant?To understandthese observations,we have run a simulationof the 1992 australfall usingthe Goddard Global SpectralMechanistic Model (GSMM) and the three-dimensional Chemistryand TransportModel (CTM). Model tracer fieldsshowgood agreementwith the observationsover a 4-month period beginningin mid-February.Both the observationsand the simulationshowthat the apparentdifferentialdescentoccursduring periodsof wave activity,while during quiet vortexperiodsN20 and CH 4 descendat approximatelythe samerate. To understandhow wave activitymay drive the observeddifferencesbetweenN20 and CH 4 behavior,we calculatethe terms of the zonal mean tracer tendencyequationin the transformed EulerianMean (TEM) formulationof Andrewset al. [1987].Analysisof the termsof the tracer tendencyequation showsthat each tracer's responseto eddy forcing is the sourceof the apparentdifferentialdescent.While descentis the dominantmotion insidethe vortex, horizontalmixingbecomessignificantduringperiodsof wave activityand differencesin the tracers'meridionalgradientsaffectthe relative amountsof each tracer transported into the vortex.This analysisdemonstratesthe relationshipbetweenwave activity,eddy transport,and tracer mixingratiosinsidethe vortex throughoutthe fall. In addition, CLAES observations deep in the vortex (70ø-80øS)showgraduallyincreasingCH 4 mixing ratios from March to September,implyingthe importanceof eddy-drivenmixingwithin the vortex in winter.