OF TRACE GASES. IN THE MIDDLE. ATMOSPHERE. L.J. Gray and M.P. Chipperfield. Rutherford Appleton Laboratory. â¢. A parameterization of the mechanisms ...
GEOPttYSICAL RESEARCH LETTERS, VOL. 17,
ON THE
NO. 7,
PAGES 933-936,
INTERANNUAL VARIABILITY OF TRACE IN THE MIDDLE ATMOSPHERE
JUNE 1990
GASES
L.J. Gray and M.P. Chipperfield Rutherford Appleton Laboratory
•.
A parameterization of the mechanisms be-
Eeyedto be responsiblefor the quasibiennialoscillation
(QBO)in thezonalwindsoftheequatorial lowerstratosphere has_been addedto a fully interactivedynamical
radiativephotochemicaltwo dimensionalmodel of the atmosphere.In addition to a well definedQBO in the zonalwinds,temperatureand columnozone,the model e•bits a strongQBO signalin many of the othermodegedtrace gases.Large year to year variations,in some cas• up to 70 %,axepredicted.The importanceof taking thispredictioninto accountwhen measuringand inter-
ozone.The purposeof thispaperis to examinethe effects of the dyna.mlcal equatorialQBO on the distributionof a sampleof the widerangeof othertracegasesthat axe included in the model.
The QBO in Trace Gases Mechanisms
preflngthe distributions of thesegasesis emphasized. Two distinct mechanisms exist which may result in a
QBO signalin n-dddleatmospheretrace gases:(1) in the caseof trace gasesthat have a long photochemical
Introduction
A quasibiennialoscillation(QBO) in the zonalwinds andtemperaturesof the lower stratospherehas been observedfor nearly 40 years and is well documented(see GrayandPy!e1989,and references therein). The oscillationconsistsof alternating easterliesand westerlieswith a period of approximately 28 months. The maximum amplitudeof about 20 ms-• occursat 25 - 30 kin. An accompanying temperature osdllation, which is in thereal windbalanceand hasan amplitudeof approximately2 K, is .alsopresent;the positivetemperatureanomalyoccurs i.uzt•ow the level of maximin westerliesand the negafivetemperatureanomalyoccursbelowthe easterlywind maxi ß .mum.The amplitude of both wind and temperature •ations is a maximum at the equator and dropsoff
r•idly with latitude. There is evidenceof a changein phase in thesubtropics resultingin a mid-latitudeoscillatianof the oppositesign. A QBO hasalsobeenobserved in .measurementsof column ozone at all latitudes.
In a recentstudy, Gray and Pyle (1989), hereafter r•erredto as GP89,includeda parameterization of the processes believedto give rise to the QBO in equatorial •nds in a two dimensional model that extended from
lifetime in the lower stratosphere, and can therefore be consideredto be 'passive' tracers in this region, an inducedmeridiona/circulation at the equator axSsingas a result of the QBO in zonal winds would result in a modification of the normal pattern of advection of those
gasesby the mean drculation; (2) in the caseof trace gasesthat have short photochemicallifetimes and whose distributionsare temperature dependent,a QBO signal may result from the presenceof the temperature QBO in
the lowerstratosphere and/or a QBO in the distribution of their sourcegases. The lifetime of ozone in the lower stratosphereis of 'theorder of months to years and it can therefore be considereda long-lived gas below about 25 kin. A detailed descriptionof the modelled QBO in colum• mounts of ozonewas providedin GP89; a brief descriptionwill be givenhere, as an illustration of the mechanismsinvolved. Figure 1 showsthe time-seriesof column ozone anomaly in the model; the appropriate monthly mean column ozoneaveragedover the wholemodel run has been subtractedfrom eachdata point to obtain the anomaly.
Notethat thisfigureis notidenticalto figure7 in GP89 ue to the updatingof someof the chemicalreactionrates
.,'•e to pole and from the groundto approximately in the model,whichhave affectedthe depletionrates of 90 km. The model calculates zonal mean values oœ ozoneat high latitudes). A QBO signalis presentat lemperatur%wind componentsand chemicalconstituent
all latitudes;the equatorialamplitudeof 3 DU and the phasechangeat approximately 15ø latitude are in good
?•xingratioswith a resolutionof •r/19in latitude, 0.5
Lu(•/r) in the vertical(wherep is pressure and p0is
[he. surface value)and a 4 hourtimestep (Harwood and
Fyie1975,1977,1980,Gray and Pyle 1986,1987,1989). The QBO parameterizationwas basedon a mechanism inmlvlngthe transferof momentumto the zonalflow
agreementwith observations.
The presenceof the equatorialozoneQBO is believed to be a direct effect of the meridional circulations that
as•aied with verticallypropagating equatorialwaves
are inducedin the equatorialregionsas a result of the maintenance of the thermalwind balance(Hasebe !983. 1984, Ling and London 1986, Gray and Pyle
i r,ea•nab!esimulation of the QBO in zonalwinds,
QBO a circulation is induced with downward motion
phaseof the (•ndzen andHolton, 1968,Høl[on andLindzen 1972). !989i. Duringa westerlyacceleration •emperaturesand column amounts of ozone resulted from
at the equatorand rising motionin the subtropicsof t.heinclusion of theseequatorial dynamical effects in both hemispheres (seefigure2). A similarcirculation the .•.el. GP89provided a .detailed analysis of the of the oppositesenseis inducedduring an easterly :.• :.'•ed QBOin the dynamical ilddsandin column accelerationphase. This inducedmeridionalcirculation modulatesthe strengthof the Hadleycirculationin the
lowerstratosphere. Duringa westerly acceleration phase
Co-pyright 1990by the AmericanGeophysicalUnion.
the strength of the equatorial ascent is reduced and
duringan easterlyphaseit is increased.The mixing ratio of equatorialozoneincreases with height up to approximately 32 km at the equator.Hence,duringan
'number89GL03738
'•:"94-8276/90/89GL-03738503 . 00 933
Grayand Chipperfield: InterannualVariabilityof TraceGases
934
present at theequatorduringthewesterly phases of the' oscillation andthenegativeanomalies arepresentduring the easterlyphases,in goodagreementwith the theory outlinedabove,andalsowith observations (Hasebe1983, 1984,Lairet al. 1989).Finally,thereis someasymmetry
in the amplitudeand timing of the maximum anomalies in the sub tropics of the two hemispheres,in good agreementwith observations.This aspectis beyond the scopeof this paper and is discussed more fully by Gray and Dunkerton(1990). l•ong-livedGases
25
4
5
6
7
8
9
T•me (Years)
Fig. 1. Latitudetime-series of the modelled column ozoneanomaly (Dobson Units).Contour interval2 DU. Dashedlinesdenotenegative values.The direction of the equatorialzonalwindsat approximately •0 mb is
indicated at the top of the figure. WESTERLY
ACCELERATION
PHASE
Height
Figure 3 showsthe percentageanomaly in N20 from the model run at 22 km as a percentageof the N20 at that level (the appropriatemonthly mean obtained by averagingover the whole model run was subtracted from each data point in order to obtain the anomaly and this was then divided by the monthly mean to obtain the percentageanomaly). Both N• 0 and Ctt• (not shown)showpeak •o peak variationsof up to 1015 %, with a phase changeat about 15 degreeslatitude. The morphologies of the quasibiennialoscillationsin N•O and CH4 are extremely similar as a result of their similar distributions in the atmosphere- both have sourcesat the surface and sinks in the s%ratosphere resulting in a distribution that decreaseswith height throughout the stratosphere. For example, both display a positive
equatorialanomalyin years2/3 followedby a negative anomalyin years3/4 with the switchover occurringat
( Ozone maximum
around June in each year. This is in the oppositesenseto
the anomalyin columnozone,which displaysa negative anomalyin years 2/3 and a positiveanomalyin years 3/4. This contrastingbehaviourbetweenozoneon the
,--,25 km --
one hand and N•O and CH4 on the other hand results
9OS
cold
warm
--re
+ Ve
from the differencesin vertical gradient of the mixing
cold
ratios.
i
!
i
i
1
60
30
O
30
60
90N
Latitude
Fig. 2. A schematicdiagram showingthe induced meridional circulation associatedwith a westerlyphase of the QBO. The positiveand negativesignsindicatethe expectedcolumnozoneanomaly.
Figures4 and 5 showthe correspondingtime-seriesof the anomalyin NO, (=NO + NO2 + 2 x N 20• + HNO3 + HO•NO• + C1ONO•) and C10• (= C1 + C10 + HC1 + HOC1+ C1ONO•) expressed as a percentageafter the monthly mean values have been subtracted. Both have
easterlyphasethe enhancedequatorialupwellingleadsto a reductionin the mixingratiosin the lowerstratosphere and thus to a reduction in the column amount of ozone
at the equator. Conversely, duringa westerlyphasethe constrained equatorialupwellingleadsto an increasein the equatorialcolumnof ozone.A similarQBO in ozone is presentin the subtropics,but of the oppositesign, as a result of the return
arms of the induced circulations.
Figure2 shows a schematic representation ofthisinduced circulationand its effectson the temperatureand column
ozone distributions.
The subtropicalQBO anomalyin columnozonein the
model(figure1) extendsfurtherpolewardthanexpected from the (linear) theoryoutlinedabove,in whichthe latitudinal
extent of the induced circulations is restricted
Time (years)
to the subtropics.This resultis in goodagreementwith observations,and is causedby a combinationof transfer windswere presentover the equator at approximately50
Fig. 3. Latitude time-seriesof the modelledQBO anomalyin N20 mixing ratio at 22 km expressed as a percentageof the ambient N30 at that level.
mb. Note tha• the positive column ozone anomalies are
Contour interval
by eddyandmeanmotions(GP89). Alsoshownin figure 1 are the periodsduring which easterlyand westerly
2 %.
Gray and Chipperfield:InterannualVariabilityof TraceGases
9N(•
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W
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935
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o','------", [',LL'" ' ....
':: '...... 'i"'"'! '' •': V.L.•-...., i / ' ..... "•'" -
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f,-.
•0
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',, •
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T,rne(years)
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Fig.4. Asfigure3 but for total oddnitrogen(NO +
Fig. 6. Latitudetime-series of the modelledQBO
NO• + HNO3 + 2 x N205 + HanNa2 + C1ONO2).
ar,omaly in temperature in degrees!•elvin. Contour
Contour interval 3%.
interval
W
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•
W
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W
9
TIME (¾•ARS)
I
E
is 1K.
I _•E
6,6""
,.-.,
,,,!
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';?':'
....:"""i"
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......
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r•me (
•:'g. •. Asfigure3 bu• for totM •ctive chlorine(C! + cJ]O+ •C1 + •OC1 + C1ONO=),Contour
4 5 T•rne(yeors)
6
7
8
Fig. 7. As figure 3 but for NO•. Contourinterwal
presentin the equatoriallower stratosphere.Figure 6
dLstfibutions thatincrease withheightin theequatorial showsthe time-seriesof the modelledtemperatureartore-
lowers•ratosphere andhencethe anomalies arecorrelated ahth•he columnozone anomaliesbut are anti-correlated
•hththe N•O and C!t• anomalies.The peakto peak •'•ations at the equator,of the orderof 20-30%,a• muchlargerthanfor N•O. The amplitudeof the QBOinduceq a.nomalies depends upontheverticalgradient of
thespecies; a steepgradient in theverticalwillresultix•.
large anomalies. Thisexplains themuch largersignal in NO•,forexample, whichhasa steeper gradient in the
vertical thanN20'•Notealso,thepotentialfor a larger •cent.•e changein rrdd-!atitudesthan at the equator
if thetracegashasa largervertical gradient in •dlatitn.des thanat theequator.
My (in degrees Kelvin)at approximately 22 km (the appropriatemonthlymeanobtainedby averaging overthe whole model run has been subtracted from each data
point to obtain the anomaly). Also indicated on the figureare the directionsof the equatorialwindsat approximately50 rob. As expected,the positivetemperature anomalies occur around the time of the maximum
westerliesand the negativetemperatureanomaliesoccur
aroundthe time of the maximumeasterlies.The amplitudeof the modelledanomalyis approximately 3 K, in goodagreement with the predictedanomalyif thermal wind balance is assumed.
Asa resultof the QBOsignalin thelonglivedspecies
and temperaturein the model, a number of shorter lived trace spedesalso displaya distinct inter-annuM
!a addition to a QBOsignal in equatorial zonal•nds aaderinran ozone, a well-observed temperature QBOis
variability. In several cases thisvariability is largerthan for thelonglivedspecies.In thelowerstratosphere the QBOsignalof NO, (=NO + NOa)closely followsthat
9
GrayandChipperfield' Int•erannual Variabilityof TraceGases
936
from oneyear only,or data whichis biasedtowards or other of the phasesof the QBO. Note also,that modelledQBO describedin this paperhasa period is almostexactlytwo years.This resultsih the anomalyin eachphaseof the QBO occurringat •he time of year. In reality,the QBO is not as r• this, and oneshouldthereforeexpecta QBO sign• is lessregularandpredictable thanthat displayed by
9O
modelled fields.
Acknowledyements. Thanksare dueto Drs. JohnPyte,
Susan Solomonand Paul Dickinsonfor usefuldiscussinn
and suggestions. This work wassupportedby the U.K. Scienceand EngineeringResearchCouncil. -6O
References -9C
Fig. 8. Asfigure3 butforN205.Contour interval 8%.
Gray, L.J., and T.J. Dunkerton,The roleof the se .• cycle in the quasi-biennialoscillationof ozone,J. Atmos.Sci.,(in press),1990. Gray, L.J., and J.A. Pyle, The semi-annualoscinati,,n and equatorialtracer distributions. Quart. J. Roy. Met. Soc.• 112,387-407, 1986.
of NO• shownin figure4. t!oweverthe QBO signalsof the NO, family membersare very different,and result from the effectof the temperatureQBO on the NO:NO2
Gray,L.J., and J.A. Pyle, Two dimensional modelstu•es of equatorialdyna;_.•cs andtracerdistributions. Quart. J. Roy. Met. Sot., 113,635-651, 1987. Gray, L.J., and J.A. Pyle, A two dimensional • of the quasi-biennialoscillationof ozone,J. Atmos.Sci.,
partitioning. Figure ? showsthe time seriesof the 4a, 203-220, 1989. modelledQBO in NO•. (Note that the rapid changes Harwood, R.S., and J.A. Pyle, A two-dimensional • at highlatitudesin thisplotandin figure8 are spurious, circulationmodel for the atmospherebelow80kin.Q. andshouldbeignored.Theyariseasa resultof the strong J. Roy. Met. Soc., •01, 723-748, 1975. seasonal variationof NO2 and N20, at highlatitudesand Harwood, R.S., and J.A. Pyle, Studiesof the omme the averaging procedure adopted.This is confirmed by budgetusinga zonal mean circulationmodel•d their absencein the corresponding plot for total NO, in linearizedphotochemistry. Quart. J. Roy. Met. Sec., themodal,shownin figure4). Thepeakto peakvariation 103,319-343, 1977. in NO2 is of the orderof 40%comparedto that for NO, beharore not shown)of about 26%. The dominanttemperature Harwood,R.S., andJ.A. Pyle,The dynamical of a two dimensionalmodelof the stratosphere.Q:.uar• • ependence in the expression for the NO:NO2ratio comes J. Roy. Met. Soc., 100,395-420,1980. from the activationenergyfor the reactionNO + Oa Hasebe,F., Interannualvariationsof globalozonere-, NO• + 02. For this reactionthe rate constant(k) yealedfrom Nimbus4 BUV and ground-based obseris givenby k=2.0x10-•exp(-1400/T)molecules-•cmas -• vations. J. Geephys. Res., 88, 6819-6834, 1983. (NASA/JPL1987).A positivetemperature anomalyat Hasebe,F., The globalstructureof the total the equator,associated with the westerlyphaseof the fluctuations observed on the time scalesof two to QBO, resultsin the NO:NO2partitioningbeingshifted several years.In Dynamics of theMiddleAtmasp•e•, in fayour of NO2. Thus the QBO signalin temperature 445-464,eds.J.R. HoltonandT. Matsuno, pb.Terra increasesthe QBO signalof NO• rdative to that of ScientificPublishing Company•1984. NO. anddecreases the signalfor NO. The corresponding Holton,J.R.,andR.S.Lindzen, An updated theory • peak to peak variationin NO (not shown)is of the order of 10%. The time seriesof the modelled QBO in
N•O5 is shownin figure8. Thereis a very largepeak to peak variationof the orderof 70%. This is much largerthanthesignalfor NO: andresults fromtheeffect of the temperature QBO via the strong temperature
dependance of the reactionNO2 + Oa --, NOs + O:.
Forthisreaction k=l.4x10-mexp(-2500/T) (NASA/JPL
thequasi-biennial cycle ofthetropical stratosphere. Atmos. Sci., 29, 1076-1080,1972.
Lait,L.R.,M.R. Schoeberl andP.A.Newman, Quadbiennialmodulationof the Antarcticozortedeptetica, J. Geophys.Res.,94,11559-11572,1989.
Lindzen, R.S.,andJ.R.Holton, A theory ofthequadbiennialoscillation. J. Atmos.Sci.,$,0.0, 513-515, 1•.
Ling,X. -D.,andJ.London, Thequasi biennial os of ozonein the tropicalmiddlestratosphere: a change of 6K canaccount for the increased QBO signal dimensional model,J. Atmos. Sci.,4_!3, 3122-313.'7, of N•O5 relative to NO2. The modelled QBO in trace 1987). With this largeactivationenergya temperature species will be discussed furtherin a moredetailedpaper currently in preparation. Discussion
Suchlargepredictedyearto year variationsin ozone, CIO•, NO,,, NO•, N•O• and o[her trace gasesare
important f•ctors tobetakenintoaccount, bothbythose who seek to make accurate measurementsof the gases
and thosewhointerpretsuchdata using,perhaps,data
1986.
NASA/JPL Chemical kinetics andphotochemical da• forusein stratospheric modelling, Evaluation No. JPL publication87-41, 1987.
L.J.Grayand" M.P.Chipperfield, Rutherford Appl•'0•
Laboratory, Chilton, Didcot, Oxon, OXll0QX,•.K. (ReceivedJune12, 1989.;
Accepted December 14,1989.)
