Dynamical Perturbations to the Ozone LayerMurry L. Salby and Rolando R. Garcia Citation: Phys. Today 43(3), 38 (1990); doi: 10.1063/1.881228 View online: http://dx.doi.org/10.1063/1.881228 View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v43/i3 Published by the AIP Publishing LLC. Additional resources for Physics TodayHomepage: http://www.physicstoday.org/ Information: http://www.physicstoday.org/about_us Daily Edition: http://www.physicstoday.org/daily_edition

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DYNAMICAL PERTURBATIONSTO THE OZONE LAYER

Global-scale disturbancesin the stratosphere produce

substantial changes in the abundanceand distribution of ozone and

prevent the formation overthe North Pole of an ozone hole

like that observed over Antarctica.

Murry L. So I byand Rolando R. Garcia

Murry Salby is an associate professor in the department ofastrophysical, planetary and atmospheric sciences at theUniversity of Colorado at Boulder. Rolando Garcia is a

scientist at the National Center for Atmospheric Research, inBoulder. Both are members of the Center for Atmospheric

Theory and Analysis at the University of Colorado.

Several years ago spectrophotometer measurements re-vealed substantial depletions of atmospheric ozone overAntarctica, reviving concerns over damage to Earth'sozone layer by human activities.1 These ground-basedobservations were subsequently verified in global ozonedata that had been collected by satellite for years.2 The si-zable ozone depletions over Antarctica are now believed toarise from complex chlorine chemistry, stimulated ulti-mately by the emission of man-made chlorofluorocarbonsinto the atmosphere.3

Ozone occurs naturally in the stratosphere, that layerof the atmosphere extending from about 10 km to 50 kmabove the surface of the Earth. During Antarctic spring,as much as half of the ozone over the South Poledisappears, with a nearly 100% loss in the layer between10 and 20 km, where ozone is normally concentrated atthese latitudes. Initially, several explanations were pro-posed for these dramatic reductions. However, recentexpeditions to the Arctic and Antarctic have pointed toheterogeneous chemistry as the mechanism responsible.3(See PHYSICS TODAY, July 1988, page 17.) Inside polarstratospheric clouds the presence of both solid and gasphases permits certain chemical reactions to proceedmuch faster than is possible in the gas phase. For this rea-son, much of the recent interest in ozone has focused onchemical issues. However, to assess man-made changes todate and to understand how the ozone layer may respondto future perturbations of human origin requires anunderstanding of how the ozone layer functions and theprocesses that contribute to its natural variability. Inparticular, anthropogenic variations must be consideredin the context of naturally occurring changes in ozone.

An important source of natural variability in ozone isa class of disturbances known as planetary waves, whichperturb the circulation of air in the stratosphere. (Seefigure 1.) These waves, which are possible in a rotatingfluid, occur routinely in the northern winter stratosphere,where they disrupt the circumpolar flow and exert aprofound influence on ozone and other chemical species.It now appears that these global-scale disturbuances alsoplay a key role in shaping ozone depletions at the poles.

This article presents an overview of these dynamicaldisturbances, focusing on how they alter the globaldistribution of ozone through horizontal and verticaltransport. To set the stage, we review the basic climatolo-gy of ozone: its origin, global distribution and seasonal

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Motion of polar air (blue) and tropical air (orange) after adisturbance to the circulation of the stratosphere, as given bya numerical calculation. Colors indicate values of potentialvorticity, which is conserved for individual bodies of air andtherefore traces out their motion (see figure 3). The colorcode ranges from orange, which indicates small values ofpotential vorticity and weak absolute angular momentum, todeep blue, which indicates large values of potential vorticityand strong absolute angular momentum. The sequenceshows tropical air being drawn poleward in the form of atongue of low potential vorticity. The folding of air contours,which develops in the first panel, is dynamically unstable andleads to the formation of a large anticyclone along theperiphery of the vortex. This planetary-scale eddy drawsmidlatitude air from the vortex and entrains it with tropicalair, stirring bodies of air down to small dimensions. Air in theresulting anticyclone has characteristics similar to air found atmuch lower latitudes in the undisturbedcirculation. Figure 1

variation. This is followed by a description of the essentialmechanics of planetary waves. We then use detailedtransport calculations to investigate how these distur-bances can influence the abundance and distribution ofozone.

Climatology of stratospheric ozoneFor some time ozone has been recognized as essential tothe existence of life at the Earth's surface.4 In addition toits biological importance, ozone plays a central role in theenergetics and dynamics of the upper atmosphere. Theabsorption of ultraviolet solar radiation by ozone is a heatsource for the tropical stratosphere, ultimately driving theglobal circulation of the upper atmosphere.5

Ozone, O3> is produced in the stratosphere by a seriesof reactions that begin with the photodissociation ofdiatomic oxygen. Ozone can be destroyed by recombina-tion with atomic oxygen and by a variety of catalytic cyclesinvolving hydrogen, nitrogen and chlorine. In the absenceof atmospheric motion, the distribution of ozone would bedetermined by a balance between the chemical processesthat create ozone and those that destroy it. On purely

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chemical grounds, one would anticipate ozone to be mostabundant in the tropics, where the photodissociation of O2is efficient. However, observations of stratospheric ozonereveal a very different picture.

The ozone abundance at any location over the Earth'ssurface is measured by the "total ozone column," which isthe integral over the depth of the atmosphere of theamount of ozone per unit volume, or ozone number densityP03 • The total ozone column is often expressed as thedepth that the ozone alone would occupy at standardtemperature and pressure; this depth is often measured inthousandths of a centimeter, or Dobson units, DU. Atmiddle and high latitudes, the ozone column is concentrat-ed between 10 and 20 km, where the ozone number densityincreases sharply with altitude before decreasing expon-entially with the air density. In the tropics, ozone isconcentrated at slightly higher altitudes, 20-30 km, butstill within a layer about 10 km deep.

Figure 2, which is based on the climatological recordof ozone, shows the seasonal variation of the total ozonecolumn as a function of latitude.6 Column abundances of300 DU, corresponding to a pure-ozone column depth ofjust 0.3 cm, are typical. Despite the fact that ozone isproduced at low latitudes, the largest column abundancesactually appear at high latitudes, as a result of polewardtransport out of the tropical source region. In addition,column amounts are enhanced to over 400 DU in winterand spring, when large-scale disturbances of the circula-tion are prevalent. Abundances even greater than themonthly-mean values shown in figure 2 are observedintermittently.

The chemical lifetime of ozone, which measures howquickly its concentration adjusts to changes in thephotochemical environment, is determined by the reac-tions involved in its production and destruction. In thelower stratosphere, where ozone is concentrated, itslifetime is several weeks, long enough for it to betransported passively, or "advected," by the circulation ofthe atmosphere.

The stratospheric circulation. On average, stratos-pheric air moves parallel to latitude circles and counter-

clockwise about the North Pole, or "cyclonically." Thisstrong circumpolar flow, known as the polar night vortex,is brought about by the Earth's rotation when poleward-moving air parcels are deflected toward the east by theCoriolis force. In steering the airstream parallel tolatitude circles, the Earth's rotation inhibits the transferof ozone out of its chemical source region near the equator.The strong zonal flow tends to homogenize constituentsalong latitude circles, but allows only a very gradualtransfer of species between the equator and the poles.

Historically, the stratospheric circulation has beenviewed in terms of the overturning of air in the latitude-height plane, averaged over longitude.5 This "zonal-meanmeridional circulation" is characterized by rising motionin the tropics and sinking motion at middle and highlatitudes and operates on a time scale of months. Thegradual overturning of air in the meridional plane drawsozone poleward from its chemical source region near theequator and redistributes it globally.

Although it provides a simple and conceptuallyappealing picture of stratospheric air motion, much of thiszonal-mean overturning is actually the net result ofmotions that are not zonally symmetric but rather aremore complex. Theoretical advances and large-scalenumerical modeling have underscored the intimate rela-tionship between the zonally averaged view of stratos-pheric air motions and large-scale disturbances to thecirculation (see reference 5 and references therein). Thesedisturbances, which form the subject of this article,operate on much shorter time scales than the zonallyaveraged circulation. They give rise to air motions thatare variable with longitude but that when averagedaround latitude circles produce the classical picture of themean meridional circulation in the stratosphere. Thestronger and more frequent these disturbances are, themore rapid is the zonally averaged overturning of air inthe latitude-height plane and the more quickly ozone isexported out of its tropical source region.

As the distribution of solar radiation changes duringthe year, chemical and dynamical processes alter theabundance and distribution of ozone. Figure 2 indicates

Seasonal march of ozoneabundance. The average

column abundance of ozone—the vertically integrated ozone

density—is plotted here as afunction of latitude and time.

(The abundance values aregiven here in Dobson units.)

Although ozone is produced atlow latitudes, the greatest

column abundances areactually found at middle and

high latitudes. Strong seasonaland intraseasonal variations inozone abundance also occurat middle and high latitudes

during winter and spring.(Adapted from

ref. 6.) Figure 2

90"N

60°N-

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90°SJ JMONTH

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Conservation of angular momentum ofbodies of air moving through the planetarywave pattern. Represented here are elementsof the potential vorticity (f + £)/h, whichmeasures the absolute angular momentum ofa body of air. To leading order, the planetaryvorticity f, which is twice the local verticalcomponent of angular velocity, is exchangedwith the relative vorticity £ of the atmosphericflow. Air parcels displaced meridionally reactto changes in planetary vorticity by spinningup cyclonically or anticyclonically, deflectingthe airstream back and forth across anequilibrium latitude. Figure 3

only a weak seasonal variation in column abundance atlow latitudes. However, at middle and high latitudes theseasonal variation is considerable. Maximum columnamounts appear in spring in both hemispheres, thenorthern maximum occurring in early spring, whereas thesouthern maximum is delayed until late austral spring.Except during southern spring, when ozone is depleted inthe Antarctic stratosphere, chemical processes do notdiffer appreciably between the hemispheres. Thus differ-ences in ozone abundance must follow from differences inthe dynamics of the Northern and Southern Hemispheres.

It is well known that the Antarctic vortex is strongerand less disturbed than its counterpart over the Arctic.The more organized circumpolar flow in the SouthernHemisphere is associated with a deep temperature depres-sion over the South Pole. Very cold temperatures thereare responsible for the formation of polar stratosphericclouds over the Antarctic, which are believed to haveplayed a central role in the ozone depletions observed inrecent years. The much warmer temperatures character-istic of the Arctic stratosphere do not support theformation of polar stratospheric clouds as readily andtherefore inhibit the destruction of ozone through hetero-geneous chemistry. It turns out that planetary waves arethe key to these hemispheric differences. By disruptingthe polar night vortex, these disturbances are responsiblefor the zonally averaged transfer of ozone out of thetropics. As we shall see, large-scale wave motions alsoaffect the ozone distribution in a more direct and dramaticfashion.

Stratospheric disturbancesDuring winter and spring, the circumpolar flow isdisrupted by global-scale, or planetary, waves that propa-gate upward out of the troposphere. Planetary waves arelarge-scale Rossby waves, a type of wave motion possible ina rotating fluid medium. When they amplify in thestratosphere, planetary waves displace the vortex out ofpolar symmetry, causing air to flow across latitude circlesand allowing chemical constituents to be transferredmeridionally on a time scale on the order of only a day.

When the planetary wave field amplifies, as occurssporadically during winter, the strong zonal-mean flow(the flow averaged around latitude circles) is deceleratedand temperatures over the polar cap increase. In its

extreme form, this disturbance is known as a "stratos-pheric sudden warming" because its effects were firstobserved as a reversal of the normal pole-to-equatortemperature gradient.7 During a sudden warming, stra-tospheric temperatures in the polar night increase dra-matically—by as much as 50 K in just a few days—actually becoming higher than those in the sunlit tropics.At the same time, the zonal-mean flow reverses direction,and ozone increases sharply at high latitudes.8 Althoughstratospheric warmings are often described in terms ofzonally averaged quantities, they actually involve acomplex distortion of the circulation in which air andchemical constituents are exchanged rapidly across theentire winter hemisphere.

Propagation of planetary waves. Planetary wavesare excited at the Earth's surface by the forced ascent ofair over elevated terrain, such as the Tibetan plateau andmajor mountain ranges, and also by convective heating inthe tropical troposphere, where large quantities of latentheat are released in cumulus cloud systems.

The restoring force for these disturbances is suppliedby the variation of the Coriolis force with latitude. Airparcels flowing meridionally experience a change inrotating environment reflected in the Coriolis parameter

/•=2ftsin0 (1)Here SI is the angular velocity of the Earth and <t> is latitude.The Coriolis parameter /"is the local vertical component ofplanetary vorticity (twice the local vertical component ofangular velocity), varying from 0 at the equator to + 211over the poles. Air parcels deflected across latitude circlesadjust to changes in local rotation so as to preserve absoluteangular momentum, that is, the angular momentum seenby an observer in an inertial reference frame. The latter ismeasured by the "potential vorticity,"

Q = (2)

The quantity t, is the relative vorticity of the flow, V X v, asseen by an observer fixed with respect to the Earth, and his the effective thickness of the fluid column. To leadingorder, the absolute vorticity /*+ f is constant, the relativevorticity f simply being exchanged with the planetaryvorticity f. Thus air streaming toward the equator willexperience decreasing planetary vorticity and spin up

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Equivalent barotropic system definedabove the 350-K isentropic surface,

wh ich is near the tropopause. Most ofthe ozone is concentrated in a 10-km

layer just above the tropopause. Airmoves along isentropic surfaces (solidlines), wh ich slope downward toward

the pole and intersect surfaces ofconstant ozone mixing ratio (colored

lines). The lowest colored curverepresents an ozone mixing ratio of 1part per mi l l ion. Because air density

decreases exponentially wi th altitude,the ozone density falls off sharply above

the lower boundary. Figure 4 Equator LATITUDE Pole

cyclonically (increasing y) to compensate, while air flowingpoleward will see increasing /and spin up anticyclonically(decreasing y).

Planetary waves owe their existence to the adjust-ment of air parcels to alternating changes in planetaryvorticity. As they are deflected across latitude circles, airparcels experience a decrease or increase in planetaryvorticity according to equation 1. To preserve potentialvorticity (equation 2) they spin up cyclonically or anticy-clonically, oscillating about an equilibrium latitude andgiving the circulation a wave-like appearance (figure 3).

Once excited, planetary waves propagate both hori-zontally and vertically. It turns out that these waves areable to propagate only where the zonal flow is toward theeast relative to the reference frame of the wave. Conse-quently, wave activity is excluded from the summerstratosphere, where winds are toward the west. However,wavelengths of planetary dimension are able to propagatefreely into the eastward flow of the winter stratosphere,amplifying vertically as the air density decreases exponen-tially.9

Transport by planetary waves. Planetary wavesdisrupt the circumpolar flow of the winter stratosphere sothat air streams meridionally. From a global perspective,the vortex is displaced from the pole and air flows acrosslatitude circles. In this disturbed circulation atmosphericconstituents are transferred meridionally two orders ofmagnitude faster than is possible in the unperturbed flow.

As an air parcel moves ^through the disturbedairstream, its dynamical characteristics change to pre-serve its potential vorticity. A property that is conservedfor individual fluid particles is referred to as a "materialtracer." Particular values of such a quantity trace out themotion of individual air parcels, and with them all otherconserved species. In the lower stratosphere, radiativedamping times are on the order of 50 days10 which is longenough (relative to the time scale for horizontal advection)for potential vorticity to behave as a tracer. Likewise, thelong photochemical lifetime of ozone in the lower strato-sphere means that its mixing ratio ,uO3, which is thenumber of ozone molecules divided by the number of airmolecules, also behaves as a tracer. Therefore thedistributions of both the potential vorticity and the ozone

mixing ratio in the lower stratosphere are, to a goodapproximation, rearranged by air motions.

As long as dissipation is unimportant, the motion ofair through the wavy stream pattern is completelyreversible. Air parcels simply cycle back and forth acrosslatitude circles with no net rearrangement. However,dissipation alters this picture by preventing air parcelsfrom returning to their original latitudes, leading to a netredistribution of potential vorticity and other long-livedquantities. When the wave field amplifies, excursions ofthe airstream across latitude circles steepen to the pointthat the flow becomes dynamically unstable. The wavesthen "break,"11 and secondary vortices develop throughinstability. As they amplify, these unstable eddies stirbodies of air down to small dimensions where nonconser-vative processes are efficient. Dissipation then results inan irreversible rearrangement of air and a net transport ofconstituents across latitude.

In addition to causing horizontal transport, planetarywaves also induce vertical motion. Air moving verticallyexperiences changes in pressure and temperature throughcompression and expansion. If the time scale for heattransfer is long compared with that for advection, as istrue in the lower stratosphere, the motion of an air parcelis nearly adiabatic. The pressure;? and temperature 7" of aparcel then adjust to conserve its "potential temperature"0:

(3)

The reference pressure pQ is 1000 millibars. The exponentK is the ratio of the gas constant to the specific heat at con-stant pressure; its value is 0.286 for air.

Like potential vorticity, potential temperature be-haves approximately as a tracer in the lower stratosphere.It follows that air parcels must move along surfaces ofconstant potential temperature. These quasihorizontalsurfaces, which are coincident with isentropic surfaces,undulate under the influence of planetary waves. Airdescending along an isentropic surface responds to in-creases in pressure by undergoing compression, but withits temperature changing according to equation 3 topreserve its potential temperature. Actually, diabatic, or

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OZONE COLUMN ABUNDANCE (Dobson units)

heat transfer, effects cause air to drift slightly acrossisentropic surfaces. This small residual motion turns outto be important for understanding the gradual sinking andrising of air in the mean meridional circulation (seereference 5 and references therein).

Planetary waves and the ozone distributionPlanetary waves can modify ozone in two important ways.By transporting air horizontally, planetary waves drawozone-rich air poleward from its chemical source regionnear the equator. Nonconservative influences can thenresult in a net redistribution of this air, dispersing highermixing ratios globally.8 Planetary waves can also modifythe distribution of ozone by moving air vertically. This isespecially true in the lower stratosphere, where isentropicsurfaces slope downward toward the pole. Air streamingnorthward along these surfaces descends to higher pres-sure. The attending compression of that air can increasethe ozone density and column abundance substantially, aswe will see below.

Initial conditions in a numerical calculationof the circulation, before a disturbance isapplied. Profiles of potential vorticity (a) andozone column abundance (b), both zonallysymmetric, are plotted as functions oflatitudes. Figure 5

These dynamical influences have been exploredthrough high-resolution transport calculations in whichquasihorizontal air motions are represented very accu-rately. Previous studies using equations similar to thosethat describe waves in shallow water have addressedhorizontal transport in this manner.12 However, verticaltransport by planetary waves, which can be especiallyimportant for column abundances, is not described easilyin this traditional framework.

The global transport calculations presented below areperformed with the equivalent barotropic equations,which govern column-averaged motions and reflect behav-ior in a layer about 10 km deep.13 The ozone columnabundance emerges naturally in this system when thethree-dimensional equations of motion are integratedvertically over pressure. Strictly speaking, this systemgoverns motion for which the vertical scale is largecompared with the vertical scale of the mass distribution,as is typical in the stratosphere. One feature of theequivalent barotropic system is that it accounts for thevertical displacements associated with deep atmosphericmotion.

An equivalent barotropic circulation, shown schema-tically in figure 4, is denned above a material surface thatbounds the ozone layer from below. In this system thehorizontal velocity v and ozone mixing ratio /iO3 are givenby

x = A(0)-<y>U,<f>,t) (4)Ho3 =M(6»)-</xO3 >Hi,() (5)

The potential temperature 8 is used as the verticalcoordinate; A{0) and M(9) are prescribed profiles; A, (f>, and tare longitude, latitude and time, respectively; and anglebrackets denote column-averaged quantities:

<•> = -Pf) ~"

dp (6)

Here p0 is the pressure along the lower boundary.The representation given by equations 4-6 accounts

for variations in the ozone mixing ratio along isentropicsurfaces and is in reasonable accord with observedbehavior in the lower stratosphere, where ozone isconcentrated. The column abundance m of ozone followsfrom these equations as

m(A,4,t) = (7)Because both the potential vorticity <Q> and the mixingratio < /LIO3 > are approximately conserved within individ-ual air parcels, the redistribution of one of these quantities

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determines that of the other. Thus, the distribution of <Q>predicted in the equivalent barotropic system uniquelydetermines the distribution of ( /xo > and, through equa-tion 7, the column abundance.

The numerical calculation is denned by prescribingcolumn-averaged properties above the 350-K isentropicsurface, which is roughly coincident with the tropopause.This lower boundary varies in altitude from approximate-ly 100 millibars over the equator to 200 millibars over thepoles and underlies most of the ozone layer. An initialcircumpolar flow having a column average of 20 m/sec isprescribed, giving the profile of potential vorticity shownin figure 5a. The column-averaged ozone mixing ratio(/"o3) is scaled to give the distribution of columnabundance shown in figure 5b. The values of (po >, whichrange from about 260 DU over the equator to 300 DU athigh latitudes, are representative of conditions duringautumn (figure 2), before the planetary wave fieldamplifies.

The initial zonal circulation is perturbed in thecalculation by deflecting its material lower boundary. Awavenumber-1 displacement, in which there is one oscilla-tion around the entire latitude circle, is amplified over 10days to a limiting deflection of about a kilometer, which isrepresentative of disturbed conditions14 near 100 milli-bars. Air motion resulting from this mechanical perturba-tion is revealed by the behavior of the potential vorticity<Q>, shown in figure 1. On day 16, the vortex has been dis-placed out of zonal symmetry, resulting in cross-polar flowat high latitudes. Velocities have increased along theequatorward flank of the vortex, where the meridionalgradient of potential vorticity has steepened. Shearaccompanying this jet leads to a folding of material linesacross a wide belt of latitude. Tropical air is beingadvected poleward along the periphery of the vortex, inthe form of a tongue of low <Q> values. A complementarytongue of midlatitude air, with high <Q> values, is beingdrawn equatorward from the vortex.

The folding of potential vorticity contours that hasresulted by day 16 is dynamically unstable, setting thestage for the wave field to break. Unstable disturbancesamplify near the fold in potential vorticity, leading tosecondary eddies that distort the material field further. Byday 24, the tongue of tropical air drawn poleward isbeginning to spin up anticyclonically to conserve potentialvorticity. Material lines are being wound about theinduced anticyclone, and midlatitude air drawn from thevortex is being entrained with air of tropical origin. Asecond anticyclone is forming on the globe's opposite side.

On day 32 the first anticyclone is intensified, asmarked by a sharp minimum in potential vorticity andstrong flow across the polar cap. Tropical and midlatitudeair has been stirred together within this secondary vortex,dissolving into a large pool with potential vorticity valuescharacteristic of much lower latitude. At the same time,the anticyclone on the opposite side of the globe has also

strengthened. Together, these unstable eddies draw airfrom the vortex and wind it together with air from lowerlatitudes down to very fine scales. This process continuesuntil bodies of air have been strained down to dimensionswhere background diffusion is efficient, at which pointanomalies in potential vorticity and other long-livedquantities dissolve.

By day 40 the flow has achieved a wavenumber-1pattern, with a jet flowing almost directly across the pole.This strong meridional motion transports air between lowand high latitudes in only a day. Contours of potentialvorticity at low latitudes, caught in this circulation, arequickly advected across the polar cap. The anticyclonecontinues to intensify at the expense of the polar nightvortex, which has suffered considerable erosion throughlarge-scale stirring.

Ozone mixing ratios undergo an evolution similar tothat shown in figure 1 for potential vorticity. However,the ozone column abundance evolves differently (seefigure 6). Unlike the mixing ratio, the column abundanceis not conserved for individual bodies of air. As tropical airis drawn poleward along the periphery of the vortex, itdescends along isentropic surfaces to lower altitude, whereit undergoes compression. The resulting increase in ozonedensity leads to a significant enhancements of columnabundance. Already by day 16, column abundances haveapproached 400 DU in a broad area overlying the tongue oftropical air drawn poleward, an increase of 30% over theinitial values (see figure 5b). On day 24 two column-abundance maxima exist, both associated with tropical airbeing drawn poleward along the northern flanks of thetwo anticyclones that have developed through instability.The stronger of these ozone anomalies has values in excessof 470 DU. These large column abundances surroundmuch of the polar cap, which remains under the influenceof the vortex (compare with figure 1).

By day 32, an extensive region of anomalously largeozone column abundance has formed at high latitude.Column abundances in excess of 520 DU appear, anincrease of nearly 100% over the original values. Thesemagnified ozone abundances correspond to the large pool oftropical air (shown in figure 1) that has been drawnpoleward, descended to higher pressure and spun upanticyclonically. By day 40 this secondary circulation hasstrengthened through the introduction of additionaltropical air. Corresponding ozone abundances are in-creased further as the anticyclone spreads poleward anddescends to higher pressure.

One can compare the above results with the variabil-ity representative of the middle and upper stratosphere.Satellite observations of the ozone column lying above 30mb reveal similar features." However, because isentropicsurfaces are nearly level at these altitudes, fluctuations incolumn abundance are considerably smaller (under 30%)and are due primarily to horizontal transport. The muchlarger enhancements of total column described above

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Ozone column abundance associated with the air motion indicated in figure 1. Orange represents 250Dobson units; deep blue, 600 DU. Tropical air drawn poleward along isentropic surfaces descends to higherpressure, causing increases in ozone density and column abundance. Ozone column abundance increasesmarkedly inside the anticyclone that develops alongside the vortex (compare with figure 1), exceeding 500 DUby the end of the sequence. The enhanced values result from the poleward displacement of tropical air, whichis stirred with midlatitude air, descends along isentropic surfaces and is compressed. Peak ozone columnabundances along the highlighted meridian in the four frames are 374 DU, 472 DU, 511 DUand 517 DU. Figure 6

result primarily from vertical transport and compression,when air descends along isentropic surfaces in the lowerstratosphere. Increases in column abundance of themagnitude seen in these calculations occur routinely insatellite data from the Total Ozone Mapping Spectrom-eter, which reflect the complete ozone overburden at anylocation, with the dominant contribution arriving fromthe lower stratosphere.

Implications for ozone destructionThe Antarctic ozone hole is of profound importancebecause it represents the first concrete evidence that

human activities have had a substantial impact on thisprotective layer of the atmosphere. However, largechanges in ozone abundance occur naturally, especially inthe winter stratosphere, where planetary waves arecommon. Poleward transport of tropical air in combina-tion with vertical transport and compression can increasecolumn abundances by as much as 100%. However, itshould be emphasized that there is no evidence in thesecalculations of comparable decreases in column abun-dance. The absence of such decreases follows from the factthat the intrusion of tropical air seen in the second panelof figure 1 is compensated by equatorward displacements

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of air that are distributed across much of the hemisphereand are therefore smaller" resulting in much weakervertical transport.

Global-scale disturbances such as those consideredhere are common in the Northern Hemisphere, whereelevated terrain is able to excite a strong planetary wavefield. Ozone increases induced by these disturbances canbe as large or larger than chemically induced losses overAntarctica, where a 50% reduction in the column istypical, and they can cover an area of hemisphericdimension. Like chemically induced changes, these per-turbations are irreversible. Air that is wound up in global-scale eddies such as those depicted in figure 1 is eventuallyacted upon by diffusion and other nonconservative pro-cesses. Once these processes have dissolved inhomogene-ities, a net transfer of ozone to high latitudes has takenplace.

Eddy motions such as these play a key role in drivingthe zonally averaged circulation and in producing thelarge monthly mean ozone abundances indicated in figure2, which occur at middle and high latitudes during winterand spring. When the vortex is disturbed, as in figure 1,diabatic effects introduce nonconservative behavior.Warm tropical air, displaced poleward, cools radiativelyand therefore sinks slightly across isentropic surfaces.Likewise, cold polar and midlatitude air, displaced equa-torward, warms radiatively and therefore rises slightlyacross isentropic surfaces. Each time the vortex isdisturbed, air sinks a little at middle and high latitudesand rises a little at low latitudes. Averaging these motionszonally and over many such episodes produces theclassical view of the mean meridional circulation andaverage transfer of ozone from low latitudes to highlatitudes. However, the latter that process actually occursthrough more complex behavior, such as that shown infigure 1.

Planetary waves such as those examined in thisarticle occur routinely in the Northern Hemisphereduring winter. In the Southern Hemisphere, where theEarth's surface features are smaller, the planetary wavefield is generally weaker. Cyclone-scale disturbances,decaying vertically from the troposphere in both hemi-spheres, are ubiquitous in the lowest levels of thestratosphere and also lead to fluctuations in ozone.15

Natural ozone variability due to these disturbances shouldbe considered when the implications of chemically inducedchanges are evaluated.

The air motions responsible for these natural changesin ozone also play a key role in shaping chemicaldepletions. Disturbances such as those shown in figure 1allow air to flow between the polar cap, where it is in dark-ness and cools radiatively to space, and the rest of thehemisphere, where -it can warm through absorption ofsolar radiation. Advection of midlatitude air into thepolar cap offsets radiative cooling there. By interruptingthe symmetric cooling to space, these dynamical effectskeep air inside the vortex warmer than it would beotherwise.16

This process is important in establishing the sharpdifference between the circulations over the Arctic andAntarctic and thereby mediates chemical depletions overthe North and South Poles. The relative weakness ofplanetary-wave activity in the Southern Hemisphereallows the Antarctic vortex to become more organizedabout the South Pole and colder than its counterpart inthe Northern Hemisphere. The very low temperaturesover the South Pole favor the formation of polar stratos-pheric clouds, which serve as sites for heterogeneouschemical reactions that liberate chlorine and set the stagefor the springtime depletion of ozone. The stronger

planetary wave activity in the Northern Hemispherekeeps the Arctic vortex considerably warmer. The highertemperatures over the North Pole do not support polarstratospheric clouds as readily and therefore inhibitdestruction of ozone through heterogeneous chemistry.Stronger planetary wave activity also brings about thespringtime ozone maximum sooner in the NorthernHemisphere than the corresponding maximum in theSouthern Hemisphere (figure 2). Larger and more fre-quent exchanges of air transport ozone more efficientlyinto the Northern Hemisphere than into the SouthernHemisphere.

In addition to these hemispheric differences, plan-etary waves are involved in the final breakdown of thevortex during spring in both hemispheres. When theequator-to-pole temperature contrast weakens, the vortexis disturbed out of polar symmetry one last time and is re-placed by an anticyclonic flow about the pole. In thatprocess, air is overturned across much of the hemisphere,with ozone-rich air from low latitudes replacing depletedair at high latitudes. Hence planetary waves play a keyrole in the eventual dispersion of ozone-lean air trappedwithin the Antarctic vortex and in the recovery towardnormal ozone abundances. A better understanding ofthese mechanisms and their influences on the stratos-pheric circulation will be essential to reaching a fullerappreciation of how the ozone layer functions and how itwill respond to changes brought about through humanactivities.

The numerical calculations discussed in this article were per-formed on the Cray-XMP computer at the National Center forAtmospheric Research. The computer graphics were produced byPatrick Callaghan at the Center for Atmospheric Theory andAnalysis at the University of Colorado at Boulder and assembledby the NCAR graphics department.

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