GEOPHYSICAL
RESEARCH LETTERS, VOL. 28, NO. 8, PAGES 1547-1550, APRIL 15, 2001
Tropospheric Response to Stratospheric Ozone Loss Ina T. Kindem Geophysical Institute, University of Bergen, Bergen, Norway
Bo Christiansen Danish Meteorological Institute, Copenhagen, Denmark
Abstract. The response to realistic total column ozone trends on the troposphere and the stratosphere as simulated
the stratosphericand the troposphericcirculation during the NH and the SH winter and spring months, becausefor these
by the ARPEGE General Circulation Model (GCM) has monthsboth theoreticalresults[Charneyand Drazin, 1961; Matsuno,1970]and observational EOF analysis[Thompson and Wallace,2000]allowusto expectthe strongestcoupling duringspring/earlysummer.The coolingtrend wasweaker between the circulation of the troposphere and the stratothan the observedtrend in the NorthernHemisphere(NH) , sphere. been investigated. In both hemispheres,the lower stratospherecooledand the polar vortex strengthenedsignificantly
but strongerthan the observedtrend in the Southern Hemi-
sphere(SH). In the troposphere,the changesin geopotential Description height resembledthe positive phase of the Arctic Oscilla-
of the model and the experimental setup
tion (AO) in the NH in March and the positivephaseof A closerdescriptionof the particular model versionof the the Antarctic Oscillation(AAO) in the SH during summer spectralgeneralcirculationmodel ARPEGE GCM [Ddqud (December-February). et al., 1994]usedin this study can be foundin Christiansen et al. [1997]. The T21 horizontalresolution(corresponding Introduction to a 5.6ø resolution)with 41 levelsin the verticalandthe full During the last two alecaries,stratospheric ozone decreases have been documented. The decrease is especially
annual cycle was chosen for the control experiment and the ozone perturbation experiment. Both experiments lasted for large during winter/springat high latitudes in both hemi- 10.5 years, and the first 6 months were discarded. spheres[e.g.WMO, 1998]. Over the sameperiod,trendsin In the model, ozone is a calculated variable with the both stratospheric temperatures and geopotential heights ozone distribution set up by a balance between the predicted have been negative during spring at high latitudes in both advection and the parameterised sources and sinks. Both hemispheres [e.g. Labitzkeand van Loon, 1995; Pawsonet the unperturbed and perturbed ozone fields were calculated al., 1998].It hasbeenshownthat the observedstratospheric by the ozone parameterisation representing the late 1970s ozone decline is responsible for at least part of these trends [Cariolle and D4qu4,1986]. In the ozoneperturbationrun,
[Ramaswamy et al., 1996;Graf et al., 1998;Randeland Wu, 1999;Langematz,2000]. Moreover, statistical studiesinvolving Empirical Orthog-
onal Function(EOF) analysisof observeddata haveshown that a coupling exists between the tropospheric and the stratospheric circulation, where the stratospheric polar vortex
seems to be involved
in some of the
interannual
and
decadalvariabilityof the tropospheric climate[e.g. Perlwiiz and Graf, 1995; Thompsonand Wallace,1998]. Recently, it has been speculated that various anthropogenic forcings might influence the coupling between the troposphere and
the ozonetrends were only imposed in the radiative heating calculations. This procedure has negligible feedbackson the
ozonechemistry[Christiansenet al., 1997].
Trends in total ozone (%/decade)basedon Herman e.tal. (1993)
80 ', i .-;'
:,,,•.',; %l•Xv, •' • •.•' -,i ....... I
60" '-'II ',,,",'•/ I ii //k I
i x, •, ,• i
'\\
i
i
i ',-
/I'
!
t•, ,,', , ,1
'
stratosphere[Graf et al. 1998; Volodinand Galin, 1998; Thompson et al., 2000; Shindell et al., 1999; Hartmann et
2O
aL, 2000].
.
The objective of this paper is to investigate the climatic impa•t of the observed ozone changes on the stratosphere and the troposphere when forced with realistic total column ozone trends. For this purpose, a control experiment and an ozone perturbation run each lasting for ten years with climatological SSTs and the full annual cycle in solar radiation were performed. Emphasis will be placed on the responseof
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-•o•',',,•,,, -80•,,---' •" •
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---.
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Jan Feb Mar Apr May Jun Jul Aug Sep Oc[ Nov •c
Jan
Month
Copyright 2001by theAmericanGeophysical Union. Papernumber2000GL012552.
Figure 1. Trendsin total ozone(%/decade) basedon Herman et al., (1993) as a functionof latitude and month.
0094-8276/01/2000GL012552505.00
1547
1548
KINDEM
AND CHRISTIANSEN:
RESPONSE
TO OZONE
LOSS
(SH) the temperaturedecreaseis especiallylargeduring the Antarctic spring, with a statistically significant peak tem............... ' .................... • • • / perature change of-8.5 K in November. The maximum temperature change is shifted one month from the maximum q-u::::::::::: ::::::::::: • :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :i'•!::::: :i::i:::::/•'i::: :•::iiiiiiiiii iiiiiiiiiii ii !•iiiii i•!!i!E•!!•i •:•i•:•:i:i•ii iiiii:i i:i:i i:i:i:i:i:i:i:i:i:i: ozone change in October due to the combined effects of the :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ================================================================= ======================================== ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: =============================================================== ========================================================================================================================= long relaxation time and more available sunlight in November as compared to October. ..• i•;-' • ":•iiiiiii• .... ..0 5. :.......... I:!::!:•:i::• ..... •...... :::::::::::::::::::::::: ...... ß•• O:::::::::::::::::::::::: ::::::::::::::::::::::: i • • I ::::::;. :::::' •• _ •::::::::: •:::•:•::•:•:•:•:•:E:•:•:•:•:•:•:•:•:•:•:•:•:E:•:•:E:•:?•:•:E: :E:•:•: :•: :• Compared to trends in the MSU temperature for the _ ===============================================================================================================
1979-1998period[WMO,1999]the modelledSH springtem-
2 0 """-•'"'"'"'"'"'"'"'"'""'"'"" • .... .%:.::::::•:•:•:•:•:•:E:E:•:•:•:•:•:•:•:•:E:•:E:E:•:E:•:E:•:•:•:•:•:•:•:•:•:•:•:•:E:•:•:E:•:•:•:•:•E:•:•:E:E:•:E:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:E:•:•:•:•:•:•:•:E:E:E:•.•y.•: •"'"'"'"'"'"'""'""'"'"'"'"'"'"'"'"'"'"'""'"'"'""" ........ "".............. '........'"'................. :':":''"': .... :E:•::::.•:•:.•:E:•:•:•:•:•:•:•:•:E:•:•.:.:•
perature decreaseis about twice as strong. In the NH, on the other hand, the modelled temperature decreaseis about 40 :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ======================= ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: half the strength of the observedtrend. Recall that the im:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ============================================================================================================== :::::::::::::::::::::: ........... ,.............. •:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:•:::•' _ ' ..:-'•(:•.•5•:•:-•::•:-•-•.½•5• posed ozone loss is representablefor the period 1979-1991 and that a somewhat weaker ozone trend is reported when :'•"42•g:':i ........... :::::::::::::::::::::::::::: ........................ V • '- • • .....:•:...:.:...:.:...:.:...:.:...:.:...:..:.•-•:...-'•.:..'•.•::•::•:..• •.•:•::::•::::,:::-•::-•::-•:•:-•'•24•:•t•-•-:'•:• ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • • ...........................................•............................................. ..... .............. ......... ................. ..•.. :.•...•.-:• ........... including the most recent TOMS measurementsfor the years Dec Jan Feb M• A• May Jun Jul Aug Sep •t Nov Dec after 1992. Hence, the modelled temperature trend might Month overestimate the observedtemperature trend for the period 1979-1998. This result suggeststhat ozone depletion alone Figure 2. Response of the lowerstratospheric (74.8hPa) modelled zonal mean temperatures to the imposed changesin ozone. can not explain the observed temperature decrease in the NH lower stratosphere. The temperature anomalies shown Anomalies in Kelvin. Contour interval: 0.5 K. The thick solid in Fig. 2 are well in accordancewith the results presented in line is the zero contour. The areas with light shading indicate statistically significant changesat the 95% confidencelevel as de-
Graf et al. [1998],who usedthe sameozonetrends. How-
ternfined
ever, a major difference between the present work and that
from
a Student
t-test.
of Graf et al. [1998]is that their midwinterwarmingat the The ozone trends in the perturbation experiment, depending on month, latitude and height, were based upon
the Herman et al. [1993]monthlymeantotal ozonetrends, which were taken from Total Ozone Mapping Spectrometer
(TOMS) measurements in the period1979-1991.To provide changesin the polar night, the TOMS trend data was extrapolated linearily into the winter poles. A similar ozone
forcingwasfirst appliedby Graf et al. [1998].The decadal trend
in total
ozone as a function
of month
and latitude
is
shownin Fig. 1. In the equatorial region there is only a weak ozone depletion throughout the year. However, both the strength and the seasonality of the trend increase towards the poles. For the Antarctic and Arctic spring months, the
trendsreachrespectively38%/decadeand 14%/decade.It shouldbe noted that when includingthe most recent TOMS measurements for the years after 1992, a somewhat weaker
ozonetrend is reported[WMO, 1999]. In the perturbation experiment, the decadal trends shown
in Fig. 1 weremultipliedby two (to get a 20 yeartrend) and then distributedhomogeneously (on a percentagebasis)in the lower stratosphere between the two pressure levels 240 and 25 hPa. This change was then repeated from year to year. This choice is simple and consistent with the observations. A similar strategy was adopted in previousstudies and is justified by the uncertainty in the ozone decline below
15 km [ WMO, 1999].
Response of the ozone perturbation Figure 2 showsthe long term averaged,zonal-mean temperature anomalies between the perturbation and the con-
NH high latitudes was not seenin the present study. Long-term, monthly mean anomaly fields for the NH and the SH were constructedby subtracting the long-term monthly mean fields of the perturbation run from the correspondingfields of the control run. For the NH winter months from December to February, only parts of the anomalies can be consideredas statistically significantdue to the high year-to-year variability during the NH winter months. The resulting anomaly field for the 50 hPa geopotential height for the NH in March are shownin Fig. 3a. The maximum in the zonally symmetric geopotentialheight anomaliesreaches
more than 160 geopotentialmeters (gpm) over the North Pole. Statistically significanttemperature changesin March
reach1-2 K (not shown). Overall, theseanomalypatterns indicate a stronger and colder polar vortex. These simulated changesare in accordancewith the structure of the observed trends in the geopotenitalheight and temperature, although
the magnitudeof the observedchangesare larger(maximum reductions over the North Pole reaching 400 gpm and 9 K
at 50 hPa accordingto Graf et al. [1998]). In the SH, the polar vortex deepeningmaximizes in December, at the end of the Antarctic ozonehole season,reach-
ing 300 gpm (Fig. 3b). Weaker,but still significantchanges were seen until
March.
From
December
to March
statisti-
cally significant temperature decreasesalso occurred, reach-
ing 3 K overthe SouthPole in January(not shown). Accordingto the statistical connectionbetweenthe stratosphere and the troposphere cited in the Introduction, a stronger and colder polar vortex in the lower stratosphere
trol experimentin the lowerstratosphere (74.8 hPa) as a
will correspondto anomaly patterns related to a stronger NAO/AO in the NH troposphere.The 500 hPa geopotential height changesin March are shown in Fig. 4a. The
function of month and latitude. In general the lower stratosphere cools, and the cooling increasesfrom the tropics towardsthe poles. In the NH the negativetrends are strongest in January, April and May and amount to -2 K. The shaded areas show that only the changesduring spring are statistically significant above the 95% confidencelevel, as determined from a Student's t-test. In the Southern Hemisphere
geopotential over the polar region strengthens,whereasthe geopotential at mid latitudes weakens,particularly over the North Atlantic and the Pacific oceans. The similarity with the'AO pattern increasesfrom December to March, but it is only in March that the changesare consideredas statistically significant in the centre of the main anomalies. Over the polar regionthe decreasein geopotentialheight reaches
KINDEM AND CHRISTIANSEN:RESPONSETO OZONE LOSS Discussion
(a)
and
1549
conclusions
The model runs have shown that lower stratospheric ozone lossesresult in a stronger and colder polar vortex in both hemispheres.The magnitude of the modelled temperature change in the NH stratosphere was weaker than the observedtrend in spite of the strong modelledozonedecline. Similar resultsare reported in other model studiesincluding
stratospheric ozonedecreases [Langeraatz, 2000; Rosierand Shine,2000]. The inconsistency betweenthe modelledand the observed NH temperature trend is speculated to be due
to increasingamountsof water vapor rather than the neglect
of greehouse gases[Rosierand Shine,2000]. Additionally, the temperature trend in the NH is also heavily dependent on the period studied. Preliminary results indicate that the
(a)
'!
.
'1':
I::II .
ß
Figure 3. (a) The 50 hPa geopotentialheightfield modelresponse for the NH in March averaged over ten years. Negative anomalies are dashed and the contour interval is 40 gpm. Shading indicate 95% statistical significanceas determined from a Stu-
d½•'• •-•½•. (b) • (•) bu• fo• •h½Sn i• •½•b½•.
===================================== :::::::::::::::::::::::::::::::::
"::•i•ii!iiiii:•:iii•i ....
Co•ou•:
t50 gpm.
80 gpm, while the mid latitude increasesover the North Atlantic amount to 60 gpm, in accordancewith the observed
trends[Graf et al., 1998,Fig. 5a]. Figure 4b illustrates the SH tropospheric response of the 500 hPa geopotentialheight in December, at the time when the stratospheric vortex responsemaximized. Here, a statistically significant pattern indicative of a strengthened AAO with geopotentialheight falls over Antarctic amounting to 100 gpm and geopotentialheight increasesreaching
80 gpmoverthe mid-latitudesare seen(Fig. 4b). Similar, Figure 4. a) As Fig. 3a, but for the 500 hPa geopotential but weaker statistically significant patterns were found in
height. Contours: 20 gpm. (b) as Fig. 3b, but for the 500 hPa
January,Februaryand March (not shown).
geopotential height. Contours: 20 gpm
1$$0
KINDEM
AND CHRISTIANSEN:
long term mean temperature responsein the NH is weaker in a 30 years model run. In the troposphere, statistically
significantanomalypatternssimilar to the NAO/AO were foundin the NH in March. Graf et al. [1998]did not find a tropospheric response in their model study, due to the too weak polar vortex resulting from the late cooling of the polar area in their model run. In the SH, the strongesttropospheric responsewith a pattern similar to a strengthened AAO wasseenduring summer. Theseresultssuggestthat in order to simulate the coupling between the stratosphereand the troposphere a strong polar vortex is needed. The tropospheric responsethen results from planetary wave-mean flow interaction.
Lower stratospheric ozone change results in an overall
coolingof the troposphere [Christiansen, 1999]becausethe negative longwaveforcing is larger than the positive shortwave forcing. In our model runs a part of the positive shortwave forcing at the surface is lost due to climatologically fixed SSTs. However, climatologically fixed SSTs will contribute to a relaxation of the expected tropospheric cooling. Thus, there are two competing mechanismsat work. Antic-
ipating the effectof prognosticSSTs on the AO/AAO will be speculative.
The total ozonetrends,takenfrom Hermanet al. [1993], were implemented in a thick vertical layer extending from
the upper troposphere/lower stratosphereat 240 hPa to 25 hPa. Concentrating ozone perturbations in a thinner vertical level right above the tropopause did not have a big impact on the model results, confirming the robustnessof the results to the badly known vertical profile of the ozone perturbation. Acknowledgments.
We would like to thank the two
anonymous reviewers for constructive suggestions.
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B. Christiansen, Danish Meteorological Institute,
I. T. Kindera, Geophysical Institute, University of Bergen, 5007
Bergen,Norway. (e-mail:
[email protected])
The
AP•PEGE/IFS atmosphere model: A contribution to the French community climate modelling, Clim. Dyn., 10, 249266, 1994.
DK-2100
Copenhagen0, Denmark. (e-mail:
[email protected])
(ReceivedOctober30, 2000; revisedJanuary 26, 2001; acceptedJanuary 29, 2001.)
