Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289www.elsevier.com/locate/jastp

Low ozone event at Madrid in November 1996

A. P-erez ∗, I. Aguirre de C-arcer, F. JaqueDepto. F�s�ca de Materiales, C-IV, Universidad Aut�onoma de Madrid, 28049 Madrid, Spain

Received 6 September 2000; received in revised form 10 July 2001; accepted 2 November 2001

Abstract

A sudden transient increase of the solar ultraviolet (UV) radiation was detected with a ground-based photometer in Madrid(Spain, 41◦N, 3◦W) on 25 November 1996 (Cordoba et al., 2000. UV-B irradiance at Madrid during 1996, 1997 and 1998.J. Geophys. Res. 105 (D4) (2000) 4903–4906). The data obtained by the TOMS satellite instrument revealed that this eventwas related to a reduction of the total ozone column values. This low ozone episode has been analysed. The ozone decreasewas accompanied by a reduction of air temperature in the lower stratosphere. The anti-correlation between total ozone andair parcel height along the three-dimensional isentropic back-trajectories suggests that the adiabatic uplift of air contributedsigni@cantly to both the ozone and the temperature decreases observed. The uplift could be caused by the propagation intothe lower stratosphere of the high-pressure cell located over the mid-Atlantic region. c© 2002 Elsevier Science Ltd. All rightsreserved.

Keywords: Trajectory; Stratosphere; Spain; Geopotential; Wind; Isentropic; Atmosphere dynamics

1. Introduction

Since the discovery of the springtime Antarctic ozonehole, the possibility of the development of a similar phe-nomenon in the Northern Hemisphere is a matter of concern(Newman et al., 1997). Photochemical processes, enhancedby the particular conditions inside the Antarctic polar vor-tex are considered to be the cause of the dramatic ozonedepletion during spring over Antarctica (Solomon, 1990).However, out of the springtime Antarctic polar vortex, theshort-term ozone variability in the lower stratosphere ismainly driven by the atmosphere dynamics. Then, to assessozone depletion, it is important to @nd out whether ozonecolumn minima observed at a particular location are driveneither by photochemical or by dynamical processes.

The link between total ozone and tropospheric weathersystems is known since the pioneering work by Dobson(Dobson and Harrison, 1926, 1927). Reed (1950) showedthat meteorological patterns are capable of producing most

∗ Corresponding author. Current address: EUMETSAT,Am Kavelleriesand, 31, 64295 Darmstadt, Germany. Fax:+49-6151-807-656.

E-mail address: albinana@eumetsat.de (A. P-erez).

of the short-term variability of total ozone. Vaughan andPrice (1991) derived a relationship between absolute vor-ticity in the lower stratosphere and the total ozone column.

Sudden transient variations of the ozone-column valuesover a particular location are usually related to atmosphericdynamics. For that reason, the study of those events canprovide an insight into the inHuence of dynamics on ozone.Recently, P-erez et al. (2000) have studied the inHuence ofthe dynamics in a number of low ozone events detectedin the mid-latitudes of South America. They employedback-trajectory analyses to study the correlation betweentotal ozone and geopotential height following the motionof air parcels in the lower stratosphere. The same noveltechnique has been employed in this work to study an im-portant reduction of the total ozone column detected overMadrid (Spain, 3◦W, 41◦N) on 25 November 1996 by aground-based instrument (C-ordoba et al., 2000).

2. Data source and analyses

Satellite total-ozone data presented in this work were ob-tained by the Total Ozone Mapping Spectrometer (TOMS)instrument onboard Earth Probe spacecraft and were pro-

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284 A. P�erez et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289

vided by the Ozone Processing Team of the Goddard SpaceFlight Center of the National Aeronautics and Space Ad-ministration (NASA, USA) (McPeters et al., 1998).

Temperature, geopotential height and wind @elds wereprovided by the National Center for Atmospheric Research(NCAR, USA) on a regular latitude=longitude grid of 2:5◦×2:5◦ every 24 h at 00 h UTC (Kalnay et al., 1996). Temper-ature and geopotential height @elds on the 100 and 20 mbarisobaric surfaces have been used to construct the geopoten-tial height @elds for the 450 K isentropic surface by verticalinterpolation.

The isentropic back-trajectory analyses have been widelyused to study the transport of ozone in the stratosphere(Atkinson et al., 1989; KirchhoJ et al., 1996; P-erez andJaque, 1998). This technique allows tracing back thepath followed by the air parcel that arrived at a locationwhen a total low ozone event took place. The isentropicback-trajectory calculation is based on the assumptionof adiabatic and inviscid How of air in the lower strato-sphere over time spans of approximately ten days (McIntyre and Palmer, 1983). Under these conditions, potentialtemperature is conserved following the motion. Thus, ifpotential temperature is used as the vertical co-ordinate,motion of air can be described in a bi-dimensional refer-ence frame (Holton, 1992). Back-trajectory calculationswere carried out by Lagrangian integration (following themotion) of analysed meridional and zonal winds on theisentropic surface of 450 K using a fourth-order Runge–Kutta scheme. In addition, interpolated data of the geopo-tential height, and total ozone are provided for everypoint of the trajectory. This allows obtaining informationon the vertical displacement of the air parcels along thetrajectory.

3. Results

On 25 November 1996, a ground-based ultraviolet (UV)photometer located at Madrid (Spain 41◦N, 3◦W) detectedan increase of the solar UV radiation intensity. The analysisof satellite data revealed that the UV increase over Madridwas related to a large sudden decrease of the total ozonecolumn value.

The evolution of the total ozone column values as mea-sured by the TOMS satellite instrument over Madrid (Spain41◦N, 3◦W) during November 1996 (crosses) is shown inFig. 1. The 450 K isentropic surface temperature data arealso represented (solid circles) in the same @gure. Two rel-ative ozone minima on 7 and 25 November, respectively,are apparent in Fig. 1. The ozone column value over Madridon 7 November was 245 Dobson Units (DU). Two days be-fore, on 5 November, the value of the total ozone columnwas 276 DU. Thus, from 5 to 7 November, the ozone col-umn over Madrid decreased by 31 DU. The second eventon 25 November was of greater magnitude. The total ozonecolumn value over Madrid during 25 and 26 November was

5 10 15 20 25 30200

225

250

275

300

325

350

DAY OF NOVEMBER 1996

TO

TA

L O

ZO

NE

CO

LUM

N (

DU

)

OZONE

200

205

210

215

AV

TE

MP

ER

AT

UR

E (K

)

TEMPERATURE

Fig. 1. Total ozone data from TOMS satellite instrument (×) andtemperature of the 450 K isentropic surface (•) during November1996 at Madrid (Spain, 41◦N, 3◦W).

218 DU. However, on 24 November, the ozone column overMadrid was 286 DU, so that the total ozone column reduc-tion was of 68 DU in only one day.

To provide an insight into the magnitude of this event,the Nimbus 7 TOMS version 7 data record at the locationof Madrid for the period 1979–1993 was examined. Theminimum total ozone record over Madrid in the mentionedperiod occurred on 29 November 1989 with a column valueof 221 DU, very close to the value on 25 November 1996. Inaddition, the average value of the ozone column over Madridin November 1996 was of 274 DU. This value is of thesame order of magnitude as the mean November total ozonecolumn for Madrid in the period 1979–1993, of 282 DUwith an estimated standard deviation of 27 DU. Thus, itcan be concluded that though the average ozone column inMadrid was not signi@cantly small during November 1996,the ozone decrease observed on 25 November produced atotal ozone value as low as the lowest recorded in the period1979–1993 by the TOMS satellite instrument.

A correlation between the temperature values of the 450 Kisentropic surface and total ozone column along November1996 can also be observed in Fig. 1. The temperature of theair in the lower stratosphere reached a relative minimum on25 November 1996, quasi-simultaneous to the mentionedlow ozone event. As can be seen in Fig. 1, the air temperaturedecreased approximately 15 K in the time interval 16–25November. The quasi-simultaneous decrease of total ozone,and temperature in the lower stratosphere suggests that bothdecreases had a common origin.

The characteristic time constants associated with the dy-namics in the lower stratosphere are shorter than those as-sociated to ozone photochemistry and radiative processes(Solomon and Brasseur, 1986). The sudden transient de-crease of the ozone column and temperature values suggeststhat these are dynamically induced phenomena.

The geographical extension of the low ozone area can beappreciated in Fig. 2, where the total ozone distribution, as

A. P�erez et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289 285

Fig. 2. TOMS total ozone distribution for 25 November 1996 in the Northern Hemisphere from the Equator to 90◦N latitude and from90◦W to 10◦E longitude. Scale is in Dobson Units (DU).

measured by the TOMS satellite instrument for 25 Novem-ber 1996, has been represented. White areas in the @gurecorrespond to regions not covered by the satellite instrumenton that date. In Fig. 2, a large region of low ozone val-ues that extends from the Iberic Peninsula to the northwestup to latitude 60◦N approximately is apparent. The lowestozone values occurred over the Atlantic Ocean westward ofthe British Isles, where the total ozone column values werebelow 200 DU. The 225 DU contour extends as a tonguetowards the Iberic Peninsula, suggesting that the low ozoneevent detected over Madrid was related to the low ozoneregion over the British Isles.

To study the transport of the air parcels that arrived atthe proximity of Madrid on the 450 K isentropic level on25 November 1996, a number of isentropic back-trajectoriesfor those air parcels were calculated. One of these trajec-tories is shown in Fig. 3. In this @gure, the horizontal tra-jectory together with the evolution of the air parcel heightand the total ozone column for each point of the trajectoryis presented. Fig. 3 shows that the air parcel that arrived atMadrid on the 450 K isentropic surface on 25 November,was in the central region of North America on 16 Novem-ber. It can be appreciated that at the starting location, theheight of the parcel was 17:75 km and that the total ozonecolumn was 300 DU approximately. At the arrival location(Madrid), the parcel was at an altitude of approximately18:75 km and the total ozone column was, as mentioned be-fore, of 220 DU approximately. In addition, it is also ap-

parent that the values of the total ozone column and the airparcel height anti-correlated along the trajectory. From Fig.3, it can be concluded that the air parcels that arrived atMadrid in the 450 K isentropic level on 25 November 1996were displaced vertically, increasing their height by 1 kmapproximately with respect to their altitude on 16 Novem-ber. Furthermore, the anti-correlation between total ozoneand air parcel height shown in Fig. 3 suggests that the ex-pansion of the air associated to the vertical displacementaJected the total ozone column.

The meteorological @elds can provide an insight into thedynamical conditions in the lower stratosphere. The geopo-tential height @eld of the 450 K isentropic surface is shownin Fig. 4 together with the wind @eld on the same surfacefor 20 November 1996 (a) and 25 November 1996 (b). Theposition of the parcel for the trajectory shown in Fig. 3 on20 November, is marked with a black dot in Fig. 4(a). Thatparcel was at a height of 17:5 km approximately in goodagreement with Fig. 3. In Fig. 4(a), it can also be seen thatthe parcel was in a trough over the South of the Labradorpeninsula. In Fig. 4(b), it can be seen that the height of the450 K isentropic surface over Madrid on 25 November be-tween 18.5 and 19 km, it is also in good agreement with thetrajectory of Fig. 3. In addition, this @gure shows a 19 kmridge of the 450 K surface over the Atlantic. The area of in-Huence of this mid-Atlantic perturbation extends to the Northup to Iceland and to the East covering the Iberic Peninsula.The comparison of Figs. 3 and 4 strongly suggests that due

286 A. P�erez et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289

16 18 20 22 24 2617.0

17.5

18.0

18.5

19.0

HEIGHT

DAY OF NOVEMBER 1996

GE

OP

OT

EN

TIA

L H

EIG

HT

(K

m)

200

250

300

350

OZONE

TO

TA

L OZ

ON

E (D

U)

Fig. 3. Isentropic back-trajectory of the air parcel arriving at Madrid at 00 h UTC on 25 November 1996 on the 450 K isentropic level.

to the presence of the mentioned ridge in the mid-Atlanticregion, the air parcels in the lower stratosphere were forcedto rise as shown in Fig. 3.

Total ozone is not an air-mass tracer and it can be diJer-ent at the initial and @nal points of an isentropic trajectory.In particular, total ozone is sensitive to vertical motion(Vaughan and Price, 1991; Petzoldt et al., 1994; P-erezet al., 2000). When an air parcel rises in the atmosphere, itexpands and the number of molecules per unit area in theparcel would decrease. Thus, the vertical displacement of

an air parcel in the lower stratosphere (where most of theozone in the atmosphere vertical column lies) could reduceits contribution to the total ozone column. The rapid verticalmotion of an air parcel could also modify its temperatureby the associated adiabatic pressure change.

To investigate the origin of the above-mentioned ridge onthe 450 K isentropic surface, the meteorological conditionsnear the surface level were examined. The 1000 mbar geopo-tential height @eld for 25 November is plotted in Fig. 5. This@gure covers the same area as Figs. 2 and 4. The ridge

A. P�erez et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289 287

Fig. 4. Geopotential height and wind @eld on the 450 K isentropic surface at 12 h UTC for 25 November 1996 in the Northern Hemispherefrom the Equator to 90◦N latitude and from 90◦W to 10◦E longitude. Height scale is in kilometres.

located westward of the Iberic peninsula indicates thepresence of the Azores high-pressure cell, a subtropicalpermanent warm anticyclone. Warm anticyclones intensifywith height and develop ridges on upper-level surfaces like

the one previously discussed in Fig. 4(b) (Dutton, 1986).Air Howing on these surfaces could be forced to rise andexpand. Thus, the Azores high-pressure cell would haveperturbed the lower stratosphere How forcing the adiabatic

288 A. P�erez et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 283–289

Fig. 5. Geopotential height of the 1000 mbar isobaric surface at 12 h UTC for 25 November 1996 in the Northern Hemisphere from theEquator to 90◦N latitude and from 90◦W to 10◦E longitude. Height scale is in metres.

vertical displacement of air parcels as observed in thethree-dimensional trajectory in Fig. 3. The adiabatic upliftof the air parcels would have contributed signi@cantly to thetotal ozone and lower stratosphere temperature decreasesobserved over Madrid on 25 November 1996.

4. Conclusions

The low ozone event detected in Madrid (Spain, 3◦W,41◦N) on 25 November 1996 was accompanied by air tem-perature decreases in the lower stratosphere. The total ozonecolumn over Madrid on 25 and 26 November was as low asthe lowest value recorded by the TOMS satellite instrumentin the period 1979–1993 over that location.

The anti-correlation found between total ozone andgeopotential height following the motion in the lowerstratosphere suggests that the vertical adiabatic displace-ment of the air parcels contributed to the low ozone eventover Madrid on 25 November 1996. As a consequence ofthe adiabatic expansion associated to the uplift, the tem-perature of the air parcels decreased as it was observed.The perturbation of the mid-Atlantic lower stratosphereby the Azores warm anticyclone to the West of the Ibericpeninsula forced the vertical displacement of the air parcelscausing the ozone and temperature decreases observed.This suggests that the low ozone event in Madrid (Spain,41◦N, 3◦W) on 25 November 1996 was not driven by thechemical destruction of ozone.

Acknowledgements

This work has been partially supported by CICYT, Co-munidad de Madrid and Programa de Cooperaci-on conIberoam-erica (Spain).

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