Journal of Atmospheric and Terrestrial Physics, Vol. 54, No. 1 I/'12, pp. 1447-1451, 1992. 0021 9169;92 $5.00+ .(X) Printed in Great Britain. i 1992 Pergamon Press Ltd

Ozone 'minihole' over Northern Scandinavia

ilk. RABBE and S. H. H. LARSEN

Department of Physics, University of Oslo, Oslo, Norway

( Receired in final/orm 24 January 1992 ; accepted 27 March 1992)

Abstrae~During the first week of February 1990 the total ozone values over England and Scandinavia dropped to a minimum of approximately 200DU. This minimum lasted, however, only for a couple of days at each of the stations, which are measuring ozone in the area. The ozone 'minihole' was not stationary, but it moved from the North Atlantic towards Northern Scandinavia and the Kola peninsula during a 4- day period. From the ozone observations its path was traced over England towards Norway, Sweden and Russia.

In this paper we propose that the "minihole' was mainly created by a strong cyclonic development in the North Atlantic, which moved towards Scandinavia. The cyclone development resulted in ascending air ahead of the cyclone in the troposphere and the lower stratosphere. The total ozone column was thereby reduced, However, this does not exclude the possibility that ozone also could have been reduced due to heterogeneous photochemical processes. The events took place during the CHEOPS Ill-Campaign, at Kiruna in Sweden, and gave most valuable information.

I N T R O D U C T I O N

Ozone variations in the Northern Hemisphere during the wintertime are greatest along Polar front latitudes. This indicates that dynamical processes are the main cause of these variations. Chemical processes may also create variations in ozone amounts, but these processes are generally slower than the dynamical pro- cesses. It is possible that dynamical and chemical pro- cesses interact, for example, through lowering of stratospheric temperature by adiabatic processes to an extent that PSC-clouds form. Most scientists agree that the ozone depletion, which takes place in the Antarctic spring, is caused by chemical reactions. It is therefore reasonable to look for similar effects on ozone in the Arctic.

NEWMAN and LAIT (1988) found moving miniholes in the Antarctic ozone distribution. They ascertained that miniholes appeared close to the Antarctic pen- insula. McKENNA (1989) also studied the Antarctic miniholes during the Airborne Antarctic Ozone Experiment. He concluded that the miniholes are created by synoptic-scale tropospheric advection, both horizontal and vertical.

Very low ozone values were observed over Scan- dinavia during the first week of February 1990. At that time, an extensive ozone observation campaign, the CHEOPS III, was conducted, based at Kiruna in Sweden.

In this paper we attempt to show that the rapid change in the ozone amount over Scandinavia in this case was mainly due to dynamical processes in the

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atmosphere. We assume that ascending and descend- ing motions in the atmosphere are primarily initiated in the troposphere and that the resulting waves and perturbations are extended to the lower stratosphere.

For our analysis we have used 500 hPa weather- maps from Deutsche Wetterdienst in Berlin, as well as maps of vertical velocities at 100 hPa from the European Centre for Mediumrange Weather Fore- casting at Reading in England.

DYNAMICAl, EXPLANATION OF THE 'MIN1HOLE'

It is well known that the Scandinavian sector has an ozone minimum during the winter as compared with other sectors along the same latitudes. In Fig. 1 we compare long-term monthly mean ozone values at Oslo ( 6 0 N , 10°E) and Nagaevo (60'N, 15ff E). Note that Nagaevo has almost 100DU more ozone than Oslo during most of the winter. The explanation might be sought in the general circulation. In the Nor th

DU

500

400

300

r , , ~ - ~ ~ NAGAEVO 1974 ** 1995

. . . z osLo ,978

i i i i i i

Fig. 1. Mean monthly ozone values in DU.

1448 /~k. RABBE and S. H. H. LARSEN

9 o O w , m - P O L , : " so",=

Fig. 2. Solid lines: 500 hPa contours, at intervals of 160 m. Stippled lines: vertical velocity at 100 hPa in hPa. h-~ +descent -ascent. 4-7 February 1990, 00Z.

Atlantic the circulation in the winter is generally southwesterly. In this airstream developing cyclones appear continually, and move quickly towards Scan- dinavia. The cyclonic activity brings about ascending motion ahead of the surface low in the troposphere and this is extended to the lower stratosphere. The ascending air is cooled adiabatically, and the result is a high and cold tropopause and a cold lower strato- sphere. The total ozone column is thereby diminished.

The opposite occurs over Eastern Siberia, East of the Siberian high, where the westerly and north-

westerly airstream has a generally descending motion, which is also partly caused by the mountain range east of the Baikal Sea. The descending motion warms the lower stratosphere adiabatically and the tropo- pause is also lowered. The stratospheric ozone column is stretched downward and ozone rich air flows into the area at higher levels.

In Figs 2-5 we have drawn the 500 hPa contours in solid lines and the vertical velocities at 100 hPa in stippled lines. The contour lines have an equidistance of 160 m, and the vertical velocities are given in

N-POLE 90°W, * ~ 90°E

Fig. 3. Solid lines: 500hPa contours, at intervals of 160 m. Stippled lines: vertical velocity at 100hPa in hPa. h-~ +descent -ascent. 4-7 February 1990, 00Z.

Ozone 'minihole'

90°W' ~,~

(

N-POLE

Fig. 4. Solid lines: 500hPa contours, at intervals of 160 m. Stippled lines: vertical velocity at 100hPa in hPa. h ~ +descent -ascent. 4-7 February 1990, 00Z.

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hPa .h- ' (1 hPa .h- ' equals approximately 70-80 m.h at 100 hPa level).

It can be seen from the figures that the maximum of the vertical velocities is moving in the SW-windfield from the Nor th Atlantic on 4 February to the Kola peninsula on 7 February. The maximum vertical vel- ocities lie a little ahead and to the right of the surface low. This is illustrated in Fig. 6 where trajectories of the maximum vertical velocities and corresponding low pressures are shown. This is in good agreement with REEO'S (1950) finding that ozone minima are

positioned close to high pressure ridges ahead of sur- face lows.

In Fig. 7 we show that the ozone minimum first occurred at Camborne in England, later on in Oslo, Norrkoping, Leningrad and Murmansk. The figures show also that the ozone minimum lasted only for two days at each of the measuring stations. In Fig. 8 we have plotted the - 7 0 ° C isotherm at 100 hPa. This isotherm advanced towards England on 5 February. The next day it retreated toward the Kola Peninsula. This is explained by the adiabatic

N-POLE

Fig. 5. Solid lines: 500 hPa contours, at intervals of 160 m. Stippled lines: vertical velocity at 100 hPa in hPa. h ~ +descent -ascent. 4~7 February 1990, 00Z.

1450 ,~k. RABBE and S. H. H. LARSEN

90°Wl / L [~o. N-P+OLE ] / 190o E

/

o o

Fig. 6. Positions of maximum vertical ascent at 100 hPa in hPa.h- ~ during 4-7 February 1990. [] Positions of surface low.

cooling which took place ahead of the surface low. The ozone values recovered to approximately the

same values as they were before the depression. This rapid rise and fall in ozone strongly suggests that dynamical processes were largely responsible for the observed low ozone values. The temperature at 30 hPa went down to -83°C at Oslo on 6 February, which is below the limit for PSC to form. This indicates that chemical processes could have contributed to the low ozone value. The low temperature lasted only for 1-

D.U,

400

80

60

40

20

300

80

6O

40

20

2OO

80

00

40

20

t00

Oslo / \\~. Norrkeping

',, i /\ ° / f •

/ / '\ / /"

"' 7~-~I / ' , / Leningrad Camborne '~,~fi ~,,,"

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='~ Murmansk

I I I I I I I I I 1. 2. $. 4, & $. T, 8. 9. F F J I R U A l l y 19110

Fig. 7. Ozone variations at ground stations.

2 days which indicate that adiabatic processes were involved.

OZONE MINIHOLE DURING 31 JANUARY 1989

The situation described above is very similar to that which occurred on 31 January 1989. On that date an ozone 'minihole' was also observed with its centre over the Norwegian Sea. It occurred toward the end of a long lasting southwesterly windflow in the North Atlantic associated with cyclonic activity and strong ascending motion in the troposphere and the lower stratosphere. The 'minihole' was replaced by a very strong ozone increase during the two first weeks of February 1989. The increase consisted most likely of three components : (1) as the ascending motion in the Atlantic ceased the horizontal transport of ozone rich air from Canada was allowed, (2) thejetstream, which earlier passed south of Greenland, now crossed the Greenland ice shelf causing strong mountain wave activity and descending motion towards Scandinavia, and (3) sudden stratospheric warming took place, more or less at the same time. All these activities increased the ozone values over Scandinavia from 175DU by the end of January to approximately 600 DU at mid-February.

In contrast, during February 1990, when ozone values did not increase following the passage of the 'minihole' no subsidence in the atmosphere was observed in the area. Also sudden warming in 1990 occurred later on in time.

CONCLUSION

The ozone 'minihole' observed over Scandinavia in the first week of February 1990 was primarily a

Ozone 'minihole'

6 0 ° W ' 6 0 ° E

Fig. 8. Variations of the - 70'C isotherm at 100 hPa.

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meteorological phenomenon . However, since the tem- perature at 30 hPa went below the limit for the for- mat ion of PSCs one canno t exclude the possibility tha t heterogeneous photochemical processes tha t destroy

ozone may have cont r ibu ted to the overall depth of the hole. The low stratospheric tempera tures were, however, most likely created by adiabat ic processes in the a tmosphere .

MCKENNA D. S.

NEWMAN P. A. and LA1T L. R.

REED R. J.

REFERENCES

1989 Diagnostic studies of the Antarctic vortex during the 1987 airborne Antarctic ozone experiment: ozone miniholes. J. geophys. Res. 94, 11,641-11,668.

1988 The morphology and meteorology of southern hemi- sphere spring total ozone mini-holes. Geophys. Res. Lett. 15, 923-926.

1950 The role of vertical motions in ozon~weather relation- ships. J. Meteorol. 7, 263 267.