• Introduction
• Data
• Results
• Acknowledgements
• Contact
• References

FIGURE

• Figure 1

1. Introduction

Negative trends of atmospheric column ozone have been detected globally already for two decades [WMO, 1999]. The reduction is characterised by an increasingly heavy chemical ozone loss during late winter and spring in the Southern Hemisphere (SH) polar vortex and a lesser and a more varying loss during late winter and spring in the Northern Hemisphere polar vortex. The reasons to these negative trends are rather well understood. There is nevertheless a lesser negative trend detected also in the mid-latitude lower stratosphere, which is not yet understood. Several possible explanations have been introduced.

These include:

1) heterogenic chemistry on sulphate particle surface,
2) dilution and mixing of processed and ozone poor air from a polar vortex towards mid-latitudes [Brasseur et al., 1997; Eckman et al., 1996],
3) formation of PSC-clouds just outside a polar vortex,
4) homogenic chemistry still missing in the models [Solomon et al, 1994] and natural variation in the stratospheric circulation [Callis et al., 1997; Jackman et al., 1996; Schneider et al., 1991; Hadjinicolaou et al., 1997]. Dilution is believed to happen after the breakdown of a polar vortex in late spring. Mixing is suggested to happen at some extent through the edge of a polar vortex and to a larger extent below the edge of a vortex at the lowermost stratosphere. Better understanding about the chemical and physical structure and mixing properties of the vortex edge region is thus needed.

In Antarctica an abrupt ozone depletion is observed starting in August [e.g. Uchino et al., 1999]. Vömel et al. [1995] and Roscoe et al. [1997] nevertheless reported evidence for an early start already in June of ozone destruction over Antarctica. According to Lee et al. [2000] this gradually progressing depletion starts and stays confined in the edge region of the vortex until the end of July.

In this study, vertical distribution of ozone in Antarctic peninsula station of Marambio is presented. Annual behaviour and long-term ozone changes during 1987-1999 of ozone inside, outside and on the edge region of the SH polar vortex are studied.

2. Data

Long term ozone changes over Marambio, Antarctic peninsula (64.2°S, 57.7°W) have been studied using Dobson spectrophotometer total column measurements for the period of 1987-1999, and ozone mixing ratios at 435 K, 475 K and 550 K isentropic surfaces from ozone sonde measurements for 1990-1999. A classification of Marambios situation in relation to the polar vortex has been made using the potential vorticity and potential vorticity gradient analysis. Three classes have been selected: (i) inside vortex, (ii) outside of vortex and (iii) at the edge of vortex. The method is compared with some generally used other methods, and it seems that the method used in this study gives good results. The classification for November, and for December is characterised by the fact that the vortex is normally in a state of breakdown during these months. During May, June and July there is markedly less data available than during the other months. Eleven year (1957-67) means of total ozone at Argentine Islands (AI) (400 km SW from Marambio) are used as a pre-depletion reference.

A twofold procedure for the ozone sonde data quality assurance has been applied. First the ozone and temperature profiles were inspected visually to detect any obvious failures and anomalous spikes. After removal of the erroneous data the total ozone integrated from the profile was compared to spectrophotometric total ozone from the same day. On the basis of visual and quality check 55 out of total 436 ozone soundings were rejected. The data itself and a more detailed description of the data treatment procedure can be found in ozone.fmi.fi anonymous ftp-server from the /pub/Marambio directory.

3. Results

Long term ozone change over Marambio was studied with linear regression analysis from monthly means of the classified and unclassified data. Standard error of the slope of the fit estimates the uncertainty of the fit. The results are presented in Table 1. The predominant feature of the ozone behaviour over Marambio is decrease. 4/5 of the changes when all cases are included (Table 1 a) are negative and all changes larger than two times their standard error are negative. The decrease is most pronounced in spring during the chemical

During months of January, February, March and April when there is no strong vortex the mean total column ozone at Marambio has been 277 Dobson units (DU), which is 6 % less than the long term mean at AI.

During the August-December period the total column ozone outside the vortex in Marambio has been 305 DU, which is 9 % less than in AI during 1957-67. The Marambio total ozone inside the vortex has been on average 41 % less than in AI. Significant linear ozone trends inside the vortex are found in September at 435 K and 475 K surfaces (-10%/year) and in total ozone (-2 %/year), in October and November in total ozone (-3 %/year) and in November at 435 K surface (-6%/year).

The overall feature of long term ozone over Marambio during 1987-1999 is decline. This is especially so during spring and for the situations when Marambio situates inside the vortex. There is decline to be seen also for the situations when the vortex edge is above Marambio, but it is less pronounced and actuates little earlier in the season (especially during August). The decline is characterised by a strong decline in the beginning of the period and less or absent decline in the latter half of the period. This is very much comparable to the springtime long term lower stratospheric ozone at Syowa, Neumayer and South Pole presented by Uchino et al. [1999]. Absent decline in the latter half of the period imply to the possibility that the chemical ozone depletion might already have reached its maximum in lower stratosphere. The first results from the early spring of 2000 (not shown) nevertheless do not speak in favour of this.

There is no negative trend seen for the situations when Marambio is outside the vortex at lower stratosphere, which does not speak in favour of substantial mixing through the vortex edge or substantial processing of air just outside the vortex. If substantial mixing had been happened, a long-term decline in ozone would have been expected outside the vortex as well. Strong conclusion is nevertheless hampered by scarcity of the outside vortex data.

The earliest significant linear trends in ozone are seen in August at the edge region (Table 1c) and on the other hand already in May (Table 1a) when all cases are included. According to the sonde temperature measurements in Marambio PSC I treshold temperature can be reached occasionally already in May, more often in June and constantly in July. Thus the suggested early start (May, June, July) for the chemical ozone depletion especially in the vortex edge region [Vömel et al., 1995; Roscoe et al., 1997; Wauben et al., 1997; Lee et al., 2000] is supported by the Marambio data.

4. Acknowledgements

Authors wish to address special thanks to all those ozone sounding operators in Marambio, who have worked with dedication in sometimes harsh conditions. We also wish to thank Mayor Mario Garcia for splendid logistical support for the sounding programme.

5. Contact

For further information, please contact: <mailto:JuhaA.Karhu@fmi.fi>JuhaA.Karhu@fmi.fi

6. References

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Callis, L. B., M. Natarajan, J. D. Lambeth, and R. E. Boughner, On the origin of midlatitude ozone changes: Data analysis and simulation for 1979-1993, J. Geophys. Res., 102, 1215-1228, 1997.

Eckman, R. S., W. L. Grose, D. E. Turner, and W. T. Blackshear, Polar ozone depletion: A 3-dimensional chemical modeling study of its long-term global impact, J. Geophys. Res., 101, 22977-22989, 1996.

Hadjinicolaou, P., J. A. Pyle, M. P. Chipperfield, and J. A. Kettleborough, Effect of interannual variability on middle latitude ozone, Geophys. Res. Lett., 24, 2993-2996, 1997.

Jackman, C. H., E. L. Fleming, S. Chandra, D. B. Considine, and J. E. Rosenfield, Past, present and futuremodeled ozone trends with comparison to observed trends, J. Geophys. Res., 101, 28753-28767, 1996.

Lee, A. M., H. K. Roscoe, A. E. Jones, P. H. Haynes, E. F. Shuckburgh, M. J. Morre and H. C. Pumphfrey, The impact of the mixing properties within the Antarctic stratospheric loss in spring, submitted to J. Geophys. Res., 2000.

Roscoe, H. K., A. E. Jones, and A. M. Lee, Midwinter Start to Antarctic Ozone Depletion: Evidence from Observations and Models, Science, 278, 93-96, 1997.

Schneider, H. R., M. K. W. Ko, and C. A. Peterson, Interannual variations of ozone: Interpolation of 4 years satellite observations of total ozone, J. Geophys. Res., 96, 2889-2896, 1991.

Solomon, S., R. R. Garcia, and A. R: Ravishankara, On the role of iodine in ozone depletion, J. Geophys. Res., 99, 20491-20499, 1994.

Uchino, O., R. D. Bojkov, D. S. Balis, K. Akagi, M. Hyashi, and R. Kajihara, Essential characteristics of the Antarctic-spring ozone decline: Update to 1998, Geophys. Res. Lett., 26, 1377-1380, 1999.

Wauben, W.M.F, R. Bintaja, P.F.J. van Velthoven, and H. Kelder, On the magnitude of transport out of Antarctic polar vortex, J. Geophys. Res. 102, 1229-1238, 1997

World Meteorological Organization (WMO), Scientific Assessment of Ozone Depletion: 1998, Rep. 44, Global Ozone Res. and Monit. Proj., Geneva, 1999.

Vömel, H., D. J. Hofmann, S. J. Oltmans, and J. M. Harris, Evicence for midwinter chemical ozone destruction over Antarctica, Geophys. Res. Lett., 21, 2381-2384, 1995.