Characteristics of Episodes with Extremely Low Ozone Values in the Northern latitudes 1957-1999
Bojkov R.D. and D.S. Balis
[abstract] On the back of the well established atmospheric ozone decline in the northern mid-and polar latitudes in the autumn, a number of episodes are observed when the total ozone for two-three days has fallen bellow 220 matm-cm, with absolute minima in most cases below 200 matm-cm. The occurrences of such episodes represent ozone deviations of about one third from the pre-1976 Oct-Nov-Dec monthly mean! Extremely low ozone (ELO3) episodes are registered, almost always, first over the Atlantic at ~400N/600W, moving then northeastward frequently reaching Scandinavia and Northern Russia. In order to look at these events in retrospective the entire data sets for the ground stations and TOMS archives were revisited. Clear identification was made of not less than three dozen short spells with extremely low ozone in the 1957-1978 period. In the next twenty-one years (1979-1999), using mainly TOMS data, one can identify close to 50 cases with ozone values falling below 220 matm-cm for longer than 1 day, each time over an area greater than 500,000 km2. The Ozone Mass Deficiency (O3MD) from the pre-1976 average ozone values over the affected area was 2.8 Mt per day, that is four to seven times greater than would be assuming only long-term trend in the Oct-Nov-Dec period. The ELO3 events on the day of their appearance over the N. Atlantic-European region contribute O3MD representing 16% of the deficiency due to the Oct-Nov trend, and ~8% of that for December in the entire 40-650N latitudinal belt. (<220 matm-cm). The O3MD of the greater pool with low ozone (here taken as <260 matm-cm) surrounding the area of the lowest events could contribute in Oct-Nov up to 60% and in December 31% to the deficiency due to the trend. An analysis of the synoptic charts of the upper troposphere and lower stratosphere supported by trajectory analyses on isentropic surfaces 350 and 380 K, show that in most of the events sub-tropical air masses with low ozone content were transported from the Atlantic toward the UK - Scandinavia, and in many cases further to the western sub-polar regions of Russia. This transport was sometimes combined with upward motions above a tropospheric anticyclone lifting low ozone mixing ratios to higher altitudes. The ELO3 events are having a pronounced effect on the vertical ozone distribution profile showing a significant deficiency above the tropopause where, in general, the sub-tropical air is injected. The seasonally averaged area with ELO3 and the associated O3MD has increased during the 1990s, which could be an indication of stronger and/or more frequent sub-tropical air intrusions. Their occurrences could contribute noticeably to the ozone deficiency of the middle latitude ozone during that period.
[keywords] Atmospheric composition and structure; middle atmosphere - composition and chemistry; Meteorology and atmospheric dynamics ; middle atmosphere dynamics
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