Characteristics of episodes with extremely low ozone values in the northern middle latitudes 1957-2000

A number of episodes are observed when the total ozone for 2 to 3 days has fallen below 220 matm-cm in the northern mid- and polar latitudes in autumn . The occurrences of such episodes represent ozone deviations of about one- third from the pre-1976 Oct-Nov-Dec monthly mean! By using primarily qualit y checked Dobson data, a clear identification was made of more than three d ozen short spells with extremely low ozone in the 1957-1978 period. In the following twenty-two years (1979-2000), using mainly TOMS data, one can ide ntify similar to 46 cases with ozone values falling below 220 matm-cm for l onger than I day, with each time over an area greater than 500,000 km(2). T he Ozone Mass Deficiency (O3MD) from the pre-1976 average ozone values over the affected area was similar to2.8 Mt per day, i.e. four to seven times g reater than it would be, assuming only a longterm trend in the Oct-Nov-Dec period. The Extremely Low Ozone (ELO3) events on the day of their appearanc e over the N. Atlantic/European region contribute to the O3MD by representi ng 16% of the deficiency due to the Oct-Nov trend in the entire 40-65 degre es N latitudinal belt. The O3MD of the greater pool with low ozone (here ta ken as <260 matm-cm) surrounding the area of the lowest events could contri bute on the day of their appearance in Oct-Nov up to 60% and in December, < similar to>30% to the deficiency due to the trend over the entire 40-65 deg rees N belt. Analysis of synoptic charts, supported by a back-ward trajecto ry on the isentropic surfaces 350 and 380 K, shows that in most of the even ts, subtropical air masses with low ozone content were transported from the Atlantic toward the UK, Scandinavia, and in many cases, further to the wes tern sub-polar regions of Russia. This transport was sometimes combined wit h upward motions above a tropospheric anticyclone which lifted low ozone mi xing ratios to higher altitudes. The ELO3 events cause a significant defici ency above the tropopause where, in general, the subtropical air is injecte d. In fact, the overall amount of ozone is not depleted, but redistributed on the hemispheric scale. Review of low ozone events, defined as days with negative deviations from the pre-1976 averages greater than 25% show, in ge neral, similar origin. The seasonally averaged area with ELO3 and the assoc iated O3MD, as well as for the cases with deviations > -25%, has increased during the 1990s, which could be an indication of stronger and/or more freq uent subtropical air intrusions. Their occurrences could contribute noticea bly to the ozone deficiency of the middle latitude ozone during the days of ELO3 appearances; however, their contribution to the long-term trend of th e ozone seasonal decline is of the order of similar to 10%.