An update on the dynamically induced episodes of extreme low ozone values over the northern middle latitudes

Over the middle latitudes of the Northern Hemisphere (NH), especially during the months of October-December, a number of episodes are observed when the total ozone for more than 2-3 days falls below 220 DU. This value has been introduced as the threshold for the Antarctic ozone hole but also represents ozone deviations of about one-third from the pre-1976 October-November-December monthly mean for the middle latitudes of the NH. Earlier studies have shown that the most common pattern of these events indicates transport of subtropical air from the Atlantic to the northeast frequently reaching Scandinavia and Northern Russia, sometimes combined with upward motions above a tropospheric anticyclone lifting low ozone mixing ratios to higher altitudes. In this study we report on the frequency and spatial extend of such extreme events using the newly available Multi Sensor Reanalysis (MSR) satellite total ozone data set for the period 1978-2008. During the autumn months considered in this study these events result to an ozone mass deficiency (O(3)MD) of 5 tonnes km(-2) and their areal extend on a daily basis often exceeds 1.5 million km(2). More and larger in area events were observed during the 1990s and were possibly influenced by changes in the meridional circulation triggered by the maximum ozone depletion observed then over the middle latitudes of the NH.