The cold Arctic winter 1995/96 as observed by GOME and HALOE:: Tropospheric wave activity and chemical ozone loss

The Global Ozone Monitoring Experiment aboard the European Remote Sensing Satellite ERS-2 was the only satellite instrument measuring total ozone on a near-global scale during the extremely cold Arctic winter 1995/96. Extremely low total ozone was observed within the Arctic vortex during February and March. The lowest value in this winter was 178 DU (Dobson units) over Greenland on 19 February, which was about 160 DU below the February Arctic vortex mean of total ozone. Although severe chemical ozone destruction occurred in late winter 1995/96, the extremely low values in total ozone observed after the middle of February were, in all cases, related to mini-hole events, where large horizontal divergent transport of ozone from the lower-stratospheric layer above a high tropopause rapidly reduced the total column in a localized region. The observed total-ozone minima were located near the vortex edge and in the region of minimum lower-stratospheric temperatures that were, in selected cases, sufficiently low for the formation of polar stratospheric ice clouds (PSC type 11) below 188 K at the isentropic level of 475 K. Coincident ozone profile observations in early March from the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite indicate that the strong chemical ozone loss was mainly confined to the polar vortex region, and that the extremely low total-ozone values below 250 DU were mainly caused by short-term reversible dynamical reductions superimposed upon chemical ozone loss occurring on longer timescales. Enhanced OClO and chlorine activation, due to strong tropospheric wave activity associated with an ozone mini-hole event, was only observed in early March following a stratospheric temperature drop below the ice frost point. In general, however, the observation of very low total ozone in mini-hole events does not necessarily point to significant additional chemical depletion.