The mid-latitude total ozone trends in the northern hemisphere

The total column ozone trends derived from satellite measurements at northern mid-latitudes during winter and spring seasons appear to be significantly larger than predicted by recent 2-D photochemical models. Analyses of 13 to 14 years of the Nimbus-7 TOMS total ozone measurements suggest that satellite-derived trends during winter and spring months are influenced by interannual variability associated with dynamical perturbations in the atmosphere. Such perturbations cause large longitudinal spread in total ozone trends at mid-latitudes. For example, the decadal trends in column ozone during February over the 10 degrees latitude band centered at 40 degrees N vary from about -9% to about -4% with error bars of 4 to 6% at the 2 sigma level. During the same month the zonally averaged trend is about -6 +/- 3%. The trends in certain geographical regions or locations, such as the Mediterranean region (35 degrees to 45 degrees N, 0 degrees to 30 degrees E) and Athens, Greece (38 degrees N, 24 degrees E), track the seasonal trends derived from the zonally-averaged data. Given the large uncertainties of the regional and local trends, such similarities may be fortuitous. By using the lower stratospheric and tropospheric temperatures as indices of dynamical variability in the ozone trend analysis, the total ozone trends are reduced by 1 to 3% per decade. For the winter months (December, January and February), the midlatitude trends (zonally averaged) are respectively -3.7 +/- 2.5 and -5.1 +/- 2.8% per decade with and without using the lower stratospheric temperature as an index of dynamical variability.