The long-term variability of atmospheric ozone from the 50-yr observations carried out at Belsk (51.84°N, 20.78°E), Poland

Total ozone (TO₃) and ozone vertical profile (by the Umkehr method) have been measured at Belsk (51.84°N, 20.78°E), Poland, since March 1963. The monthly mean data are analysed for the long-term changes in the period 1975–1996 and 1997–2012, that is, in the increasing and decreasing phases of the ozone-depleting substances (ODS) concentration in the mid-altitude stratosphere over the NH mid-latitudes. Standard explanatory variables are selected for the ozone variability attribution to chemical and dynamical processes. A triad of regression models with various formulae for the trend term is examined to get a synergetic effect. The trend term could be: (1) proportional to ODS, (2) piecewise linear (with the turning points in 1975 – the trend onset and in 1997 – the trend overturning), (3) represented by any smooth curve fitted to the ozone time series having ‘natural variations’ removed. Confirming the results from previous studies on the midlatitudinal ozone, the analyses show a weakening of the TO₃ trend and the statistically significant positive trend in the upper stratospheric region (33–43 km) since 1997. The TO₃ depletion in summer and autumn for the period 1997–2012 is found in the Umkehr data due to the ozone decrease in the lower and mid-stratosphere. A novel statistical-simulation-based test is proposed. It uses the bootstrap sample of the smooth trend pattern to calculate statistical significance of hypotheses for the trend variability. The test corroborates the results of the regression models and shows strengthening of the ozone negative trend in summer and autumn, disclosed in the Umkehr data, since about 2005.