Seasonal Variations of Ozone Trends

Abstract The long-term trends (least-square linear regression with time) of ozone content at seven European, seven North American, three Japanese and two tropical stations during 21 years (1964–1984) are analysed. In all regions negative trends are observed during the seventies, but are partly compensated by limited periods of positive trends during the late sixties and late seventies. Solely the North American ozone data show negative trends in all 10-year periods. When the long-term ozone trends are evaluated for each month of the year separately, a seasonal variation is revealed, which in Europe and North America has largest negative trends in late winter and spring. While in Europe the negative trends in winter/spring are partly compensated by positive trends in summer, in North America the summer values reach only zero, retaining the significant negative trend in annual mean values. In contrast to the Antarctic ozone hole, the spring reduction of ozone in Europe and in North America is associated with stratospheric temperatures increasing in the analysed period and therefore is consistent with the major natural ozone production and loss processes.