A seasonal trend analysis of Dobson total ozone data that have been critically reevaluated and revised is performed for 29 northern hemisphere stations located between 19¿N and 64¿N latitude using data through 1986. The trend model considered for these data allows for a different linear trend for each month of the year, so that the seasonal as well as the latitudinal and regional nature of the total ozone trend behavior can be examined. The trend model also incorporates the 10.7-cm solar flux series and the 50-hPa equatorial zonal wind series as additional explanatory factors for solar and quasi-biennial oscillation induced ozone variations. Regression random effects models are then used for the individual station seasonal trend estimates to obtain trend estimates as function of latitude for different seasons of the year.

The results of this seasonal trend analysis indicate significantly more negative trends during the winter months (December--March) than during the summer months (May--August), notably at higher latitudes, with the trends in winter becoming more negative with increasing latitude. The trends in the winter are estimated to be of the order of -1.2%-2.1%, and -3.0% per decade for latitudes 35¿N, 45¿N, and 55¿N, respectively, while trends during the summer are of the order of -0.6% per decade with no distinct pattern as a function of latitude. The year-round or annual trend over all latitudes is estimated to be about -0.84¿0.82% per decade. The trends are found to display some regional variation, with trends in Japan being considerably less negative than those in North America and Europe. Sensitivity studies are also performed to investigate the effects on ozone trend estimates due to certain factors such as abnormal ozone behavior in 1983 and 1985, the use of ozone data prior to 1965, and nuclear weapons testing in the early 1960s. The seasonal trend analysis is also performed using published (unrevised) Dobson data. Trend results based on published data are on average less negative than trends from revised Dobson data for European stations, by about 1.0% per decade across all seasons, with only small average differences for stations in North America and Japan. ¿ American Geophysical Union 1990