Update of Umkehr ozone profile data trend analysis through 1997

An extended update of both nonseasonal and seasonal trend analysis for northern hemisphere Umkehr ozone profile data from 1977 through 1997 is presented. The nonseasonal or year-round trend analysis reaffirms the previous results of significant negative ozone trend in the upper stratospheric Umkehr layers 7, 8, and 8-10 (33-48 km) of about -0.5% per year, with little trend in the middle stratospheric layer 5 (24-28 km). In addition, the Umkehr data show a substantial negative trend in the lower stratospheric Umkehr layer 2-3 (10-19 km) of about -0.4% per year. Two procedures for the correction of the Umkehr measurements for errors due to volcanic aerosols are considered, and trend results are quite similar using both aerosol-correction procedures. With respect to the seasonal trend aspects, the analysis indicates relatively small trend differences over seasons in the upper layers, where trends in the winter and spring seasons are only slightly more negative than those in the summer and fall seasons, by about -0.1% per year. In the lower stratospheric layer 2-3, however, a strong seasonal pattern in trends is observed in the midlatitude Umkehr data, with the most extreme negative trends, of about -0.6% per year, occurring in the spring and winter, the least negative trends, of about -0.2% per year, occurring in the fall, and the summer trends of about -0.4% per year are intermediate. Also, the trends in layer 2-3 are substantially more negative for the higher latitude range of 40-52°N than for the lower latitude range of 26-39°N, by about -0.3% per year during all seasons, with even larger differences in the spring.