Uncertainties in upper stratospheric ozone trends from 1979 to 1996

The time series of differences in coincident measurements of ozone by Stratospheric Aerosol and Gas Experiment (SAGE) and by Solar Backscattered Ultraviolet (SBUV), SBUV/2, Umkehr and Microwave Limb Sounder (MLS) are analyzed, and the slopes in the differences are calculated. SAGE ozone measurements are also compared against those by HALOE. The purpose of these comparisons is to look for statistically significant nonzero slopes which could indicate long-term calibration problems in one or more of the measurement systems. It is found that the slopes are remarkably similar between the Northern and Southern Hemisphere midlatitudes, and, apart from a few exceptions, the slopes are also similar in the tropics. Slopes of MLS-SAGE differences and HALOE-SAGE trends from approximately 1992 to 1996 have values of approximately 20.5 6 0.4% yr (95% confidence limits) in Umkehr layers 7–9 (which are centered at ;37, 42, and 47 km altitude). Umkehr-SAGE slopes for 1979–1996, however, are almost all positive and in the range 20.1–0.4% yr for Umkehr layers 4–8, while SBUV-SAGE slopes for 1979–1989 are essentially zero in layers 4–7 and 0.3–0.4% yr in layers 8 and 9. Averaging all these results with SBUV-SAGE II slopes from 1985 to 1989, the other sensors minus SAGE slopes are most likely between 0.2 and 20.2% yr from ;20 to 40 km altitude. The results indicate slightly negative slopes in Umkehr layers 5–7 and positive slopes in the other three layers. There thus appears to be no overall drift in the SAGE ozone measurements from 1979 to 1996, but SAGE sunrise/sunset trend differences .40 km altitude, combined with the more accurate SBUV-SAGE slopes for 1979–1989, suggest a most likely slope range of 0.4 to 20.4% yr between 40 and 50 km altitude. SBUV/2 measurements from 1989 to 1994 have an upward trend with respect to SAGE measurements of ;0.7% yr with some altitudinal structure; this slope exceeds the estimated 95% uncertainties on the SBUV/2 trends.