Ozone trends deduced from combined Nimbus 7 SBUV and NOAA 11 SBUV/2 data

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Abstract
The long‐term time series of global ozone from the Nimbus‐7 SBUV (Nov. 1978–June 1990) are extended through June 1994 by using measurements from the NOAA‐11 SBUV/2. The data sets are merged based upon comparisons during the 18‐month overlap period in which both instruments were operational. During this period, the average offset between the two time series is less than 2% in total ozone, and less than 6% in Umkehr layers 1–10. A linear‐regression trend model is applied to the extended time series to calculate updated trends as a function of latitude and altitude. Trends through June 1994 are 1.5‐2% per decade less negative than through June 1990 in the tropical middle stratosphere (35–40 km) and in the upper stratosphere (45–50 km) at mid‐latitudes. In the lower stratosphere, the trends are nearly 1.5% per decade more negative in the southern hemisphere tropical regions to 25°S, but remain relatively unchanged elsewhere. The seasonal structure of the total ozone trends is similar to past trend study results, but the magnitude of the seasonal trend can vary by 2% per decade depending on the length of the time series. Both TOMS (through April 1993) and SBUV total ozone time series show small negative trends in the equatorial region, though they are not statistically significant at the 2‐σ level.