Search for evidence of trend slow-down in the long-term TOMS/SBUV total ozone data record: the importance of instrument drift uncertainty

摘要

We have developed a merged ozone data set (MOD) for the period October1978 through June 2006 combining total ozone measurements (Version 8retrieval) from the TOMS (Nimbus 7, Earth Probe) and SBUV/SBUV2 (Nimbus 7,NOAA 9/11/16) series of satellite instruments. We use the MOD data set tosearch for evidence of ozone recovery in response to the observed levelingoff of chlorine and bromine compounds in the stratosphere. A crucial step inany time series analysis is the evaluation of uncertainties. In addition tothe standard statistical time series uncertainties, we evaluate the possibleinstrument drift uncertainty for the MOD data set. We combine these twosources of uncertainty and apply them to a cumulative sum of residuals(CUSUM) analysis for trend slow-down. For the extra-polar mean between60° S and 60° N, the apparent slow-down in trend is found to beclearly significant if instrument uncertainties are ignored. When instrumentuncertainties are added, the slow-down becomes marginally significant at the2σ level. For the mid-latitudes of the northern hemisphere (30° to 60° N)the trend slow-down is highly significant at the 2σlevel, while in the southern hemisphere the trend slow-down has yet to meetthe 2σ significance criterion. The rate of change ofchlorine/bromine compounds is similar in both hemispheres, and we expect theozone response to be similar in both hemispheres as well. The asymmetry inthe trend slow-down between hemispheres likely reflects the influence ofdynamical variability, and thus a clearly statistically significant responseof total ozone to the leveling off of chlorine and bromine in thestratosphere is not yet indicated.