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Observations of stratospheric ozone from multiple instruments now span threedecades; combining these into composite datasets allows long-term ozonetrends to be estimated. Recently, several ozone composites have beenpublished, but trends disagree by latitude and altitude, even betweencomposites built upon the same instrument data. We confirm that the maincauses of differences in decadal trend estimates lie in (i) steps in thecomposite time series when the instrument source data changes and (ii) artificialsub-decadal trends in the underlying instrument data. Theseartefacts introduce features that can alias with regressors in multiplelinear regression (MLR) analysis; both can lead to inaccurate trend estimates.Here, we aim to remove these artefacts using Bayesian methods toinfer the underlying ozone time series from a set of composites by building ajoint-likelihood function using a Gaussian-mixturedensity to model outliers introduced by data artefacts, together with adata-driven prior on ozone variability that incorporates knowledge of problemsduring instrument operation. We apply this Bayesian self-calibration approachto stratospheric ozone in 10° bands from 60° S to 60° N and from 46 to 1 hPa (∼ 21–48 km) for1985–2012. There are two main outcomes: (i) we independently identify andconfirm many of the data problems previously identified, but which remainunaccounted for in existing composites; (ii) we construct an ozone composite,with uncertainties, that is free from most of these problems – we call thisthe BAyeSian Integrated and Consolidated (BASIC) composite. To analyse the new BASIC composite, we usedynamical linear modelling (DLM), which provides a more robust estimate oflong-term changes through Bayesian inference than MLR. BASIC and DLM,together, provide a step forward in improving estimates of decadal trends.Our results indicate a significant recovery of ozone since 1998 in the upperstratosphere, of both northern and southern midlatitudes, in all fourcomposites analysed, and particularly in the BASIC composite. The BASICresults also show no hemispheric difference in the recovery at midlatitudes,in contrast to an apparent feature that is present, but not consistent, inthe four composites. Our overall conclusion is that it is possible toeffectively combine different ozone composites and account for artefacts anddrifts, and that this leads to a clear and significant result that upperstratospheric ozone levels have increased since 1998, following an earlierdecline

Reconciling differences in stratospheric ozone composites