Abstract
Abstract. Observations of stratospheric ozone from multiple instruments now span three decades; combining these into composite datasets allows long-term ozone trends to be estimated. Recently, several ozone composites have been published, but trends disagree by latitude and altitude, even between composites built upon the same instrument data. We confirm that the main causes of differences in decadal trend estimates lie in (i) steps in the composite time series when the instrument source data changes and (ii) artificial sub-decadal trends in the underlying instrument data. These artefacts introduce features that can alias with regressors in multiple linear regression (MLR) analysis; both can lead to inaccurate trend estimates. Here, we aim to remove these artefacts using Bayesian methods to infer the underlying ozone time series from a set of composites by building a joint-likelihood function using a Gaussian-mixture density to model outliers introduced by data artefacts, together with a data-driven prior on ozone variability that incorporates knowledge of problems during instrument operation. We apply this Bayesian self-calibration approach to stratospheric ozone in 10° bands from 60° S to 60° N and from 46 to 1 hPa (∼ 21–48 km) for 1985–2012. There are two main outcomes: (i) we independently identify and confirm many of the data problems previously identified, but which remain unaccounted for in existing composites; (ii) we construct an ozone composite, with uncertainties, that is free from most of these problems – we call this the BAyeSian Integrated and Consolidated (BASIC) composite. To analyse the new BASIC composite, we use dynamical linear modelling (DLM), which provides a more robust estimate of long-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 upper stratosphere, of both northern and southern midlatitudes, in all four composites analysed, and particularly in the BASIC composite. The BASIC results also show no hemispheric difference in the recovery at midlatitudes, in contrast to an apparent feature that is present, but not consistent, in the four composites. Our overall conclusion is that it is possible to effectively combine different ozone composites and account for artefacts and drifts, and that this leads to a clear and significant result that upper stratospheric ozone levels have increased since 1998, following an earlier decline.
Highlights
The ozone layer in the stratosphere protects the Earth’s biosphere from harmful solar ultraviolet (UV) radiation
The operating periods of all the instrument datasets used for either SWOOSH or GOZCARDS are presented as a spectrum of colours between them; the same is done for the SBUV composites, where we show information related to the time of day at which Equator crossings occur, which will be important later
That we have established the validity of the BAyeSian Integrated and Consolidated (BASIC) approach and constructed an ozone composite from GOZCARDS, SWOOSH, SBUV-MOD, and SBUV-MER, we turn to analysing trends and modes of variability
Summary
The ozone layer in the stratosphere protects the Earth’s biosphere from harmful solar ultraviolet (UV) radiation. The use of ozone-depleting substances (ODSs), including chlorofluorocarbons (CFCs), led to a decline in ozone globally over the latter half of the 20th century Crutzen, 1971; Molina and Rowland, 1974), in the polar regions (WMO, 2011, 2014). The implementation of the Montreal Protocol (MP), which banned the use of most ODSs, has halted this decline, and in some regions there has been a recovery in total column ozone (Solomon et al, 2016). There is large uncertainty in the sign and magnitude of recent trends depending on altitude and latitude, and a clear signal is difficult to determine (Harris et al, 2015)
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