Multi-decadal trend analysis of total columnar ozone over Thessaloniki Fragkos K.1, Taylor M.2, Bais A.F.1, Fountoulakis I.1*, Tourpali K.1, Meleti C.1, Zempila M.M.1,2 1 Aristotle University of Thessaloniki, Laboratory of Atmospheric Physics, Thessaloniki, Greece 2 Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, USA

COMECAP 2016 Thessaloniki 19 - 21 September 2016 Motivation Total ozone column has undergone a large decline on a global scale due to the effect of ODS. Successful implementation of Montreal’s Protocol led to decline in ODS But, the rate of TOC recovery is low Other factors, mostly connected with anthropogenic-induced climate change are expected to influence future TOC recovery Continuous and high quality TOC measurements are of great importance for monitoring the evolution of the ozone layer and for understanding its variability by dynamical and chemical processes New techniques (non-linear) for time series analysis could provide better insights, in conjunction to the classical linear techniques. The most important message has to do with our much clearer understanding that we're dealing with a coupled and quite complex ozone layer and climate change issue. Between these two, there are connections in science, in human emissions that cause them, and hence also in the policy options for dealing with them. A. Steiner (UNEP Executive Director) COMECAP 2016 Thessaloniki 19 - 21 September 2016

Ground-based Instruments Satellite Instruments TOC measurements TOC measurements are based on remote sensing techniques from the ground and space Ground-based Instruments Satellite Instruments (+) (-) Very high lifetime-> ideal for trend analysis Poor spatial coverage Global coverage Low temporal resolution Relatively simple calibration Their calibration is always based on the “accuracy” of the standard instruments Little influence from weather conditions Homogeneity problems Simple geometric approximations Creation of a dense monitoring network is costly Many different algorithms -> complicated geometry Simple maintenance & High accuracy Clear sky conditions Limited lifetime (& challenging to repair) Ground-based instruments are essential for satellite validation and the detection of possible changes in ozone trends COMECAP 2016 Thessaloniki 19 - 21 September 2016

Brewer Spectrophotometers in LAP The first commercial Brewer spectrophotometer operates at LAP since March 1982. LAP has one of the largest Brewer time series of total ozone column (TOC), suitable for the study of the ozone layer and validation of satellite retrievals. COMECAP 2016 Thessaloniki 19 - 21 September 2016

TOC Time Series in Thessaloniki Mean Value: 326.9 DU, Minimum: 243.9 DU (12/3/1999), Maximum: 509.1 DU (3/7/1987). Minimum during October: 295.99 ± 21.84 (DU), Maximum during April: 365.34 ± 28.28 (DU) COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 MLR Model The change in the trend is on Jan 1997. PWLT (Source: Reinsel et al., 2002) COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 SSA Model SSA is a spectral method like Fourier analysis but its basis functions are not sinusoids (instead they are calculated from the time series itself by embedding) We set the embedding dimension = N/2 (192 months) and extract nonlinear trend, quasi-periodicity and noise components The SSA nonlinear trend resembles long period moving averages (>96 months) COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 Results (1/3) MLR model captures quite well the variability on TOC The SSA reconstruction (including noise term) is lossless and reproduces the TOC time series with zero error to machine precision Noise-free SSA correlates higher with observations than noise-free MLR. COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 Results (2/2) EESC -> Equivalent effective stratospheric clorine. The SSA nonlinear trend very strongly anti-correlates with EESC The minimum in the SSA trend coincides with the maximum of EESC and also the a priori PWLT change-point COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 Results (2/3) TOC trend (DU/year) Period PWLT 1982-1996 -1.26±0.18* 1997-2014 0.52±0.13* * Statistically-significant at the 95% level Pre-turnaround (1997) trend is statically-significant for January and March. There is no statistically-significant post-recovery trend for individual months Trend coefficients are lower during autumn COMECAP 2016 Thessaloniki 19 - 21 September 2016

COMECAP 2016 Thessaloniki 19 - 21 September 2016 Conclusions Thessaloniki has a very long TOC time series, suitable for long-term trend analysis Both models (PWLT-MLR and SSA) have good performance SSA correlates better with observations The selection of 1997 as the pre-turnaround year in the MLR model is verified by SSA Moderate correlation of the MLR trend component with the EESC curve Very high anti-correlation of the SSA trend with EESC The multi-decadal TOC variations in Thessaloniki are mostly driven by ODS. COMECAP 2016 Thessaloniki 19 - 21 September 2016

And we go global… THANK YOU FOR YOUR ATTENTION! Taylor et al (2016) A global description of oscillations and trends in multi-decadal ozone from spectral analysis of zonally-averaged merged satellite data. QOS 2016, 4-9 September, Edinburgh. THANK YOU FOR YOUR ATTENTION! COMECAP 2016 Thessaloniki 19 - 21 September 2016