1. 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

2. 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 September 2016

3. 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 September 2016

4. 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 September 2016

5. TOC Time Series in Thessaloniki
Mean Value: DU, Minimum: DU (12/3/1999), Maximum: DU (3/7/1987).
Minimum during October: ± (DU), Maximum during April: ± (DU)
COMECAP 2016 Thessaloniki September 2016

6. 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 September 2016

7. 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 September 2016

8. 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 September 2016

9. 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 September 2016

10. COMECAP 2016 Thessaloniki 19 - 21 September 2016
Results (2/3)
TOC trend (DU/year)
Period
PWLT
-1.26±0.18*
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 September 2016

11. 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 September 2016

12. 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 September 2016