1. Federal Departement of Home Affairs FDHA Federal Office of Meteorology and Climatology MeteoSwiss Revisiting Swiss temperature trends 1959-2008* Paulo Ceppi, Simon C. Scherrer, Andreas M. Fischer, Christof Appenzeller Federal of Office of Meteorology and Climatology MeteoSwiss 16 July 2010 11 th International Meeting on Statistical Climatology, Edinburgh UK * submitted to the International Journal of Climatology

2. 2 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Motivation Temperature trends: The hallmark of climate change IPCC 2007, WG1  seasonal differences  different processes on different spatial scales  local climate change = large scale + local processes new Swiss grid

3. 3 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Outline  Geographical setting, input data and gridding  What are the linear temperature trends (yearly / seasonal)?  Can regional climate models explain the observed trends?  What is the trend contribution of … large scale circulation? … local factors?  Conclusions

4. 4 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Geographical setting and input data  91 homogeneous station series, 1959-2008  station altitude range: 203 to 3580 m asl  ~2 km x 2 km gridded data set Edinburgh Switzerland

5. 5 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Monthly anomaly gridding climatological distance λ  spatial interpolation of altitude corrected residuals  influence of topography determined independently every month C. Frei, MeteoSwiss valley setting λ = 0.01 λ = 0.05λ = 0.1 summit setting

6. 6 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Gridded temperature anomalies Example December 2009 good representation of local/altitude effects! C. Frei, MeteoSwiss topography

7. 7 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Yearly temperature trends 1959-2008 (°C/10yrs) average OLS trend: +0.35°C/10yrs  stronger trends than in global mean (+0.13°C/10yrs)  highly significant trends everywhere (p < 0.0005)  small spatial variability  no altitude dependence

8. 8 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Seasonal temperature trends 1959-2008 (°C/10yrs) °C/10yrs  trends positive, (almost) everywhere significant  large seasonal differences of the trends [°C/10yrs]  autumn weak (+0.02-0.38), summer strong (+0.34-0.62)  clearly larger than NH-land (exception autumn)  altitude depences in autumn (stronger for low altitudes) NH-land trend [°C/10yrs] density SON JJA MAM DJF

9. 9 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Can climate models reproduce warming? Northern Switzerland ENSEMBLES models (smoothed) observed change (smoothed) anomaly [°C] wrt 1961-1990 20yr Gaussian smoothing but: models are NOT forced with observed circulation!  Can observed circulation changes explain the differences?

10. 10 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Different warming by circulation changes? Linear regression model Quantify effect of circulation on T via regression model: principal components (PCs) of geopotential height at 500 hPa over N-Atlantic/Europe model calibration: 1959-83 / validation: 1984-2008 choice of PCs by „stepwise selection“ observed temperatures “modell error” other effects atmospheric circulation

11. 11 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Circulation effects on temperature Swiss mean 1959-2008 trend lines 1959-2008 observed / modelled trend (°C/10yrs) observed / modelled temperatures bright: calibrationdark: validation 0.35 0.13 0.39 0.21 0.46 0.07 0.39 0.08 0.17 0.02

12. 12 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Summary temperature trend analysis Switzerland 1959-2008 [°C/10yrs] DJFMAMJJASONYEAR observed 0.400.39 0.46 0.17 0.35 circulation 0.21 (54%) 0.08 (21%) 0.07 (15%) 0.02 (12%) 0.13 (37%) residual 0.19 0.31 0.39 0.15 0.22  Can regional climate models account for the residual (circulation corrected) trends?

13. 13 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Summary temperature trend analysis Switzerland 1959-2008 [°C/10yrs] DJFMAMJJASONYEAR observed 0.400.39 0.46 0.17 0.35 circulation 0.21 (54%) 0.08 (21%) 0.07 (15%) 0.02 (12%) 0.13 (37%) residual 0.19 0.31 0.39 0.15 0.22 RCM 0.160.180.16 0.24 0.18

14. 14 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch trend (°C/10yrs) Altitude dependence gridded temperature trends 1959-2008 negative anomaly °C/10yrs altitude [m asl] positive anomaly 0.6 0.4 0.2 0 -0.2

15. 15 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Swiss temperature trend anomalies 1959-2008 [°C/10yrs] month-to-month evolution altitude [m asl] month of the year more trend than all altitude average less trend than all altitude average 3000 2500 2000 1500 1000 500 DecJanFebMarAprMayJunJulAugSepOctNovDec changes in fog/visibility?

16. 16 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Swiss temperature trend anomalies 1959-2008 [°C/10yrs] incl. mean temperature evolution [°C] altitude [m asl] month of the year more trend than all altitude average less trend than all altitude average 3000 2500 2000 1500 1000 500 DecJanFebMarAprMayJunJulAugSepOctNovDec

17. 17 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch altitude [m asl] month of the year Swiss temperature trend anomalies 1959-2008 [°C/10yrs] … and mean snow pack [cm] more trend than all altitude average less trend than all altitude average 100 30 5 100 30 5 snow-albedo effect? 3000 2500 2000 1500 1000 500 DecJanFebMarAprMayJunJulAugSepOctNovDec

18. 18 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Conclusions  Swiss temperature trends stronger than NH-land trend (  1.6)  Large seasonal differences, altitude dependence in autumn  ~50% of the trend in winter due to changes in circulation – much less in other seasons  Regional climate models underestimate circulation corrected trends in spring & summer  Local processes important for local trends, especially in autumn (fog), spring (snow-albedo effect) and summer

19. 19 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch 21 RCM-GCM-chains from the ENSEMBLES project mean of 21 ENSEMBLES runs Altitude dependences observed, circulation-corrected, regional climate models (RCMs) trend (°C/10yrs) observed trends circulation corrected (residual) trend altitude [m asl]

20. 20 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Circulation corrected series 1959-2008 Signal of global forcing & local effects (e.g. visibility, snow-albedo, soil-atm.)  Can climate models account for these circulation corrected trends?

21. 21 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Observed time series 1959-2008 Swiss mean  Can changes in circulation explain these differences?

22. 22 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Northern Switzerland temperature changes Observation and models (ENSEMBLES RCM) IA range ENSEMBLES models smoothed ENSEMBLES models IA observed changes 20yr smoothed observed change anomaly [°C] wrt 1961-1990

23. 23 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch Altitude dependences of circulation effect observed vs. circulation contribution trend (°C/10yrs) altitude [m asl] observed trend circulation trend