federal departement of home affairs fdha federal office of meteorology and climatology meteoswiss...

Federal Departement of Home Affairs FDHAFederal 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 201011th International Meeting on Statistical Climatology, Edinburgh UK

* submitted to the International Journal of Climatology

2 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

MotivationTemperature 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 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 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 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Monthly anomaly griddingclimatological distance λ

spatial interpolation of altitude corrected residuals influence of topography determined independently every month

C. Frei, MeteoSwiss

valley setting

λ = 0.01 λ = 0.05 λ = 0.1

summit setting

6 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Gridded temperature anomaliesExample December 2009

good representation of local/altitude effects!

C. Frei, MeteoSwiss

topography

7 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Yearly temperature trends1959-2008 (°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 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Seasonal temperature trends1959-2008 (°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]

dens

ity

SONJJA

MAM

DJF

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)

ano

mal

y [°

C]

wrt

196

1-19

90

20yr Gaussian smoothing

but: models are NOT forced with observed circulation! Can observed circulation changes explain the differences?

10 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

)(tTOBS = + )(tε)(tTCIRC

Different warming by circulation changes? Linear regression model

Quantify effect of circulationon 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

)(10

1

tPCi

ii∑=

β

atmospheric circulation

11 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Circulation effects on temperatureSwiss mean 1959-2008

trend lines 1959-2008observed / modelled trend (°C/10yrs)

observed / modelled temperatures

bright: calibration dark: validation

0.35 0.13

0.39 0.21

0.46 0.07

0.39 0.08

0.17 0.02

12 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Summary temperature trend analysisSwitzerland 1959-2008

[°C/10yrs]DJF MAM JJA SON YEAR

observed 0.40 0.39 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 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Summary temperature trend analysisSwitzerland 1959-2008

[°C/10yrs]DJF MAM JJA SON YEAR

observed 0.40 0.39 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 0.16 0.18 0.16 0.24 0.18

14 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

trend (°C/10yrs)

Altitude dependencegridded temperature trends 1959-2008

negative

anomaly

°C/10yrs

altitude [m

asl]

positive

anomaly

0.6

0.4

0.2

0

-0.2

15 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Swiss temperature trend anomalies1959-2008 [°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

Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

changes in fog/visibility?

16 Revisiting Swiss temperature trends | 11th IMSC, 16 July 2010, Edinburgh simon.scherrer@meteoswiss.ch

Swiss temperature trend anomalies1959-2008 [°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

Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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 anomalies1959-2008 [°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

Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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