The spread amongst ENSEMBLES regional scenarios: Regional Climate Models, driving General Circulation Models and interannual variability

Various combinations of thirteen regional climate models (RCM) and six general circulation models
(GCM) were used in FP6-ENSEMBLES. The response to the SRES-A1B greenhouse gas
concentration scenario over Europe, calculated as the difference between the 2021-2050 and the
1961-1990 means can be viewed as an expected value about which various uncertainties exist.
Uncertainties are measured here by variance explained for temperature and precipitation changes
over eight European sub-areas. Three sources of uncertainty can be evaluated from the
ENSEMBLES database. Sampling uncertainty is due to the fact that the model climate is estimated
as an average over a finite number of years (30) despite a non-negligible interannual variability.
Regional model uncertainty is due to the fact that the RCMs use different techniques to discretize
the equations and to represent sub-grid effects. Global model uncertainty is due to the fact that the
RCMs have been driven by different GCMs. Two methods are presented to fill the many empty
cells of the ENSEMBLES RCMxGCM matrix. The first one is based on the same approach as in
FP5-PRUDENCE. The second one uses the concept of weather regimes to attempt to separate the
contribution of the GCM and the RCM. The variance of the climate response is analyzed with
respect to the contribution of the GCM and the RCM. The two filling methods agree that the main
contributor to the spread is the choice of the GCM, except for summer precipitation where the
choice of the RCM dominates the uncertainty. Of course the implication of the GCM to the spread
varies with the region, being maximum in the South-western part of Europe, whereas the
continental parts are more sensitive to the choice of the RCM.