Climate change and circulation types in the Alpine region

The frequency of circulation types over the Alpine region is explored using 20 different global and regional climate model chains. The projected changes in these circulation types are investigated for the 21st century using the SRES A1B scenario. The multi-model approach relies on the climate models from the ENSEMBLES project and circulation type classifications provided by the COST Action 733. For the latter, the two circulation type classifications GWT (Grosswetter-types) and CAP (Cluster Analysis of Principal components) are selected. GWT is applied to sea level pressure and geopotential height at 500 hPa whereas CAP is applied to sea level pressure. Overall, the ensemble of climate models captures the frequency of individual circulation types well, as shown by the comparison of circulation types from climate models and re-analysis data between 1980 and 2009. Discrepancies occur during winter (DJF) when westerlies are overestimated both at the sea level and at the 500 hPa geopotential height level. The model spread is largest during summer. The frequency of circulation types is simulated best during spring and autumn irrespective of the applied circulation type classification. The analysis of circulation types in the climate projections indicates that in winter easterlies are expected to decrease mostly at the benefit of westerlies until the end of the 21st century. In summer projected changes depend on the height level considered. At sea level westerlies are projected to decrease while easterlies increase markedly in their frequency. This change is not occurring on the 500 hPa geopotential height level.