A weather-type approach to analyzing winter precipitation in France:: Twentieth-century trends and the role of anthropogenic forcing

The relationship between large-scale atmospheric circulation and November-March precipitation over France during the twentieth century is investigated. A long daily MSLP dataset is used to derive daily weather types that are discriminant for precipitation. A linear regression model is then used to relate the November-March-accumulated precipitation amount and the occurrence frequency of the weather types. This simple model shows that an important part of the interannual variability of precipitation is directly linked to large-scale circulation changes. Trends in observed precipitation and precipitation series reconstructed by regression are computed and compared. Spatially coherent trends in November-March precipitation during the second half of the twentieth century are observed, with an increase in the north and a decrease in the south. The spatial pattern of the trends in reconstructed precipitation is very similar to that observed, even if an underestimation of the positive trends in the north is seen, indicating that other mechanisms play a role. A detection study then leads to a better understanding of the respective roles of anthropogenic forcing ( greenhouse gases and sulfate aerosol) and sea surface temperature in the evolution of the weather-type occurrence. Finally, it is shown that intratype dynamical variability has also played a role in precipitation changes in northern France, whereas no impact of temperature changes is seen.