Predictability of seasonal precipitation in the Nordic region

Predictability of seasonal precipitation in the Nordic countries (Denmark, Finland, Iceland, Norway and Sweden) is investigated using a nine-member ensemble of atmospheric general circulation model simulations with prescribed sea-surface temperature from October 1950 until March 1999. The simulations and corresponding observations from 65 stations in the Nordic countries are used to identify large-scale patterns of seasonal precipitation, the predictability of which is investigated. Subsequently, the identified large-scale patterns are used in a statistical downscaling of the model simulated precipitation. The downscaling, which is of the model output statistics type, yields seasonal predictions for the individual stations. The model simulations of precipitation are compared to predictions of precipitation directly from observed sea-surface temperature using a statistical prediction method and no dynamic model. The two different methods give consistent results. It is demonstrated that seasonal precipitation in the Nordic region contains a weak predictable signal in several seasons. The most skilful predictions can be made in spring, especially in the April–June season when precipitation appears to be influenced both by tropical and North Atlantic sea surface temperature. In particular, the North Atlantic Oscillation in winter appears to influence the North Atlantic sea-surface temperature in spring, which in turn has an effect on precipitation in Scandinavia.