PATTERNS OF LOW-FREQUENCY VARIABILITY IN A 3-LEVEL QUASI-GEOSTROPHIC MODEL

To assess the extent to which atmospheric low-frequency variability ca n be ascribed to internal dynamical causes, two extended runs (1200 wi nter seasons) of a three level quasi-geostrophic model have been carri ed out. In the first experiment the model was forced by an average for cing field computed from nine winter seasons; in the second experiment we used a periodically variable forcing in order to simulate a season al cycle. The analysis has been focused on the leading Northern Hemisp here teleconnection patterns, namely the Pacific North American (PNA) and the North Atlantic Oscillation (NAO) patterns, and on blocking, bo th in the Euro-Atlantic and Pacific sectors. The NAO and PNA patterns are realistically simulated by the model; the main difference with obs ervations is a westward shift of the centres of action of the NAO. Rel ated to this, the region of maximum frequency of Atlantic blocking is shifted from the eastern boundary of the North Atlantic to its central part. Apart from this shift, the statistics of blocking frequency and duration compare favourably with their observed counterparts. In part icular, the model exhibits a level of interannual and interdecadal var iability in blocking frequency which is (at least) as large as the obs erved one, despite the absence of any variability in the atmospheric e nergy sources and boundary conditions on such time scales.