Patterns of low-frequency variability in a three-level quasi-geostrophic model |
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| Authors: | S Corti A Giannini S Tibaldi F Molteni |
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| Institution: | (1) Atmospheric Dynamics Group, Department of Physics, University of Bologna, Italy, IT;(2) CINECA, Casalecchio di Reno, Bologna, Italy, IT;(3) Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA, US |
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| Abstract: | To assess the extent to which atmospheric low-frequency variability can be ascribed to internal dynamical causes, two extended
runs (1200 winter seasons) of a three level quasi-geostrophic model have been carried out. In the first experiment the model
was forced by an average forcing field computed from nine winter seasons; in the second experiment we used a periodically
variable forcing in order to simulate a seasonal cycle. The analysis has been focused on the leading Northern Hemisphere teleconnection
patterns, namely the Pacific North American (PNA) and the North Atlantic Oscillation (NAO) patterns, and on blocking, both
in the Euro-Atlantic and Pacific sectors. The NAO and PNA patterns are realistically simulated by the model; the main difference
with observations is a westward shift of the centres of action of the NAO. Related 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 particular, the
model exhibits a level of interannual and interdecadal variability in blocking frequency which is (at least) as large as the
observed one, despite the absence of any variability in the atmospheric energy sources and boundary conditions on such time
scales.
Received: 30 January 1997 / Accepted: 17 June 1997 |
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