LOW-FREQUENCY VARIABILITY IN A GCM - 3-DIMENSIONAL FLOW REGIMES AND THEIR DYNAMICS

General circulation models (GCMs) can be used to develop diagnostics f or identifying weather regimes. The author has looked for three-dimens ional (3D) weather regimes associated with a 10-yr run of the U.K. UGA MP GCM with perpetual January boundary conditions; 3D low-pass empiric al orthogonal functions (EOFs), using both the 500- and 250-mb streamf unctions (psi) have been computed. These EOFs provide a low-order phas e space in which weather regimes are studied. The technique here is an extension to 3D of that of Haines and Hannachi. They found, within th e 500-mb psi EOF phase space, two local minima of area-averaged psi-te ndency (based on barotropic vorticity dynamics), which were identified as +/-Pacific-North America (PNA). In this work, the author demands t hat both the flow and its tendency be within the phase space spanned b y the 3D EOFs. The streamfunction tendency is computed from the two-le vel quasigeostrophic potential vorticity equation and projected onto t he EOF phase space. This projection produces a finite dynamical system whose singular points are identified as the quasi-stationary states. Two blocking solutions and one zonal solution are found over the Pacif ic. The first blocking solution is closer to the west coast of North A merica than the other blocking, which is shifted slightly westward and has a larger scale, rather similar to the +PNA pattern, indicating th at blocking over the Pacific may have two phases in the model. Further investigation of the GCM trajectory within the EOF phase space using a mixture analysis shows the existence of realistic three-dimensional weather regimes similar to the singular points. The same solutions wer e found when the transient eddy contributions to the climatological qu asigeostrophic potential vorticity budget were included. It is also sh own that this extended technique allows a direct study of the stabilit y of these quasistationary states and helps in drawing transition pict ures and determining the transition times between them.