Toward a dynamical understanding of planetary-scale flow regimes

Abstract

The method assumes a nonlinear dynamical model of the atmospheric motion, and determines a subspace of the phase space of the model in which multiple quasi-stationary solutions of the equations of motion are likely to be located. The axes that generate this subspace are the vectors that possess the smallest amplitude of the time derivative computed from a linearized version of the model, using the time-mean state of the system as a basic state. These vectors are called here "natural vectors' and are shown to be eigenvectors of a self-adjoint operator derived from the linearized model. A three-level quasigeostrophic model in spherical geometry is adopted as the dynamical model. Neutral vectors are computed using the observed mean atmospheric state in winter as a basic state; alternative basic states, in which the eddies in the time-mean state are partially or fully removed, are also used in sensitivity experiments. The apatial patterns of the leading neutral vectors are relative insensitive to variations in some model parameters, but are strongly controlled by the form of the basic state; such dependence can be understood in terms of linear planetary-wave theory. -from Authors