Toward a dynamical understanding of planetary-scale flow regimes
- Imperial College (United Kingdom)
- European Centre for Medium-Range Weather Forecasts, Reading (United Kingdom)
A strategy for diagnosing and interpreting flow regimes is presented and applied to the study of observed and modeled planetary-scale regimes of the wintertime circulation in the Northern Hemisphere. 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. As a prototype of a dynamical system with quadratic nonlinearity relevant to atmospheric dynamics, the three-variable convection model that generates the well-known Lorenz attractor is investigated. It is shown after using the proper time filter that the presence of two unstable stationary solutions generates a strong bimodality in the projection of the state vector of the system onto the most neutral vector. Adopting a three-level quasigeostrophic model in spherical geometry as the dynamical model, neutral vectors are computed using the observed mean atmospheric state in winter as a basic state. The spatial 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. The neutral vectors of the wintertime climatology are then used to analyse a 32-winter sample of observed atmospheric fields. It is found that the time series of the projection of these fields onto one particular neutral vector has a significantly bimodal probability density function, suggesting the existence of (at least) two separate flow regimes associated with anomalies of opposite sign. Finally, if an appropriate forcing function is employed, the quasigeostrophic model is able to generate a very realistic climatology in a long nonlinear integration and, furthermore, two regimes similar to the observed ones. Modeled and observed regimes have not only similar spatial patterns but also an almost identical distribution of the residence time. 42 refs., 17 figs., 3 tabs.
- OSTI ID:
- 6019759
- Journal Information:
- Journal of the Atmospheric Sciences; (United States), Vol. 50:12; ISSN 0022-4928
- Country of Publication:
- United States
- Language:
- English
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