A low dimensional model of extratropical low frequency variability

It is shown that the low-frequency and large scale variability of an intermediate complexity reference model can be reproduced faithfully by a simplified model of 10 independent variables and 10 equations.

The reference model is a quasi-geostrophic baroclinic model. The low-order model is based on the truncated projection of the reference model equations on the empirical orthogonal functions of its output. A closure term is shown to be essential for good performance of the low-order model. This closure term is meant to reproduce all the neglected scales and all the scale interactions, mainly baroclinic eddy forcing, that drive the large scale flow. The closure is built as an empirically defined function of the large scale flow of the model, relying on a long previously computed library of tendency differences between the full and truncated model. Other closure schemes, a constant and a stochastic one, are also used and compared to the empirical one. The low-order model exhibits the same climate as the reference as well as the same low-frequency variability and weather regimes.

The linearized version of this model around different basic states is also used. The analysis of the eigenvector spectrum and of the singular spectrum of the linear operators permits different consideration on the slow modes of variability of the modes and on their forcing mechanisms. Results are discussed.