A Qualitatively Realistic Low-Order Model of the Extratropical Low-Frequency Variability Built from Long Records of Potential Vorticity

Abstract

A low-order deterministic qualitative model is formulated in order to simulate extratropical low-frequency variability. This deterministic model is based on a filtering of the potential vorticity equation on the 315-K isentrope and a projection onto its leading empirical orthogonal functions. The model has an empirical formulation, and the feedback of unresolved scales is taken into account. The model building procedure is novel, since it is not based on a severe truncation of the physical evolution equations but on an empirical analog averaging of each relevant dynamical process. It can be applied to any geophysical system for which long observational data series are available. The model is used to diagnose weather regimes with its multiple equilibria and intraseasonal oscillations as periodic orbits. These equilibria result from the balance between large-scale advection, transient feedback, and residual forcing. The authors analyze their forcing budgets and show in particular that transient feedback tends to amplify and advect upstream the regime anomaly patterns. However, the key forcing turns out to be the large-scale advection, since the other forcing terms only reshape the regime anomalies. The maintenance of observed intraseasonal oscillations is also examined by means of forcing budgets. Results show that large-scale advection and transient feedback are also key dynamical factors in the maintenance of their life cycle. Finally, the low-order model is integrated and qualitatively simulates two of the three identified oscillations, those with periods of 70 and 28 days. The intraseasonal oscillations show up as unstable periodic orbits in the low-order model. This indicates that these oscillations are mostly driven by the internal dynamics of the extratropical atmosphere.