Extratropical low-frequency variability as a low-dimensional problem. II: Stationarity and stability of large-scale equilibria

Stationarity and stability properties of large-scale persistent anomalies in the northern hemisphere are addressed. The low-order model developed in Part I is used for this purpose. It was obtained as the projection of a three-level quasi-geostrophic system on the ten leading empirical orthogonal functions. Three global quasi- stationary states are identified, which represent the Arctic high and the positive and negative phases of the main teleconnections (Pacific North American and North Atlantic oscillation). The quasi-stationary solutions have only a partial correspondence to the weather regimes found in Part 1. Stability analysis shows a growing mode that describes an oscillation between the two phases of the teleconnections. The negative teleconnection state is also shown to be much more stable than the other two. Possible decay mechanisms are also discussed.