Preferred Regime Transition Routes and Evidence for an Unstable Periodic Orbit in a Baroclinic Model

Abstract

The phase space structure of the attractor of a baroclinic quasigeostrophic model is analyzed in terms of mean tendencies in dominant EOF space. The dominant EOFs as well as the probability density functions in the planes spanned by the dominant EOFs are similar to corresponding features of the NCAR general circulation model, the Community Climate Model (CCM0). It is found that the mean motion is characterized by a cyclic motion involving the Northern Hemisphere annular mode (NAM), the Pacific–North American (PNA) pattern, and a third pattern. States with positive (negative) projection onto this third pattern are characterized by a mean tendency toward increasing (decreasing) NAM and PNA amplitudes. The time scale of the cycle is about 20 days, and it compares well with an observed westward-propagating wave. Three regimes are identified in the space of the dominant EOFs. Preferred transition routes between these regimes exist and correspond to particular phases of the cycle. The authors hypothesize that the cycle corresponds to an unstable periodic orbit that is a remnant of a heteroclinic connection between the stationary points underlying the regimes.