The transition to baroclinic chaos on the β-plane

Experiments on two-layer β-plane flows are described. The regime diagrams for both easterly and westerly forcing indicate complex scenarios by which baroclinically unstable flows can become chaotic as the forcing is increased. The transition sequence can involve as many as three different vacillation mechanisms, but also exhibits the periodic window phenomena prevalent in many model dynamical systems. The fractal dimension of the chaos at low rotational Froude number F is measurable and is somewhat less than 3. The dimension increases as F is raised. A six-wave low-order model, while successfully predicting some of the observed vacillations, gives a relatively poor description of the chaos.