Wave interactions and baroclinic chaos: a paradigm for long timescale variability in planetary atmospheres

Baroclinic instability is the principal mode of non-axisymmetric flow in the large-scale atmospheric circulation at mid-latitudes, and is responsible for organising the structure and behaviour of major weather systems. This instability can also be fruitfully studied in the laboratory under controlled conditions. In this paper, we review recent work carried out by the authors and collaborators on various routes to chaotic behaviour in rotating, stratified flows. Results include the discovery of new multi-mode regimes in which small ensembles of baroclinic waves interact in a nonlinear mode competition with the thermally-driven axisymmetric component of the flow, generating chaotic oscillatory variability on very long timescales. We discuss various attempts to capture this type of behaviour in simple models, and consider the significance of the phenomenon as a paradigm for understanding the nature of long timescale variability in the climates of the Earth and Mars.