Peter L. Read, Dept. of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU. UK. email: p.read1@physics.ox.ac.uk
The spontaneous emission of inertia-gravity waves (IGWs) in the presence of a time-dependent, baroclinic flow is an important paradigm for the emergence of spontaneous imbalance in the atmosphere and oceans. However, such a phenomenon, and the mechanisms which initiate it, are difficult to study in observations and numerical models of the atmosphere or oceans, and is also highly challenging to analyse realistically in idealised theoretical models. Conditions conducive to this process can be obtained, at least under some circumstances, in rotating, stratified flows in the laboratory, though it has only been within the past 5-10 years that substantial evidence has begun to emerge which confirms that it occurs on a laboratory scale and which have enabled its behaviour to be characterized.
In this presentation we will give an overview of the discovery and subsequent characterisation of inertia-gravity wave generation in rotating, stratified flow in a rotating annulus experiment. The phenomenon is most clearly seen in a cylindrical tank with two-layer stratification, in which two immiscible fluids of different density were placed in a tank and rendered baroclinically unstable by differential rotation of a lid in contact with the upper fluid layer. The appearance of IGWs in the flow became apparent when using a novel polarimetric measurement technique to visualize and map perturbations on the interface. Short wavelength wave trains are typically found at particular locations within the large-scale baroclinic wave flow, and propagate azimuthally at a phase and group velocity which is consistent with interfacial inertia-capillary-gravity waves (ICGWs, for which interfacial tension plays a significant modifying role) in the deep limit. More recent work has begun to elucidate the dependence of ICGW emission on parameters such as the Rossby and Froude numbers, indicating significant differences from dependencies anticipated from shallow-water theory, and to investigate possible mechanisms for their generation. These and other properties of these waves will be reviewed in the context of these two-layer experiments. The prospects for finding analogous IGW emission in other experimental configurations, e.g. with continuous stratification, will also be considered, and possible generalizations to systems of geophysical significance will be discussed.