SURFACE QUASI-GEOSTROPHIC DYNAMICS

The dynamics of quasi-geostrophic flow with uniform potential vorticit y reduces to the evolution of buoyancy, or potential temperature, on h orizontal boundaries. There is a formal resemblance to two-dimensional flow, with surface temperature playing the role of vorticity, but a d ifferent relationship between the flow and the advected scalar creates several distinctive features. A series of examples are described whic h highlight some of these features: the evolution of an elliptical vor tex; the start-up vortex shed by flow over a mountain; the instability of temperature filaments; the 'edge wave' critical layer; and mixing in an overturning edge wave. Characteristics of the direct cascade of the tracer variance to small scales in homogeneous turbulence, as well as the inverse energy cascade, are also described. In addition to its geophysical relevance, the ubiquitous generation of secondary instabi lities and the possibility of finite-time collapse make this system a potentially important, numerically tractable, testbed for turbulence t heories.