Generation of Mean Flows and Jets on a Beta Plane and over Topography

Abstract

This paper proposes and discusses mechanisms whereby mean flows and jets are produced by differential rotation and by topographic effects. It is shown that, in general, a mean gradient of potential vorticity not only inhibits the cascade of energy to large scales but directly produces anisotropic structures. Scalings for this are examined on the β plane using ideas from classical phenomenology. The scalings are naturally anisotropic and predict the formation of zonal flows directly through a turbulent cascade. Numerical simulations and two-point closure calculations qualitatively confirm the predictions. Also, simulations of barotropic flow on the β plane can produce zonal jet structures of exceptional persistence over many eddy turnover times. Unsteady flow over topography generally produces a mean flow with a correlation between streamfunction and topography, with anticyclonic motion over humps. If the topography is shallow (or the flow sufficiently energetic) the mean streamfunction will be of a scale similar to that of the topography. For sufficiently steep topography, shelf (topographic Rossby) waves prevent the flow from achieving the scale of the topography, and may lead to the formation of jets parallel to but with a narrower scale than the topographic slope, superimposed on the topographically rectified mean flow. Such mechanisms can produce poleward undercurrents along oceanic boundaries and equatorward undercurrents on western boundaries. Similar mechanisms can produce alternating jets, superimposed on a mean vortex, around isolated seamounts.