Zonation

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Zonal jets are the result of a poorly understood process via which spontaneously arising Reynolds stresses destroy an initial state of spatially homogeneous beta-plane turbulence. The ultimate equilibration of this instability results in a mature pattern of zonal jets, co-existing with turbulence. All properties of the turbulence, particularly the transport of scalars and potential vorticity, are substantially modified by the jets. This project will develop new approaches to quantitatively analyzing the jet-forming instability and parameterizing transport in these flows. The main hypothesis is that the instability is a transfer of energy, non-local in wave-number, from the forcing scale directly to the jet scale. Preliminary results are supportive of this hypothesis and suggest that negative eddy viscosity may connect the momentum fluxes to mean shear. Essential to the proposal is understanding and modeling the nonlinear feedbacks required to saturate the instability. In physical oceanography there are several proposals for the parameterization of eddy heat and scalar fluxes. Lateral or layerwise momentum fluxes have been almost ignored in this discussion, with the assumption that this processes is unimportant on large scales. Compelling observational evidence that zonal jets exist in the ocean, imply a failure of this approximation. The pressing oceanographic problem addressed by this proposal is the origin of negative viscosity momentum fluxes, and implications for the transport of tracers.The dynamics of zonal flows has important implications for oceanography, meteorology, planetary physics and plasma physics. Structure formation in the midst of a turbulent flow is a striking physical phenomenon, and zonal flows driven by turbulent Reynolds stresses are a foremost example. The mechanism of zonal flow saturation and the question of how, quantitatively, zonal flows figure in the regulation of heat and potential vorticity fluxes are not well understood. The project will provide physically-rooted parameterizations of eddy momentum fluxes, which might be used in coarse resolution climate models. A graduate student at Scripps Institution of Oceanography will receive training in the areas of theoretical problem formulation, geophysical fluid dynamics, applied mathematics, analysis and scientific computation.