PASSIVE SCALAR TRANSPORT IN BETA-PLANE TURBULENCE

Evaluation of spectral closure theory and direct numerical simulation are used to examine the eddy transport of a passive scalar in barotropic beta-plane flow. When a large-scale gradient of scalar concentration is imposed, the implied scale separation between fixed background gradient and eddies supports the concept of 'eddy diffusion'. The results can be cast in terms of an eddy diffusion tensor K, whose behaviour as a function of mean vorticity gradient beta is examined. Earlier theoretical work by Holloway & Kristmannsson (1984) is extended to include cases where strong vorticity-scalar correlations are observed, and corrected in order to restore random Galilean invariance. The anisotropy of eddy energy and the direct influence of Rossby wave propagation contribute to the overall anisotropy of K. The resulting suppression of meridional diffusivity K(yy), and enhancement of zonal diffusivity K(xx), with increased beta is examined. The variation in simulation K(yy) is closely reproduced in the closure equations. However, the increased K(xx) is the result of zonal jets whose persistence is not accounted for in the statistical theory.