Passive Tracer Dispersion by Idealized Flows across Rossby Numbers

Abstract In this work, we compare and contrast the phenomenological changes in passive tracer dispersion at asymptotically small to O(1) Rossby numbers using idealized two-dimensional flows generated by a two-vertical-mode model. With increasing flow Rossby numbers, we find that the forward flux of tracer variance increases monotonically and the tracer variance spectra show severe depletion of the tracer field, indicating enhanced stirring of the tracer by higher Rossby number flows. On examining the physical structure of the tracer flux and its connection to strain- and vorticity-dominant regions in the flow, we find that a major share of the tracer flux is located in high-shear, strain-dominant regions between coherent vortices at low Rossby numbers, while at higher Rossby numbers, the tracer flux is primarily located in vorticity-dominant regions that are composed of fragmented bits of vorticity. The tracer field is anticorrelated with the tracer flux, i.e., tracer variance is higher in physical regions where tracer flux is lower and vice versa. Our results highlight multiple anisotropic features of submesoscales that enhance tracer dispersion at O(1) Rossby numbers and emphasize the need to take these into account while developing parameterizations for large-scale models.