Mesoscale Eddy-Induced Sharpening of Oceanic Tracer Fronts

Abstract

Oceanic fronts are ubiquitous and important features that form and evolve due to multiscale oceanic and atmospheric processes. Large-scale temperature and tracer fronts, such as those found along the eastward extensions of the Gulf Stream and Kuroshio currents, are crucial components of the regional ocean environment and climate. This numerical study examines the relative importance of large-scale currents and mesoscale currents (“eddies”) in the front formation and evolution. Using an idealized model of the double-gyre system on both eddy-resolving and coarse-resolution grids, we demonstrate that the effect of eddies is to sharpen the large-scale tracer front, whereas the large-scale current counteracts this effect and acts to create a broader front. The eddy-driven frontogenesis is further described in terms of a recently proposed framework of generalized eddy-induced advection, which represents all those eddy effects on tracers that are not due to eddy-induced mass fluxes and are traditionally parameterized by isopycnal diffusion. In this study the generalized advection is formulated using an effective eddy-induced velocity (EEIV), which is the speed at which eddies move large-scale tracer contours. The advantage of this formulation is that the frontal sharpening can be readily reproduced by EEIVs. A functional form of EEIV in terms of large-scale variables effectively represents the frontogenesis in a coarse-resolution simulation. This study shows promise for using an advective framework to parameterize eddy-driven frontogenesis in numerical models that are not eddy-resolving.