Yueyang Lu, Igor Kamenkovich

Oceanic fronts are ubiquitous and important features that form and evolve due to multiscale oceanic and atmospheric processes. For example, large-scale temperature and tracer fronts along the eastward extensions of the Gulf Stream and Kuroshio currents play key roles in the regional ocean environment and climate. The fronts cannot be realistically simulated by numerical models at spatial resolutions that do not resolve the oceanic mesoscale.
This numerical study examines the relative importance of large-scale 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 front, whereas the large-scale current counteracts this effect. 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 represented using an effective eddy-induced velocity (EEIV), which is the speed at which eddies move tracer contours. The advantage of this formulation is that the frontal sharpening can be readily reproduced by EEIVs, whereas it cannot be modeled as a diffusive process. A proposed closure (“parameterization”) for EEIV based on large-scale properties shows promise in representing frontogenesis in coarse-resolution simulation. This study demonstrates advantages of using an advective rather than diffusive framework for representing eddy effects in coarse-resolution models.