Tidal diffusivity: A mechanism for frontogenesis

Abstract

It is hypothesized that tidal mixing may provide a "diffusivity" mechanism for frontogenesis. It stems from the fact that tidal diffusivity varies in the opposite sense from the water depth, so the vertically integrated diffusivity may exhibit a minimum at midshelf, thus giving rise to a maximum in the property gradient-even in the absence of flow convergence. An analytical model assuming a tidal diffusivity dominated by shear dispersion is used to elucidate the mechanism, which shows additionally that the front is located at a water depth that is about twice the tidal frictional depth-a prediction not inconsistent with some observed fronts. The proposed frontogenesis is demonstrated by numerical calculations using the Princeton Ocean Model (POM), which show the emergence of a front from an initial field of uniform gradient after tides are switched on, and the diagnosis of the numerical solution and its parameter dependence has corroborated the analytical model. It is suggested moreover that this diffusivity mechanism may be extended to the wind-induced mixing to explain the shellbreak front off of the northeastern United States.