Abstract

Density fronts are common features at the ocean surface. Field observations and numerical simulations have shown that the sharpening of frontal gradients, or frontogenesis, can spontaneously generate inertia-gravity waves. Although significant progress has been made in describing frontogenesis using quasi-balanced approximations, most notably semi-geostrophic theory (Hoskins, 1982), these models omit waves. In this talk I will present a new model of frontogenesis that describes the spontaneous generation of inertia-gravity waves from an initially balanced front. A key feature of the model is that it permits large strains (i.e. on the order of the inertial frequency), meaning that it is applicable to the ocean submesoscale. The model predicts wave emission due to frontogenesis to be exponentially small for small values of strain but significant for larger strains. Furthermore, generated waves are trapped by the oncoming strain flow and can only be released when the strain field weakens, leading to wave emission that is strongly localised in both time and space. The analytical results compare well with the output of fully non-linear numerical simulations.