A numerical study of a shallow sea front generated by the buoyancy flux

Abstract

We numerically investigated the physical process of water exchange caused by fluctuations of the front. This front is formed in a vertically two-dimensional NH-model (non-hydrostatic model) under steady forcing and simulates well the front observed during winter in the Kii Channel, Japan. The velocity field in the model has two kinds of oscillations. The first has a period of 6∼12 hr and is caused by intermittent gravitational convection in the frontal zone. The period and the intensity of intermittent convection are determined by buoyancy flux through the side boundaries as well as surface cooling. The other is associated with large scale circulation driven at the side boundaries and is controlled by the Coriolis force and the bottom stress. Its period of 3∼4 days is determined by the sum of the inertial period and the spin down time for the baroclinic mode of the along-front velocity component. These oscillations make the position of the front fluctuate with the same periods.

We next examined water exchange across the fluctuating front by numerically tracking a number of labelled particles. Intermittent convection induces exchange of particles in the frontal zone and large scale circulations transport the exchanged particles toward offshore or onshore through the lower layer. The exchange rate and the dispersion coefficient are calculated in the NH-model as 0.85 and 2.3×10³ cm² sec⁻¹, respectively. On the other hand, in the H-model (hydrostatic model) parameterizing gravitational convections with a convective adjustment method, these values are reduced to 0.68 and 3.2×10² cm² sec⁻¹, respectively. This result implies that intermittent convections in the frontal zone have a large effect on water exchange across the front, and that no little water is exchanged across the fluctuating front in an actual shallow sea, such as observed in the Kii Channel.