A Numerical Study of a Shallow Sea Front Generated by Buoyancy Flux: Generation Mechanism

Abstract

We studied numerically the frontogenesis of shallow sea fronts such as are observed in the Kii Channel, Japan, during winter, under conditions of sea surface cooling and buoyancy influx from coast and open ocean. Numerical experiments were carried out in a vertically two-dimensional basin with a new model (NH-model), without using the hydrostatic approximation and the convective adjustment. Considering the vertical acceleration term in the momentum equation, intermittent gravitational convections with a large aspect ratio were produced in the frontal region to intensify the horizontal convergence and to strengthen the horizontal density gradient. Consequently, a front in the tracer distribution had a sharpness comparable to the observed front in the Kii Channel and 3.6 times the sharpness in an H-Model, using the hydrostatic approximation and the convective adjustment. In the present model situation, this effect of gravitational convections on sharpening a front in the NH-model is equivalent to that of the 10-time cooling rate in the H-model on the time average. Moreover, the convections intermittently intensified the sharpness of the front up to 2.4 times the time-averaged value: In the H-model, such a sharp front could not be formed even if the cooling rate was increased 10 times. Further, we discussed the effect of eddy viscosity and diffusivity on frontogenesis.