The Role of Stratification in the Formation and Maintenance of Shelf-Break Fronts

Abstract

A mechanism is described for the formation or a front at the edge of a continental shelf in an initially linearly stratified fluid lacking horizontal density gradients. A primitive equation numerical model is used with a specified vertically uniform inflow imposed at the upstream boundary, and the flow is allowed to evolve alongshelf under the influence of bottom friction. As the flow progresses downstream, the shelf water moves steadily offshore due to the Ekman flux concentrated in the bottom boundary layer. This offshore flow transports light water under heavier water, which leads to convective overturning and ultimately a vertically well-mixed density field over the shelf. Large cross-shelf density gradients appear along the bottom at the shelf break where the vertically well-mixed shelf water abuts the linearly stratified water on the upper slope. At the shelf break, the bottom boundary layer detaches and continues offshore along upward sloping isopycnals. Neutrally buoyant particles in the bottom boundary layer over the shelf move offshore until reaching the shelf break, where they ascend along isopycnal surfaces. Near-surface particles over the outer shelf move alongshelf rapidly with small offshore velocities, while near-surface particles over the inner shelf are drawn shoreward and downwell at the coastal boundary to feed the bottom boundary layer. The basic frontogenesis process is quite robust over a wide range of values of model parameters. The frontogenesis mechanism has important implications for observations of shelf-break fronts as well as for the biology and chemistry of these fronts.