Formation and Maintenance of Shelfbreak Fronts in an Unstratified Flow

Glen Gawarkiewicz Woods Hole Oceanographic Institution, Woods Hole Massachusetts

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David C. Chapman Woods Hole Oceanographic Institution, Woods Hole Massachusetts

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Abstract

A depth-averaged model with no density variations was used by Chapman to describe the formation of a passive tracer front at a shelfbreak. The relevance of this frontogenesis mechanism to cases that allow vertical variations is examined by considering the three-dimensional structure of a passive tracer front with explicit finite vertical mixing and bottom boundary layer dynamics. A three-dimensional primitive-equation numerical model is configured in a channel with a continental shelf, slope, and abyssal plain running the length of the channel. A vertically and horizontally uniform inflow is imposed over the shelf, with a large horizontal velocity shear near the shelfbreak. In the primitive-equation model, the offshore flow is concentrated in the bottom boundary layer while the alongshelf flow distribution is similar to the depth-averaged case; the presence of the bottom topography maintains a strong horizontal shear near the shelfbreak above the bottom boundary layer. This velocity shear causes a smooth passive tracer distribution imposed at the inflow boundary to develop strong cross-shelf gradients near the shelfbreak (i.e., a passive tracer front) within a rather short downstream distance, as in the depth-averaged model. Neutrally buoyant Lagrangian particles initialized above the bottom boundary layer are rapidly advected along the shelf with little cross-shelf motion. However, particles initialized within the bottom boundary layer move quickly offshore toward the shelfbreak and beyond while being advected alongshelf relatively slowly. The shelfbreak does not act as a barrier to the offshore transport of neutrally buoyant particles despite the presence of the passive tracer front. This results in a continuous net offshore transport from the shelf to the deep ocean due to the effects of bottom friction.

Abstract

A depth-averaged model with no density variations was used by Chapman to describe the formation of a passive tracer front at a shelfbreak. The relevance of this frontogenesis mechanism to cases that allow vertical variations is examined by considering the three-dimensional structure of a passive tracer front with explicit finite vertical mixing and bottom boundary layer dynamics. A three-dimensional primitive-equation numerical model is configured in a channel with a continental shelf, slope, and abyssal plain running the length of the channel. A vertically and horizontally uniform inflow is imposed over the shelf, with a large horizontal velocity shear near the shelfbreak. In the primitive-equation model, the offshore flow is concentrated in the bottom boundary layer while the alongshelf flow distribution is similar to the depth-averaged case; the presence of the bottom topography maintains a strong horizontal shear near the shelfbreak above the bottom boundary layer. This velocity shear causes a smooth passive tracer distribution imposed at the inflow boundary to develop strong cross-shelf gradients near the shelfbreak (i.e., a passive tracer front) within a rather short downstream distance, as in the depth-averaged model. Neutrally buoyant Lagrangian particles initialized above the bottom boundary layer are rapidly advected along the shelf with little cross-shelf motion. However, particles initialized within the bottom boundary layer move quickly offshore toward the shelfbreak and beyond while being advected alongshelf relatively slowly. The shelfbreak does not act as a barrier to the offshore transport of neutrally buoyant particles despite the presence of the passive tracer front. This results in a continuous net offshore transport from the shelf to the deep ocean due to the effects of bottom friction.

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