Separation of an Advectively Trapped Buoyancy Current at a Bathymetric Bend

Abstract

The coastal current system along the east coast of North America, from the Labrador shelf to Cape Hatteras, must negotiate complex bathymetry with numerous sharp bends and large cross-shelf channels. The behavior of these shelf currents is studied here using an advectively trapped buoyancy current (ATBC) model, in which a coastal buoyancy current on a sloping bottom forms a surface-to-bottom density front that becomes trapped along an isobath by offshore advection of buoyancy in the bottom boundary layer. Alongfront flow is concentrated in a nearly geostrophic surface-intensified frontal jet. Trapping occurs where the cross-shelf bottom velocity vanishes on the shoreward side of the front, thus terminating offshore buoyancy advection and causing the bottom boundary layer to detach and to flow upward along frontal isopycnals. The dynamics of an ATBC at a sharp bend in bathymetry and at a cross-shelf channel are investigated using a primitive equation numerical model, focusing on the separation of the frontal jet from the topography. The response depends on the relative size of the buoyant inflow that creates the ATBC and a weak alongshelf background current that is typically imposed to ensure the downstream propagation of the buoyant inflow. With no background current, the ATBC separates at either the single bend or the channel, regardless of the strength of the buoyant inflow. With a background current, the ATBC follows the isobaths around the bend until the bottom velocity parallel to the front vanishes, after which the frontal jet separates and flows freely away from the topography while the foot of the front remains attached near the trapping depth. The separation point is highly sensitive to the magnitude of the background current, with changes of a few centimeters per second having a major impact on the response. Unlike most geophysical flows, the separation process is basically linear and does not require large nonlinear inertial contributions to the momentum balances. The model suggests that substantial losses of buoyant Labrador shelf water are likely to occur at the southern tail of the Grand Banks and that ambient/offshore currents probably control the penetration and crossover of the shelfbreak front at the Northeast Channel.