Trapping of a Coastal Density Front by the Bottom Boundary Layer

The dynamics of a surface-to-bottom density front on a uniformly sloping continental shelf and the role of density advection in the bottom boundary layer are examined using a three-dimensional, primitive equation numerical model. The front is formed by prescribing a localized freshwater inflow through the coastal boundary. The resulting freshwater plume turns anticyclonically and moves along the coast, generating offshore transport in the bottom boundary layer, which advects freshwater offshore and creates a sharp surface-to-bottom density front with a surface-intensified alongshelf jet over the front. The offshore buoyancy flux in the bottom boundary layer moves the front offshore until it reaches a depth where the vertical shear within the front leads to a reversal in the cross-shelf velocity at the shoreward edge of the front. Consequently, the offshore buoyancy flux in the bottom boundary layer vanishes shoreward of the front. Within the front, a steady balance is established in the bottom boundary layer between vertical mixing and onshore advection of density. At this point, the front is “trapped” to an isobath; that is, the front remains parallel to the isobath and does not move farther offshore. The location of the trapped front is consistent with simple thermal wind dynamics. The basic frontal-trapping mechanism dominates the dynamics for a wide range of inflow velocities and densities (including very weak density anomalies), indicating that the advection of density in the bottom boundary layer may play a major role in the circulation on many continental shelves, even when the bottom boundary layer is thin compared to the total water depth.