Response of the bottom boundary layer over a sloping shelf to variations in alongshore wind

Abstract

Rapidly repeated transects of currents, density, and turbulence through the bottom boundary layer across a relatively uniform stretch of the continental shelf off Oregon reveal the response to a sequence of strong upwelling followed by relaxation and thence a resumption of upwelling. Several definitions of boundary layer thickness are employed to describe the evolution of the bottom boundary layer. Well‐mixed and turbulent layers were typically confined to 10 m from the bottom. However, boundary layer thicknesses were greatest during relaxation from upwelling (when mixed layer and turbulent layer thicknesses exceeded 20 m), and turbulence in the bottom boundary layer was most intense at this time. Dense, near‐bottom fluid was observed to move upslope with upwelling and back down the slope with relaxation from upwelling. By tracking the intersection of near‐bottom isopycnals with the bottom over successive transects, we estimate the cross‐shore speed of fluid in the bottom boundary layer. Cross‐shore speed agrees well with dynamical estimates of cross‐shore velocity in the bottom Ekman layer derived from bottom stress measurements. This leads to a confirmation of the Ekman balance of alongshore momentum in the bottom boundary layer across the full width of the shelf. Good correlation exists between alongshore velocity at the top of the bottom boundary layer and cross‐shore velocity of dense fluid in the bottom boundary layer. Application of a derived proxy for bottom stress to moored velocity observations indicates Ekman balance of alongshore momentum at a midshelf location (81 m depth) for a 3 month period in spring/summer 2001.