Bottom Boundary Layer Structure and Detachment in the Shelfbreak Jet of the Middle Atlantic Bight*

Abstract

The hydrographic properties of the bottom boundary layer (BBL) are investigated in a synoptic cross section of the Middle Atlantic Bight shelfbreak frontal jet. The dataset consists of closely spaced conductivity–temperature–depth stations and concurrent shipboard acoustic Doppler current profiler measurements. An extremum in BBL properties occurs in the frontal region where the layer becomes thinner (disappearing briefly), more stratified, and more strongly capped. These changes are apparently related to the significant cross-slope variation in interior stratification. Where the BBL vanishes, at the shoreward edge of the front, it detaches into the interior along an (nearly) isopycnal layer. This is revealed both by weak vertical stratification as well as weak isopycnal gradients of potential temperature (θ) and salinity (S) along the layer. An advective–diffusive model of the detachment in density space is used to explain the observed θ, S distribution as well as estimate the pumping speed along the detached BBL. The detided ADCP velocity fields are analyzed in light of the observed detached BBL. The mechanism of detachment is discussed in relation to existing models, and the secondary circulation in the cross-stream plane is inferred. This reveals a deep interior upwelling cell, apparently tied to the local bathymetry, which enhances the flow along the detached BBL.