A Benthic Front in the Straits of Florida and Its Relationship to the Structure of the Florida Current

Abstract

Observations from CTD tow-yos and microstructure profiles indicate the presence of a benthic front near 450-m depth on the western side of the Straits of Florida at 27°N. The front is at midslope and approximately beneath the axis of the Florida Current. There is a 0.2 kg m?3 jump in density across the front, and the bottom boundary layer changes thickness from 10?20 m upslope to 50?70 m downslope of the front. Axial currents exhibit very strong vertical shear and cyclonic vertical vorticity upslope of the front in contrast to the significantly weaker vertical shears and anticyclonic vertical vorticity observed downslope. All these features, with the exception of the anticyclonic vorticity downslope of the front, are consistent with a model of barotropically forced flow through a stratified channel with a sloping sidewall. An arrested boundary layer regime is predicted to develop on the sloping sidewall upslope of a height hp ≡ fL/N, where f is the Coriolis parameter, L is the width of the flat-bottomed portion of the channel, and N is the buoyancy frequency. The arrested boundary layer supports little cross-slope transport. Below hp a cross-channel circulation cell develops in the model, driven by the bottom Ekman layer. Transition between the boundary layer regimes leads to convergence and frontogenesis near a height hp on the sloping sidewall. Strong mixing at the front acts to destroy the salinity minimum signature of Antarctic Intermediate Water moving into the North Atlantic. The development of anticyclonic vorticity in the Florida Current as it flows through the Florida Straits forces the strongest flow onto the western slope, where an arrested boundary layer develops. This configuration supports strong vertical and lateral shears, all largely in geostrophic balance.