The variability of mixing at the continental slope

Abstract

The benthic boundary layer on the continental slope is a region in which isopycnal surfaces intersect topography. It is, in consequence, particularly subject to variability caused by reflecting internal gravity waves, as well as by trapped baroclinic slope waves. Observations made using an array of moorings off the west slope of the Porcupine Bank reveal the presence there of a northerly along-slope current at depths of 3000-4000 m, which extends to some 20 km off-slope, and of waves of periods 2-9 days trapped within about 10 km ( ca. 1 Rossby radius) of the slope. Lower-frequency variations are dominant at greater distances. The tide has a significant spectral peak, with a first subharmonic apparent near the slope. Measurements of the temperature structure in the boundary layer have been made at 1705 m on the Hebrides slope and at 3447 m off the Porcupine Bank. In both areas the boundary layer structure is dominated by asymmetrical M ₂ variations in the isopycnal surfaces, with displacements of50-100 m. Uniform near-bed ‘mixed’ layers appear during the tidal cycle, reaching to some 50 m off the sea bed, but are transient, lasting for only a fraction of the tidal cycle following sharp rises in isotherm levels, later to be replaced by statically stable conditions extending to within a few metres of the sea bed. The slope-trapped waves also modify the mean temperature gradients and are responsible for variations in the locations at which reflecting internal waves of given frequency are critical or resonant.