The temperature and current structure on the sloping benthic boundary layer

Measurements of current and temperature from moored instruments on two sites on the continental slope, west of the British Isles, show that the benthic boundary layer structure is highly complex and varies on time scales ranging from minutes to several days. A variation in the background density stratification at a period between 5-8 days is attributed to slope waves. There is tidal (M2) variation in the boundary layer structure, which has the characteristics of that due to reflecting baroclinic internal waves of tidal period (internal tide). In general, vertical mixing is observed to be most intense during periods of falling temperatures, lasting typically one hour, and is associated with conditions of high current shear, low vertical stratification and a component of upslope advection. Conditions favouring convection are also observed during this period, and are dominated by structures with a timescale of order 30 minutes, less than the mean buoyancy period. Other observed high frequency structure, of which several case studies are presented, may be the result of the generation of small scale lee waves on the rough topography, that are subsequently modulated by the varying density structure due to the baroclinic motions present on the slope. This process has been demonstrated in laboratory experiments. The vertical turbulent eddy diffusivity (k) for the Porcupine Bank slope was estimated, with a mean value between k= 15-50cm2s-1, consistent with previous estimates in such regions. There is a temporal variability in the inferred diffusivity, with values averaged over 25 hours and ranging between 0.1-200cm2s-1, and is due to a combination of varying current shear due to the barotropic tidal currents and by the modulation of the boundary layer density structure by baroclinic motions. Arguments are presented to characterise this variability. It is suggested that there will be a spatial variation of the boundary layer diffusivity and that a simple parameterisation for the diffusivity is not apparent, but is a complex function of current, stratification and boundary roughness.