Tidal and subtidal variability in the sloping benthic boundary layer

Observations are presented of the benthic boundary layer (BBL) structure for two sites on the continental slope, west of the British Isles. Variability at the tidal (M₂) and subtidal frequencies is discussed. A mean poleward, along-isobath current is present at both sites, with periodic (5-8 days) reversals observed in the flow. These reversals cause a change in the water column stratification close (<100 m) to the seabed. The relationship between the direction of the along- and cross-slope flow, stratification, and the change of temperature in the frictional layer, relative to that above it, is consistent with Ekman veering induced up/downwelling close to the slope. The observations are compared to recent models of the sloping BBL and laboratory experiments, with particular emphasis on the transient nature of the BBL and the influence of internal wave reflection from the slope. -from Author