Ekman-induced turbulence over the continental slope in the Faeroe–Shetland Channel as inferred from spikes in current meter observations

Data from Aanderaa RCM-8 current meters, deployed on the continental slope of the Faeroe–Shetland Channel between depths of 471 and 1000 m, show intermittent spike-like reductions in current speed of ∼20–40%. The spikes have a duration of only one data value, corresponding to the unusually short sampling period of 1 min. Associated with the spikes are concurrent deflections in the current direction records, with a mean value of 7° and in a predominantly clockwise sense. The spikes furthermore occur only when the current direction has a poleward long-slope component when the shallow water is to the right. We reject instrumental deficiency as the cause of the spikes, and find that the negative bias is caused by the cosine response of the mechanical current meter when misaligned with the mean flow. The misalignment is due to turbulence generated by instabilities in the bottom boundary layer (BBL) caused by the downslope Ekman transport of buoyancy. The veering in current direction between 8 and 47 m above the seabed and the hourly averaged current direction are consistent with such a mechanism, whilst the intermittency is explained by the bursting phenomenon in a BBL. Both the quasi-period of ∼600 s and the duration of the spikes are in accordance with previous fieldwork and laboratory experiments and, in conjunction with a thickening of the boundary layer induced by the downwelling Ekman transport, explain the predominance of the spikes at heights of 34 and 47 m. Oblique internal wave reflection also plays a potential role in generating turbulence at the upper interface of the BBL higher up the slope where the stratification is comparatively strong, but the Ekman transport represents the dominant mechanism. Our findings cause concern for the reliability of data from traditional rotor- and vane-type current meters sampling at lower rates in turbulent near-bed regions where the turbulent properties of the flow will be averaged out in the sampling process, potentially causing an underreading of the true current speed.