Near-bed solibores over the continental slope in the Faeroe-Shetland Channel

On two occasions within a 12-day measurement period in the Faeroe-Shetland Channel, strongly nonlinear wave trains were observed at the sea-bed, propagating up the continental slope in water depths >450 m. The events were separated by a period of 4 days and, whilst resembling in appearance a density current running up a slope, are termed ‘solibores’, displaying the properties of both turbulent internal bores and nonlinear internal solitary waves (ISW). Each solibore displays a steep leading edge followed by a train of nonlinear waves with amplitudes of O(10 m) and periods of ∼5–20 min. Wave-induced particle velocities are consistent with ISW, whilst a zone of strong horizontal convergence at the leading edge of the solibores causes the formation of a rotor with flow in the opposite sense to a ‘forward overturning’ surface wave. In both cases upward vertical velocities >10 cm s−1 are immediately followed by a return downward flow of equal magnitude. A CTD and microstructure transect conducted during the passage of the first solibore illustrates its behaviour as an up-slope intrusion of cold, dense water with concurrent rates of turbulent dissipation, ε , of O(10−7 W kg−1) and resulting in short-term maximum vertical diffusivities, Kz, of O(10−1 m2 s−1). In the long-term however, Kz<10−4 m2 s−1, implying that the solibores are not important for sustaining deep-sea mixing. In contrast, sediment fluxes at 2 and 30 m above the bed during the first solibore are O(102) larger than the background value, implying solibores are the dominant sediment transport mechanism despite their intermittent occurrence. The transport of sediment, up the slope in the direction of propagation of the solibore, is contrary to the usually considered pathway for sediment from coastal seas to the abyss via the continental slope. That sediment transport is dominated by short-term events appears to be corroborated by long-term data (>140 days) which again indicate intermittency in periods of enhanced sediment flux. The solibores are consistent with the results of numerical and laboratory experiments on hydraulic jumps resulting from shoaling ISW. The influence of the slope is proposed to be the reason for their slightly different forms and effects on sediment transport; thus the first solibore, propagating directly up the slope, is proposed to result from an overturning hydraulic jump caused by kinematic instabilities and which subsequently forms a horizontal density intrusion. The second solibore represents a dispersive wave train due its oblique direction of propagation which reduces the effective bottom-slope, allowing dispersion to balance nonlinear effects and prohibiting overturning.