Sub-inertial modulation of semi-diurnal currents over the continental slope in the Faeroe-Shetland Channel

Abstract

Observations were made over the continental slope in the Faeroe-Shetland Channel with the purpose of studying motions in the semidiurnal tidal frequency band and their interaction with background conditions. Specifically, the study focused on internal (baroclinic) tidal motions associated with internal gravity waves. In the internal tide (IT) source region, kinetic energy spectra reveal a fall-off rate with frequency of σ −2 which is associated with the passage of strongly non-linear fronts at the sea-bed. At a distance of 10 km off shore from the source region spectra from both the permanent pycnocline and weakly stratified interior exhibit a fall-off rate of σ −3, indicative of a regime in which internal waves are dominated by higher harmonics of the M 2 semidiurnal tidal frequency as a result of non-linear advection. The bandwidth, Δ σ = 0.23 – 0.3 cycles per day (cpd) of the semidiurnal band determined from the distribution of baroclinic energy outside the deterministic tidal frequencies, implies a modulation period of 3.3–4.3 days and which is observed as pulses of semidiurnal energy that are not related to the spring–neap cycle. The period of modulation is related to changes in the background stratification and the low-frequency vorticity. Variations in stratification are sufficient to modify the IT path by>150 m in the vertical over a horizontal distance of 10 km.The cross-slope gradient of the low-frequency ( σ < 0.75 cpd ) long-slope velocity, ∂ V / ∂ x , which represents one component of the vorticity varies with a periodicity of 3–4 days and has a magnitude sufficient to increase the effective Coriolis frequency to a value larger than M 2 so that the IT is unable to propagate as a free internal gravity wave. The source of the subinertial variability in the background conditions is attributed to meteorologically forced continental shelf waves which respond in two ways to prevailing winds and which perturb the pre-existing geostrophic balance that exists over the slope; oscillatory, clockwise rotating currents with a frequency of 0.7× f, where f is the Coriolis frequency, result from short impulsive winds whilst a quasi-steady long-slope flow persists as long as stronger winds of longer duration prevail. The impact of the observed subinertial variability in low-frequency vorticity and stratification raises doubts as to whether an ‘attractor’, along which internal wave energy is found following repeated reflections within a confined basin, may be observed in such a dynamic environment.