Incoherent internal tidal currents in the deep ocean

'Eleven months' current meter observations from the deep Bay of Biscay were examined for the residual (incoherent internal tidal; icIT) signal, left after harmonic analysis using eight tidal constituents (large-scale barotropic or coherent baroclinic signal) within the semidiurnal band. This residual signal comprised similar to30% of the total tidal kinetic energy and, due to its flat spectral appearance, it was responsible for typically 5-7 days intermittency. Although icIT was part of the red noise internal wave band continuum, it was not attributable to instrumental noise. It consisted of quasi-harmonics at non-tidal harmonic frequencies having amplitudes larger than N-2, the third largest semidiurnal tidal constituent. It is suggested that the kinetic energy at these non-tidal frequencies reflects interaction between semidiurnal tidal motions and the slowly varying background conditions.