The Relationship of Tidal and Low-Frequency Currents on the North Slope of Georges Bank

Abstract

Measured currents at the steeply sloping north edge of Georges Bank show an unusually strong correlation between tidal and lower frequency components. The dominant current constituents are the rotary semidiurnal tides (amplitude ?30 cm s?1, ellipticity 0.6) and the mean isobath-parallel flow to the northeast (ranging from zero to 40 cm s?1). At the middle water depth in the period range of 3?5 days, the along-isobath low-frequency current component is highly coherent (?2>0.8) with the amplitude of the semidiurnal tidal current. The fact that variation of the tidal current amplitude occurs on time scales appropriate to wind-driven events suggests that the tidal structure must be significantly baroclinic, and this is supported by hydrographic data. The evidence thus suggests a local nonlinear interaction between tidal and low-frequency currents, with both the steep bottom topography and the density structure being important factors.