An Upper Bound for the Tidally Rectified Current at One Location on the Southern Flank of Georges Bank

Abstract

Long-term current observations at 45 and 75 m at one location on the southern flank of Georges Bank in water 85 m deep were examined for evidence of tidal rectification. Loder has shown analytically that rectification of the strong semidiurnal tidal current can cause a mean along-bank flow, and thus may partially drive the observed clockwise circulation around Georges Bank. The amplitude of the tidally rectified along-bank flow is proportional to the squared amplitude of the cross-bank tidal current. A simply extension of Loder's model to include the weaker N₂ and S₂ tidal components suggests that fortnightly (354 h) and monthly (661 h) variations of the square of the cross-bank tidal current should cause a modulation of the subtidal along-bank flow. The predicted ratio (R) of the fortnightly and monthly modulation of the along-bank flow to the mean along-bank flow on the southern flank was a function of position and ranged from ∼0.1–0.5. The amplitude of modulation of the along-bank flow at 360 and 648 h, estimated from the (weak) coherence between the observed along-bank flow and the subtidal envelope of a simulated surface tide, was less than ∼1.1 and 0.9 cm s⁻¹, respectively, at 45 m. The amplitude of the modulation which can be attributed to tidal rectification may be in error by the astronomically forced Mm and MSf tidal currents, which are undescribed in this region. However, the magnitude of the mean along-bank tidally rectified current determined from the observed modulation and R predicted by the analytical model was ∼2.0 cm s⁻¹ at 45 m (36% of the observed mean current in winter) and less than 1.6 cm s⁻¹ at 75 m (43% of the observed mean current). Although R may change in a more realistic model, this analysis suggests that only part of the seasonal-mean along-bank flow on the southern flank of Georges Bank may be caused by tidal rectification.