The front on the Northern Flank of Georges Bank in spring: 1. Tidal and subtidal variability

Abstract

During March–April 1999, 2 weeks of undulating CTD and shipboard acoustic Doppler current profiler surveys revealed the variability of the intense internal tide on the northern edge of Georges Bank. The nature of the internal tide was modulated by episodic surface intrusions of cool, fresh Scotian Shelf Water (SSW), stratifying the otherwise vertically well‐mixed outer bank. The introduction of SSW created a system analogous to that in summer, when the outer bank is thermally stratified and separated from well‐mixed regions by a tidal mixing front. During SSW intrusions, internal tidal behavior is characterized by tidal advection that is significantly faster than internal wave propagation speeds on the bank (supercritical flow) and slower than the lowest internal modes in deeper water to the north (subcritical flow). A large‐amplitude internal lee wave develops over the slope during off‐bank tidal flow. This stalled energy is released to propagate on‐bank as a high‐frequency internal wave when the tide reverses. It is suggested, by analogy with the summer case, that a portion of this energy is used by mixing at the on‐bank limit of SSW. The presence of SSW thus modifies the internal tidal response, enabling a mechanism that contributes to its assimilation with ambient bank water. The off‐bank‐propagating internal tide is dominated by a mode 1 internal wave at the M2 frequency, with cross‐bank velocities comparable to the barotropic tide.