THE INTERNAL TIDE OVER THE SHELF INSHORE OF CAPE POINT VALLEY, SOUTH-AFRICA

Thermistor string observations of the internal wave field over the she lf off South Africa display a whitened Garrett-Munk spectrum (log-log slopes of -1.7 to -2.0) with peaks in inertial, tidal, and high-freque ncy (>1 cph) bands. The vertical structure is dominated by lower-order modes, and the temporal signal is dominated by the internal tide. On propagating shoreward, the internal tide transports energy of the orde r of 0.04 kW m-1 onto the shelf, where it is dissipated. The resultant vertical exchange, characterized by an eddy viscosity of 10(-4) m2 s- 1, may maintain significant primary production in the absence of coast al upwelling. The internal tide is larger than expected, comparable wi th that over shelves where the forcing is twice as strong. This enhanc ed barotropic-baroclinic coupling is explained in terms of resonance b etween multiple generation sites. A linear, two-layer model is derived to investigate the enhancement due to the presence of Cape Point Vall ey, a submarine canyon that runs parallel to the shelf edge. Under res onant conditions the barotropic-baroclinic coupling may be 3 times as large as that in the absence of the canyon; this explains an internal tide amplitude enhancement of 70%.