Intense, Variable Mixing near the Head of Monterey Submarine Canyon

Abstract

A microstructure survey near the head of Monterey Submarine Canyon, the first in a canyon, confirmed earlier inferences that coastal submarine canyons are sites of intense mixing. The data collected during two weeks in August 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 103 times higher than in the open ocean. Dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere (ε = 1.1 ? 10?6 W kg?1; K? = 1.0 ? 10?2 m2 s?1). Mixing occurred mainly in an on-axis stratified turbulent layer, with thickness and intensity increasing from neap to spring tide. Strain spectra showed a gentler than k?1z rolloff, suggesting that critical reflection and scattering may push energy into high wavenumbers. Dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean, with indications that the dependence maybe as low as ε ? S. Coastal canyons may account for a small but significant fraction of the internal tide energy budget. A crude estimate of global dissipation in canyons is 58 GW, ≈15% of the estimated global M2 internal tide dissipation.