A review of the role of submarine canyons in deep-ocean exchange with the shelf

S E Allen,X Durrieu De Madron

doi:10.5194/os-5-607-2009

Abstract

Abstract. Cross shelf-break exchange is limited by the tendency of geostrophic flow to follow bathymetric contours, not cross them. However, small scale topography, such as canyons, can reduce the local lengthscale of the flow and increase the local Rossby number. These higher Rossby numbers mean the flow is no longer purely geostrophic and significant cross-isobath flow can occur. This cross-isobath flow includes both upwelling and downwelling due to wind-driven shelf currents and the strong cascading flows of dense shelf-water into the ocean. Tidal currents usually run primarily parallel to the shelf-break topography. Canyons cut across these flows and thus are often regions of generation of strong baroclinic tides and internal waves. Canyons can also focus internal waves. Both processes lead to greatly elevated levels of mixing. Thus, through both advection and mixing processes, canyons can enhance Deep Ocean Shelf Exchange. Here we review the state of the science describing the dynamics of the flows and suggest further areas of research, particularly into quantifying fluxes of nutrients and carbon as well as heat and salt through canyons.

Highlights

It has long been known that the steep topography of the continental slope steers flow along isobaths
For a number of dynamical reasons, under-water submarine canyons can act as conduits, with deep, dense ocean-water moving onto the shelf, or with dense shelf-water cascading to the open ocean, or act as regions of enhanced mixing with focusing and amplification of internal waves
The influence of submarine canyons has received particular attention in the past years, and observation as well as laboratory and numerical modelling studies clearly showed their importance in enhancing ocean-shelf exchanges

Summary

Introduction

It has long been known that the steep topography of the continental slope steers flow along isobaths. Wind driven shelf-break or slope currents lead to upwelling or downwelling flows within the canyon with the strongest effects at the canyon rim especially at shelf-break depth. Net upwelling occurs throughout the canyon for weakly stratified flows, with maximum upward velocities at the shelf-break depth (Boyer et al, 2006). For upwelling which is generated by negative along-slope flows, flow over the shelf (above the boundary layer) flows over the upstream rim of the canyon.

Results

Conclusion