The impact of locally-enhanced vertical diffusivity on the cross-shelf transport of tracers induced by a submarine canyon

TL;DR

This study uses numerical simulations to show that submarine canyons significantly enhance cross-shelf tracer transport, with locally-enhanced vertical diffusivity increasing tracer flux by up to 27%, impacting biogeochemical cycles.

Contribution

It introduces a scaling estimate for canyon-induced tracer upwelling considering local diffusivity and validates it through numerical experiments.

Findings

Canyons increase tracer upwelling by 25-89% compared to coastal upwelling.

Locally-enhanced vertical diffusivity can boost tracer flux by up to 27%.

Canyon width is about 5% of shelf length, significantly affecting cross-shelf exchange.

Abstract

The exchanges of water, nutrients and oxygen between the coastal and open ocean are key components of on-shelf nutrient budgets and biogeochemical cycles. On a regional scale, submarine canyons enhance physical processes such as shelf-slope mass exchange and mixing. There is good understanding of the flow around upwelling submarine canyons; however, the flux of biologically relevant tracers is less understood. This work investigates the impact of submarine canyons on the cross-shelf exchange of tracers and water, taking into account the impact of locally-enhanced mixing within the canyon, and develops a scaling estimate for canyon-induced upwelling of tracers, proportional to local concentration, vertical diffusivity, and previously scaled upwelling flux. For that purpose, we performed numerical experiments simulating an upwelling event near an idealized canyon, adding a passive tracer with an initially linear profile. We varied the geographic distribution of vertical eddy diffusivity and its magnitude, the initial stratification, Coriolis parameter, and the strength of the incoming flow. We find that a canyon, of width 5% of the length of the shelf, upwells between 25 to 89% more tracer mass onto the shelf than coastal upwelling. Locally-enhanced vertical diffusivity has a positive effect on the tracer that is advected by the upwelling flow and can increase canyon-upwelled tracer flux by up to 27%.