The Influence of a Submarine Canyon on the Circulation and Cross-Shore Exchanges around an Upwelling Front

Abstract

The response of a coastal ocean numerical model, typical of eastern boundaries, is investigated under upwelling-favorable wind forcing and with/without the presence of a submarine canyon. Experiments were run over three contrasting shelf depth/slope bathymetries and forced by an upwelling-favorable alongshore wind. Random noise in the wind stress field was used to trigger the onset of frontal instabilities, which formed around the upwelling front. Their development and evolution are enhanced over deeper (and less inclined) shelves. Experiments without a submarine canyon agree well with previous studies of upwelling frontal instabilities; baroclinic instabilities grow along the front in time. The addition of a submarine canyon incising the continental shelf dramatically changes the circulation and frontal characteristics. Intensified upwelling is channeled through the downstream side of the canyon in all depth/slope configurations. Farther downstream a downwelling area is generated, being larger and stronger on a shallow shelf. The canyon affects mainly the location of the southward upwelling jet, which is deflected inshore and accelerated after passing over the canyon. This process is accompanied by a break in the alongshore scale of the instabilities on either side of the canyon. Term balances of the depth-averaged cross-shore momentum equation reaffirm the downstream acceleration of the jet and the increased wavelength of the instabilities, and clarify the dominant balance between the advection and ageostrophic terms around the canyon.

Circulation and Upwelling Induced by Coastal Trapped Waves Over a Submarine Canyon in an Idealized Eastern Boundary Margin

“…The transition to a high sea level involves the advection of the previously upwelled water farther downwave, which results in an extended pool of high density water over the downwave region (Figure 4e). Comparing the CTW results to a wind-driven upwelling simulation with the same bathymetries (Saldías & Allen, 2020), we note that the adjustment time of flow over the canyon to wind-driven forcing is on the order of days. The CTW's, with a period of 7 days, are accelerating/decelerating on a time scale similar to the adjustment time.…”

Surface Thermal Structure and Variability of Upwelling Shadows in the Gulf of Arauco, Chile

“…Once the alongshore wind stress has ceased, the sloping isopycnals tend to slump back, eventually destroying the surface front and potentially re-stratifying the upper water column. The scale of the front is relevant to the local dynamics since large-scale horizontal density gradients tend to maintain a geostrophic jet that flows in the same direction as the upwelling-favorable winds (Barth et al, 2000;Saldías & Allen, 2020). Northward winds induce a southward (poleward) pressure gradient and a southward coastal current when the upwelling wind weakens.…”

Hydraulic control of continental shelf waves

“…While ZL17 report steady flow when the background current opposes CTW propagation, and a response consistent with an arrested CTW (their figure 14), Saldías & Allen (2020) find that their simulations never become steady in the forcing region and instead develop a meandering wave train upstream. The reason for this difference is not clear, although Saldías & Allen (2020) estimate that the Froude number for the first three CTW modes in their model is 2, 0.2 and 0.1, respectively, so that they may be outside the range in which hydraulic control occurs. By exploring a wider range of flow speeds in a numerical model, one could potentially identify the boundaries for critical flow, as well as boundaries at which control changes between different modes.…”

“…First and most important is to understand the regimes in which CTWs exert hydraulic control in the real ocean. Zhang & Lentz (2017) and Saldías & Allen (2020) both present numerical simulations of CTWs in a configuration very similar to that used here, albeit with sloping topography and a background flow driven by (constant) wind forcing. While ZL17 report steady flow when the background current opposes CTW propagation, and a response consistent with an arrested CTW (their figure 14), Saldías & Allen (2020) find that their simulations never become steady in the forcing region and instead develop a meandering wave train upstream.…”

“…Zhang & Lentz (2017) and Saldías & Allen (2020) both present numerical simulations of CTWs in a configuration very similar to that used here, albeit with sloping topography and a background flow driven by (constant) wind forcing. While ZL17 report steady flow when the background current opposes CTW propagation, and a response consistent with an arrested CTW (their figure 14), Saldías & Allen (2020) find that their simulations never become steady in the forcing region and instead develop a meandering wave train upstream. The reason for this difference is not clear, although Saldías & Allen (2020) estimate that the Froude number for the first three CTW modes in their model is 2, 0.2 and 0.1, respectively, so that they may be outside the range in which hydraulic control occurs.…”