Glacial troughs are flat‐bottomed, steep‐sided submarine valleys that incise the shelf and significantly alter coastal circulation. We examine how these features drive exchange between the shelf and the slope in the barotropic and linear limits. When the alongshore flow is in the Kelvin‐wave (downwave/downwelling favorable) direction, the troughs move transport from the shelf upwave of the trough to the slope downwave of the trough, diminishing wind‐driven alongshore transport on the shelf downwave of the trough. Conversely, when the alongshore flow is against the Kelvin wave direction (upwave/upwelling favorable), the troughs move transport from the slope downwave of the trough to the shelf upwave of the trough. These cross‐shelf flows are driven by the acceleration and curvature of the flows induced by the narrowing and turning isobaths around the trough, and the bottom friction experienced by these accelerated flows. These dynamics are quantified by examining the along‐isobath evolution of potential vorticity in the model's limits.
Instabilities in the Bottom Boundary Layer Reduce Boundary Layer Arrest and Stir Boundary Layer Water Into the Stratified Interior
An along‐isobath current in stratified waters leads to a bottom boundary layer. In models with no alongshore variation, cross‐isobath density transport in this bottom boundary layer reduce the velocity in the bottom boundary layer via thermal wind, and thus the bottom friction experienced by the current above the boundary layer—this is bottom‐boundary‐layer arrest. If, however, alongshore variation of the flow is allowed, the bottom boundary layer is baroclinically unstable. We show with high resolution numerical models that these instabilities reduce this arrest and allow bottom friction to decelerate the flow above the bottom boundary layer when the flow is in the Kelvin wave direction (so that the bottom Ekman transport is downwelling). Both the arrest of the bottom boundary layer and the release from this arrest are asymmetric; the friction experienced by flows in the direction of Kelvin‐wave propagation (downwave) is much greater than flows in the opposite direction. The strength of the near bottom currents, and thus the magnitude of bottom friction, is found to be governed by the destruction of potential vorticity near the bottom balanced by the offshore along‐isopycnal transport of this anomalous potential vorticity. A simple model of this process is created and used to quantify the magnitude of this effect and the resulting reduction of arrest of the bottom boundary layer.
more » « less- PAR ID:
- 10380603
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 127
- Issue:
- 4
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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