The “insulating” effect of a steep continental slope

Abstract

Geochemical evidence shows that the midcontinental slope off the Mid‐Atlantic Bight constitutes an important sink for substances attached to fine particles. It is reasonable to infer that near‐bottom currents in this region are generally much weaker than those over the adjacent continental shelf, allowing the settling out of fine particles and their incorporation into permanent sediment. A partial dynamical explanation of this phenomenon may be sought in the “insulating” effect of a steep slope. At levels below the main thermocline the bottom pressure perturbation associated with steady flow is governed by a parabolic equation which expresses vorticity tendency balance between bottom stress curl and vortex stretching. A heat conduction analogy is appropriate, in which the friction coefficient corresponds to conductivity and the bottom slope corresponds to heat capacity. Over the continental slope the latter is large, suppressing pressure gradients originating from the shelf. Thus steady, near‐bottom currents over the middle slope are generally weak. Exceptions to this rule are locations subject to intense winter cooling and deep convection, which may generate strong thermohaline near‐bottom flow. Oscillatory motions of importance (which may be taken to be coastally trapped waves and topographic waves propagating shoreward from deep water) are weak partly because an efficient local generation mechanism is lacking. Disturbances originating from the Gulf Stream decay as the depth reduces below 3 km, by a mechanism the details of which are not clear at present. The findings in the Mid‐Atlantic Bight are likely to apply to other continental slopes distant from western boundary currents and not subject to intense winter cooling.

The Origin of Along-Shelf Pressure Gradient in the Middle Atlantic Bight

“…The MAB circulation has been extensively studied through observations and modeling Beardsley and Boicourt 1981;Chapman 1986;Csanady and Hamilton 1988;Linder and Gawarkiewicz 1998;Flagg et al 2006;Lentz 2008a). Depth-averaged mean currents are predominantly along isobath directed equatorward, with speeds of 0.03-0.1 m s 21 that increase with distance offshore (Lentz 2008a(Lentz , 2010.…”

“…Lentz (2008a) discussed the possibility of other types of forcing, but the hypothesis that ASPG exists seems reasonable. The Gulf Stream (GS) and Slope Sea gyre (Csanady and Hamilton 1988) may drive an ASPG at the shelf break, but the penetration of the pressure field onto the shelf is limited (Wang 1982;Csanady and Shaw 1983;Chapman 1986). The Gulf Stream (GS) and Slope Sea gyre (Csanady and Hamilton 1988) may drive an ASPG at the shelf break, but the penetration of the pressure field onto the shelf is limited (Wang 1982;Csanady and Shaw 1983;Chapman 1986).…”

“…We will examine mean, seasonal, and interannual variability and attempt to relate them to driving mechanisms: wind stress curl, Gulf Stream's latitudinal shifts, rings, Coastal Labrador Seawater (CLSW; Csanady and Hamilton 1988) transport, and river discharge. We attempt to provide answers to the origin of ASPG: its mean as well as seasonal and interannual variability.…”

A loop current‐induced jet along the edge of the West Florida Shelf

“…At these low frequencies, the extent of influence of the imposed pressure is inversely proportional to the slope of the sea bottom [Chapman and Brink, 1987]. Thus, the steep topography of the WFE "insulates" the WFS from the influence of the LC pressure and turns the along-break north-tosouth pressure drop into a sharp drop in pressure onshore, giving rise to a focussed southward flow along the shelf edge and leaving the flow weak over the rest of the shelf [Csanady and Shaw, 1983]. …”

North Atlantic Slope and Canyon Study; Volume 2

“…Csanady and Shaw (1983) have suggested that because of the relatively weak currents on the slope, this region may be a sink for fine-grained particles. Because anthropogenic pollutants often adhere to fine-grained particles, over long periods of time the slope may be a region of accumulation for these contaminants.…”