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.