Mean circulation in shallow seas

Abstract

The mean circulation of shallow seas arises as the residue of chaotic first-order flow episodes created by winds, tides, and river inflow. The average effect of many such episodes is to generate an internal pressure and shear stress distribution which may differ significantly from a steady solution of the equations of motion. Within the coastal boundary layer (of about 10-km width), complex average effects of variable flow probably dominate. However, in mid-shelf regions of the Mid-Atlantic Bight, for example, classical steady state models adequately describe the main features of the mean circulation. The statistical effects of variable first-order flow even bring about some simplifications of the theory, i.e., linear internal and bottom friction laws and a decoupling of salt transport from the mean circulation. The mid-shelf circulation of the Mid-Atlantic Bight is thus found to consist of four additive components, each driven by offshore wind, longshore wind, longshore pressure gradient, and density contrasts caused by freshwater influx. The longshore pressure gradient enters the theory as a parameter necessary for describing the interaction of a portion of the continental shelf with the rest of the ocean. The pressure gradient driven flow component is responsible for some notable circulation features such as the line of divergence in bottom drift at about the 60-m isobath. The wind-driven and thermohaline flow components are familiar in character. Quantitatively, the four components together account satisfactorily for the observed mean shelf circulation.