Mixing and recirculation in two‐layer exchange flows

Abstract

Mixing and recirculation due to interfacial friction in two‐layer exchange flows through topographic constrictions are investigated using a nonhydrostatic, three‐dimensional numerical model. The simulated exchange flows are always lower than what inviscid hydraulic theory predicts; the reduction is greater for a contraction than for a sill, with intermediate values for sill‐contraction combinations. A large fraction of a tracer flux in one layer is lost to the other layer from an inflow section at the entrance of the contraction (foot of the sill) up to the throat of the contraction (sill crest). For a contraction this fraction is about equal for the two layers, whereas for a sill the loss from the upper to the lower layer is about 6 times as high as the loss from the lower to the upper layer. Sill‐contraction combinations show similar behavior to a contraction toward the dense reservoir and to a sill toward the light reservoir. The variation in recirculation fractions over a tidal cycle is small for flow over a sill, but greater for flow through a contraction. The peak rate of loss from a layer occurs when the flow in the other layer has its maximum, i.e., at maximum negative barotropic flow for the lower layer and at maximum positive barotropic flow for the upper layer.