Frictionally modified rotating hydraulic channel exchange and ocean outflows

Laboratory experiments of two-layer exchange through channels of circular and half-circular cross section are described. Simple theoretical limits on exchange for the circular channels are obtained from models involving either friction or rotating hydraulic control alone. Ekman and Stewartson layers are appended to the rotating hydraulic control model in a simple theory for frictionally modified rotating control. In theory and experiment, rotation and friction together exert a stronger constraint on the exchange than they do separately. Secondary cross-channel circulations were found in the rotating experiments. These secondary circulations further reduce exchange by efficiently moving fluid that is spun down at the boundaries and interface into the interior. More accurate theoretical estimates of exchange must modify rotating hydraulics to include these frictional effects. Solid-boundary Ekman layers drove some of these secondary circulations; others were driven by robust interfacial Ekman layers. The interface in the rotating experiments had a cross-channel tilt since the exchange was geostrophic to first order. In the half-circular channel, the interface was sharpened on its deep side and spread on its shallow side by the strain field of the secondary circulations. In the presence of mixing, this strain field causes a wedge-shaped density field within the channel. A similar pattern has been observed in outflows through the Vema and Faroe Bank channels.