Gravity Currents in Rotating Systems

Abstract

When a fluid spreads under gravity in a rotating system, motions normal to the rotation vector induce Coriolis forces that tend to oppose the spreading. In the absence of boundaries intersecting isopotential surfaces and of instability or viscous dissipation, the flow approaches a state of geostrophic equilibrium in which buoyancy and Coriolis forces are in balance and further release of potential energy is impossible. However, in the vicinity of a boundary, velocities normal to the boundary, and hence Coriolis forces parallel to the boundary, must vanish. The requirement that angular momentum be conserved is then removed, the constraints imposed by the rotation are broken, and a gravity current flows along the boundary. While such gravity currents have some similarities to those that form without rotation (reviewed by Simpson 1982), they involve a variety of additional characteristics. In geophysical situations the rotation vector of interest is the component of the Earth's rotation antiparallel to gravity. Boundaries may be mountain ranges, coastlines, or a sloping ocean bottom. Examples of gravity currents that are large enough to be influenced by the Earth's rotation are numerous. River outflows are localized sources of fresh, less dense water that (in the absence of advecting currents) are confined by Coriolis forces to spread in only one direction along the coast. The coast may have any orientation, but the current must have the coast on its right-hand side when looking in the direction of flow in the Northern Hemisphere, or on its left in the Southern Hemisphere. On a similar length scale are the Kyucho. These appear to be gravity currents that propagate around the perimeter of bays in Japanese islands in response to variations of a warm ocean current passing the mouth