The effect of alongshore wind stress on a buoyancy current’s stability

Buoyancy fronts reach from the surface to the bottom over continental shelves, separating light inshore water from denser offshore water, and are known to be responsive to Ekman transport (and associated return flow at depth) driven by alongshore winds. The consequent changes in frontal structure are clearly related to changes in the gravitational Available Potential Energy (APE), so it is reasonable to expect that these winds will affect the eddy field that results from baroclinic instabilities. Idealized numerical experiments and scaling analyses are brought to bear on this problem. It is found that several days of wind-driven downwelling (which creates more nearly vertical isopycnals) generally leads to an enhancement in the time maximum of volume-averaged Eddy Kinetic Energy (EKE). Upwelling-favorable winds (which tend to flatten isopycnals) usually lead to a decrease in APE, hence in eddy energy. The exception to this rule occurs when the winds are strong enough that an upwelling front forms inshore of the buoyant water, in which case APE and EKE may increase.