Sack Lunch Seminar (SLS)

SLS: David Marshall - Oxford
Date Time Location
September 28th, 2010 11:00am-12:00pm 54-1615
Momentum Balance of the Wind-Driven and Meridional Overturning Circulation



When a force is applied to a (nearly) Boussinesq fluid, such as the ocean, fluid parcels are accelerated locally by the applied force, and non-locally by the pressure gradient forces established to maintain continuity and satisfy the boundary conditions. The net acceleration can be represented through a "rotational force" in the rotational component of the momentum equation. This decomposition elucidates the correspondence between momentum and vorticity descriptions of the large-scale ocean circulation: if two terms balance point-wise in the rotational momentum equation, then the equivalent two terms balance point-wise in the vorticity equation. The utility of this decomposition is illustrated for three classical problems: barotropic Rossby waves, wind-driven circulation in a homogeneous basin, and the meridional overturning circulation in an inter-hemispheric basin. In the hydrostatic limit, it is shown that the rotational forces further decompose in depth-integrated forces that drive the wind-driven gyres and overturning forces that are confined to the basin boundaries and drive the overturning circulation. Applications to eddy forcing of the mean flow and to the formulation of a numerical ocean model based on the three-dimensional vorticity equation shall also be discussed.