On the Generation of Mesoscale Eddies and their Contribution to the OceanicGeneral Circulation. I. A Preliminary Numerical Experiment

Abstract

Numerical experiments on the wind-driven ocean circulation in a closed basin show that mesoscale eddiescan appear spontaneously during the integration of the equations of motion for a baroclinic ocean. For somevalues of the basic parameters governing the flow, the solutions reach a steady state while for other valuesfinite-amplitude eddies remain a part of the final statistically steady state. In the eddying cases the solutionscan be regarded as a mean flow upon which is superimposed a set of eddies which propagate westward at afew kilometers per day. The eddies typically have horizontal wavelengths of a few hundred kilometers. Analyses of die energetics show the eddies to be generated by the process of baroclinic instability. Thepotential energy of the mean flow is released to supply energy to the eddies. The computed Reynoldsstresses, while small compared to the terms in the geostrophic balance of the mean momentum equations, dohave a strong influence on the mean circulation and, in fact, the deep mean circulation is driven entirelyby the eddies. If the flow were steady, there would be no flow in the deep layer in this model. Finally, thecomputed curl of the Reynolds stresses shows that the vorticity balance of the mean flow is strongly affected by the presence of mesoscale eddies. In the first part of this report we describe the two-layer model and discuss its numerical formulation.Then the results of a preliminary eddy experiment are discussed in detail, showing the spontaneous growthof baroclinic eddies and describing the final statistical steady state that occurs. Energetic analyses andvorticity balances show the important role played by the eddies in determining the character of the oceanicgeneral circulation. Part II of this paper will discuss a variety of experiments which explore the dependence of results on thebasic parameters and boundary conditions governing the model. In particular the dependence of resultson wind stress magnitude and distribution, lateral viscosity coefficient, basin size, and boundary conditions(free slip and no slip) will be examined.