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 closedbasin show that mesoscale eddiescan appear spontaneously during theintegration of the equations of motion for a baroclinic ocean. Forsomevalues of the basic parameters governing the flow, the solutionsreach a steady state while for other valuesfinite-amplitude eddiesremain a part of the final statistically steady state. In the eddyingcases the solutionscan be regarded as a mean flow upon which is superimposeda set of eddies which propagate westward at afew kilometers per day.The eddies typically have horizontal wavelengths of a few hundredkilometers.Analyses of die energetics show the eddies to be generated by theprocess of baroclinic instability. Thepotential energy of the meanflow is released to supply energy to the eddies. The computed Reynoldsstresses,while small compared to the terms in the geostrophic balance of themean momentum equations, dohave a strong influence on the mean circulationand, in fact, the deep mean circulation is driven entirelyby theeddies. If the flow were steady, there would be no flow in the deeplayer in this model. Finally, thecomputed curl of the Reynolds stressesshows that the vorticity balance of the mean flow is strongly affectedby the presence of mesoscale eddies.In the first part of this report we describe the two-layer model anddiscuss its numerical formulation.Then the results of a preliminaryeddy experiment are discussed in detail, showing the spontaneousgrowthof baroclinic eddies and describing the final statistical steadystate that occurs. Energetic analyses andvorticity balances showthe important role played by the eddies in determining the characterof the oceanicgeneral circulation.Part II of this paper will discuss a variety of experiments whichexplore the dependence of results on thebasic parameters and boundaryconditions governing the model. In particular the dependence of resultsonwind stress magnitude and distribution, lateral viscosity coefficient,basin size, and boundary conditions(free slip and no slip) will beexamined.