SPECIFICATION OF EDDY TRANSFER-COEFFICIENTS IN COARSE-RESOLUTION OCEAN CIRCULATION MODELS

Parametric representations of oceanic geostrophic eddy transfer of hea t and salt are studied ranging from horizontal diffusion to the more p hysically based approaches of Green and Stone (GS) and Gent and McWill iams (GM). The authors argue for a representation that combines the be st aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) a nd adoption of a transformed Eulerian mean formalism (GM). Recommendat ions are based upon a two-dimensional (zonally or azimuthally averaged ) model with parameterized horizontal and vertical fluxes that is comp ared to three-dimensional numerical calculations in which the eddy tra nsfer is resolved. Three different scenarios are considered: 1) a conv ective ''chimney'' where the baroclinic zone is created by differentia l surface cooling; 2) spindown of a frontal zone due to baroclinic edd ies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving ca lculations, the authors recommend a form for the horizontal transfer c oefficient given by k = alpha f/root Ri l(2) = alpha M(2)/N l(2), wher e Ri = f(2)N(2)/M(4) is the large-scale Richardson number and f is the Coriolis parameter; M(2) and N-2 are measures of the horizontal and v ertical stratification of the large-scale flow, I measures the width o f the baroclinic zone, and alpha is a constant of proportionality. In the very different scenarios studied here the authors find alpha to be a ''universal'' constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the atmosphere. The magnitude of t he implied k, however, varies from 300 m(2) s(-1) in the chimney to 20 00 m(2) s(-1) in the wind-driven channel.