On the Eddy Isopycnal Thickness Diffusivity of the Gent–McWilliams Subgrid Mixing Parameterization

Abstract

Output of an eddy-resolving model of the North Atlantic Ocean is used to diagnose the eddy thickness diffusivity coefficient, ?, defined by Gent and McWilliams in their quasi-adiabatic parameterization for transports by mesoscale eddies. The results suggest that ? has large spatial and temporal variations, with negative values about half of the time. The order of magnitude of ? shows a wide range in the western North Atlantic, varying from 10 m2 s?1 to 107 m2 s?1. Also, the value of ? is considerably affected by the timescale used to define the high-frequency and low-frequency components. The results suggest that ? should be a diagnosed variable that reflects the strength of eddy mixing during a model integration.