Assessing coherent feature kinematics in ocean models

We propose a method for comparing ocean models based on their ability to reproduce the kinematics associated with coherent features such as mesoscale eddies. Of particular focus is the reproduction of transport and mixing processes around the feature in question. The feature boundaries are represented by material curves, known as effective invariant manifolds, and the error in the location of the manifolds as manifest in different models is measured here in terms of separation area and distance between them. As part of the case for such a Lagrangian discriminant of ocean models, we show that, in reference to a Rossby wave, critical-layer kinematic model, an Eulerian error metric based on energy is insufficient to determine the proximity of vortex boundaries in different perturbations. The manifold comparison technique developed here is applied to a model double-gyre flow to analyze the effects of finite resolution and uncertainty in wind forcing. The results of the Lagrangian evaluation showed that, in the resolution experiments, the manifold accuracy correlates with the average Eulerian error (L2 norm of the velocity error field). However, for the models with modulated wind forcing, the Eulerian error was found to be an inadequate indicator of the quality of transport predictions.