Eddy length scales in the North Atlantic Ocean

Eddy length scales are calculated from satellite altimeter products and in an eddy-resolving model of the North Atlantic Ocean. Four different measures for eddy length scales are derived from kinetic energy densities in wave number space and spatial decorrelation scales. Observational estimates and model simulation agree well in all these measures near the surface. As found in previous studies, all length scales are, in general, decreasing with latitude. They are isotropic and proportional to the local first baroclinic Rossby radius (L-r) north of about 30 degrees N, while south of 30 degrees N (or for L-r > 30 km), zonal length scales tend to be larger than meridional ones, and (scalar) length scales show no clear relation to L-r anymore. Instead, they appear to be related to the local Rhines scale. In agreement with a recent theoretical prediction by Theiss [2004], the observed and simulated pattern of eddy length scales appears to be indicative of two different dynamical regimes in the North Atlantic: anisotropic turbulence in the subtropics and isotropic turbulence in the subpolar North Atlantic. Both regions can be roughly characterized by the ration between L-r and the Rhines scales (L-R), with L-R > L-r in the isotropic region and L-R < L-r in the anisotropic region. The critical latitude that separates both regions, i.e., where L-R = L-r, is about 30 degrees N.