Ubiquitous, patchy, energetic upper ocean structures persist for years

We investigate the isotropy of oceanic mesoscale eddies with seven years of sea surface height data recorded by satellite altimeters. From these data, we determine a sea surface height anomaly, and surface geostrophic velocities u' and v' in the zonal (east-west) and meridional (north-south) directions, respectively. From the latter two quantities we can calculate zonal and meridional kinetic energies u'^2 and v'^2. In the extratropics, the zonal averages of u'^2 and v'^2 are nearly equal, in contrast with the results of simple beta-plane geostrophic turbulence models, which suggest that zonal motions should predominate. Although mid- and high-latitude eddies are isotropic when averaged over large spatial scales, on small scales the mesoscale eddy field is locally anisotropic almost everywhere. Maps of the quantity u'^2-v'^2 (normalized by u'^2+v'^2) show fine-scale structures that persist over times longer than the lifespan of a turbulent eddy. Further investigation into the causes of these small-scale structures is currently underway.