The degree of anisotropy for mid-ocean currents from satellite observations and an eddy-permitting model simulation

The degree of anisotropy is calculated for the mid-ocean currents estimated from satellite altimetry and simulated with a numerical model of the Pacific Ocean. A high resolution eddy-permitting model is used for its ability to simulate mid-ocean multiple zonal flows, crucial for the evaluation of the degree of anisotropy. Using a commonly defined parameter of anisotropy, a, that falls between -1 and 1 and equals 0, - 1, and 1 for an isotropic, purely meridional, and purely zonal flow, respectively, it is found that a increases from nearly zero for weekly data to significantly positive values for seasonally and annually averaged data. This tendency of increasing zonal anisotropy with averaging time is true for the surface geostrophic velocity from satellite altimetry and for both surface and deep-ocean velocities from the model simulation. The absolute value of a for the simulated surface currents agree with that derived from satellite observation, reaffirming the reliability of both data sets. In the model simulation, the behavior of a at the surface is very similar to that at 1000 m, indicating a deep structure of the zonally elongated features in the middle of the Pacific Ocean. The implications of these findings are discussed in the context of oceanic eddies, Rossby waves, and zonal jets.