Scale-dependent anisotropy of the ocean velocity field
Event: 2023 Ocean Surface Topography Science Team Meeting
Session: Science III: Mesoscale and sub-mesoscale oceanography
Presentation type: Type Poster
Contribution: PDF file
The anisotropic structure of the ocean velocity field is re-examined using ~30 years of satellite altimetry observations, combined with results from a high-resolution ocean circulation model. In particular, we try to understand if the degree of anisotropy of the flow field is scale-dependent and how it varies in time. We find that low-frequency variations (periods longer than 16 months) are characterized by large positive values of the anisotropy coefficient indicating the dominance of zonal movements. The geographical distribution of the energy containing length scales of the low-frequency variability is similar to that of the eddy length scales (from the eddy dataset), suggesting that mesoscale eddies and, in particular, eddy movements project strongly on the low-frequency variability. Higher-frequency variability (periods shorter than 9 months) is primarily isotropic (except for a very narrow near-equatorial region) and characterized by generally shorter spatial scales varying from about 400 km wavelength near the equator to 100 km wavelength towards sub-polar latitudes. The Aviso fields show spurious negative anisotropy at short temporal and spatial scales, reflecting presumably correlated errors in sea level anomaly (SLA) maps. These effects are visible mostly in sub-tropical latitudes and the velocity variance at these scales is relatively low. Averaged over all time scales, the distribution of the flow anisotropy geographically and as a function of spatial scale provides a more nuanced view on the flow regimes. An example would be elevated zonal anisotropy on the northern and southern flanks of the Antarctic Circumpolar Current (ACC) associated with relatively long spatial scales and reduced zonal anisotropy along the core of the ACC at the mesoscale. Likewise, elevated levels of zonal anisotropy in the interiors of the subtropical gyres (particularly in their eastern parts) in the mesoscale range are mainly associated with the so-called “eddy trains” and/or “polarized eddy tracks”. The effect of this kind of eddy anisotropy on the horizontal transport and mixing of tracers is assessed using numerical experiments with Lagrangian particle trajectories.