Eddy morphology: Egg-like shape, overall spinning, and oceanographic implications

Systematic tracking of individual eddies during their entire lifetimes marks a significant milestone in satellite oceanography over the first two decades of this century. Assuming that the geometry and properties of an oceanic eddy are orientation-sensitive, an angularly aligned composite analysis of over 40 million eddy-focused surface topographic "snapshots" obtained by merging tandem altimeter data from January 1993 through January 2019 reveals that oceanic vortices appear to have a characteristic surface shape of "egg" rather than circle or ellipse as previously understood. Consequently, a second-order moment in eddy morphology is revealed which leads to a similar to 10 km departure from a standard ellipse in terms of major axis. Furthermore, the sharp poles of oceanic eddies exhibit two quasi-orthogonal modes of orientation: primarily meridional and secondarily zonal. The additional submesoscale asymmetry in eddy shape is confirmed by a consistent anisotropy in a normalized eddy-centric velocity field derived from over 25 thousand drifters. The high-order moments in terms of geometric asymmetry and dynamic anisotropy associated with mesoscale eddies (which are found to be statistically significant at 99% level) may have profound geophysical and biological impacts on energy transport, substance entrainment, as well as ecosystem dynamics in the ocean. In particular, it is demonstrated that some of the previously identified patterns in eddy properties obtained by non-rotated normalization may be notably biased due to the ignorance of existing eddy orientation.