Sack Lunch Seminar (SLS)

This study is motivated by Arctic Ocean observations of sub-mixed layer eddies found at large distances from their assumed formation region of a surface ocean front. The eddy formation is explored through high resolution numerical simulations of surface fronts separating two mixed layers, with a range of configurations similar to those observed in the Arctic Ocean. The frontal instabilities lead to the development of self-propagating dipoles which have the potential to propagate far from the front provided that the interactions with other eddies are avoided. However, most dipoles are unbalanced, consisting of a dominating surface cyclone and a weaker anticyclone below, and thus propagate on curved trajectories with eventual recirculation back to the front. Their maximum separation distance from the front depends on the ratio of self-advecting velocities epsilon; balanced dipoles have epsilon~1 and the ability to propagate far from the front. For dipoles generated numerically, we estimate epsilon using analytical solutions of a 2.5 layer quasigeostrophic model for Gaussian vortices. The distribution of the ratio epsilon for these dipoles is skewed towards higher values (i.e. cyclones are dominant in dipoles). Sensitivity experiments suggest that shallow fronts that separate mixed layers of approximately equal depths favor the development of far-propagating balanced dipoles. These favorable frontal configurations also commonly appear during the melting of sea ice at marginal ice zones where mesoscale eddies transport sea ice into the open ocean affecting the melting rates.