Initialization, Asymmetry, and Spindown of Arctic Eddies

Abstract

Initialization and development of cyclonic and anticyclonic eddies under the influence of a frictional surface, as with Arctic eddies under ice cover, are examined both analytically and numerically using a three-dimensional numerical model. Solutions of the linear Rossby adjustment problem show that energy released from an initial density anomaly in the barotropic mode and the first few baroclinic modes destabilizes the numerical computation for small Arctic eddies. This result suggests the necessity of slow spinup to reduce energy release in these modes and sufficient vertical resolution to resolve higher baroclinic modes. The numerical computation shows that in an initially stratified and motionless ocean, a surface cyclone (an anticyclone) is produced by a localized salinity source (sink). In open waters, the stable flow field consists of a vertically aligned pair of counterrotating eddies. When dampened by surface friction, an eddy pair produced by deep forcing has features dissimilar to the submerged eddies under Arctic ice. In the case, of shallow forcing with depth scales about 100 m or so, surface friction can quickly eliminate the top eddy while leaving the lower eddy intact. This leads to the counterintuitive results that warming or freshening generates a submerged cyclone, while cooling or brine ejection produces a submerged anticyclone. The resulting eddies have many attributes of observed Arctic eddies under sea ice.