Eddy Maintenance and Attrition in a Vertically Sheared Current under Arctic Ice

Abstract

Steady drift of an ice cover produces a vertically sheared current in the upper ocean of the Arctic. Under the ice cover, mesoscale shallow brine and freshening sources generate submerged anticyclones and cyclones, respectively. A submerged eddy extending deep into the water column experiences differential advections by the vertically sheared current. Interaction between subsurface eddies and the sheared current is examined using a three-dimensional numerical model in a coordinate system moving with the ice. The initial salinity field is in geostrophic balance with the sheared current, and a pulse of brine or freshening forcing produces an anticyclone or a cyclone. In a coordinate system moving with the ice, eddies are in a vertically sheared backward ambient current. To an observer looking into the direction of the backward ambient current, eddies move with the current and deflect to the right (left) for counterclockwise (clockwise) rotating eddies in both hemispheres. The lower half of the eddy always moves faster. The lateral movement can be explained by the Kutta?Zhukhovski lift theorem. Differential advection produces eddy tilting and entails the development of a narrow jet following the moving eddy. The jet reduces eddy straining and tilting, and eddies disperse in cases of sizeable tilts. Driven by a vertically sheared current, cyclones are short-lived compared with anticyclones because the lateral movement of a cyclone exposes the lower part of the eddy into waters of weaker stratification. The results help explain the predominance of anticyclonic eddies under the Arctic ice.