Eddy maintenance and attrition in a vertically sheared current under Arctic ice

Steady drift of an ice cover produces a vertically sheared current in the u pper ocean of the Arctic. Under the ice cover, mesoscale shallow brine and freshening sources generate submerged anticyclones and cyclones, respective ly. A submerged eddy extending deep into the water column experiences diffe rential advections by the vertically sheared current. Interaction between s ubsurface eddies and the sheared current is examined using a three-dimensio nal numerical model in a coordinate system moving with the ice. The initial salinity field is in geostrophic balance with the sheared current, and a p ulse of brine or freshening forcing produces an anticyclone or a cyclone. I n a coordinate system moving with the ice, eddies are in a vertically shear ed backward ambient current. To an observer looking into the direction of t he backward ambient current, eddies move with the current and deflect to th e right (left) for counterclockwise (clockwise) rotating eddies in both hem ispheres. The lower half of the eddy always moves faster. The lateral movem ent can be explained by the Kutta-Zhukhovski lift theorem. Differential adv ection produces eddy tilting and entails the development of a narrow jet fo llowing the moving eddy. The jet reduces eddy straining and tilting, and ed dies disperse in cases of sizeable tilts. Driven by a vertically sheared cu rrent, cyclones are short-lived compared with anticyclones because the late ral movement of a cyclone exposes the lower part of the eddy into waters of weaker stratification. The results help explain the predominance of anticy clonic eddies under the Arctic ice.