Evidence of a topographically trapped vortex on the Norwegian continental shelf

Abstract

The Halten Bank at 64°40′N, 09°00′E on the Norwegian continental shelf is characterized by a large ratio of fractional height to Rossby number and relatively weak winter stratification so that theory predictsa trapped vortex over the bank. Current measurements show that the bank perturbs the generally northeast flow of the area. In particular, there are considerable horizontal and vertical current shears over the east slope. Hydrographic observations reveal a dome of cold heavy water over the bank; it reaches the surface in winter and is limited to below the seasonal thermocline in summer. The data are interpreted in terms of Taylor column theory and it is suggested that the winter circulationover the bank is characterized by a topographically trapped quasi-stationary vortex. Significant variations of the current direction with depth and veering of the current at 50m from west to north-northwest in fall, at the time of large temperature increases at that depth, suggest that the vertical extent of the column varies with the stratification. The anticyclonic vortex may become baroclinically unstable and play an important part in the formation of bottom water.