The large-scale time-mean ocean circulation in the Nordic Seas and Arctic Ocean estimated from simplified dynamics

Abstract

A simplified diagnostic model of the time-mean, large-scale ocean circulation in the Nordic Seas and Arctic Ocean is presented. Divergences in the surface Ekman layer are extracted from observed climatological wind stress fields. Similarly, divergences caused by the meridional thermal wind transport (relative to the bottom) are calculated from an observed climatological density field. These known quantities are then used to force the model's bottom geostrophic velocities. Both scaling arguments and direct observations show that for long time scales the bottom currents are closely aligned with contours of flH, (where f is the Coriolis parameter and H is the depth of the seabed). Due to the weak planetary vorticity gradient at high latitudes, the f/H field is dominated by topography and is characterized by multiple regions of closed isolines. The only frictional effect included in the model is bottom stress. By then integrating the depth-integrated vorticity equation over the area spanned by a closed f/H contour, and assuming that the same contour is a streamline of the bottom geostrophic flow, we derive an analytical expression for the bottom geostrophic velocity on this f/H contour. For the few contours that are not closed, current measurements are used as boundary conditions. Model results are compared with near-bottom current measurements in both the Nordic Seas and the Arctic Ocean. In addition comparison is made with observations from surface drifters in the Nordic Seas by adding the observed thermal wind shear to the modeled bottom flow. The agreement is surprisingly good, suggesting that the simple model is capturing some of the most important processes responsible for the large-scale circulation field. Features like the subgyre recirculations in the Nordic Seas, the gyres in the Canadian and Eurasian Basins, the East Greenland Current, the Norwegian Atlantic Current and the Arctic Circumpolar Boundary Current are all well reproduced by the model. The simplicity of the model makes it well suited as a dynamical framework for interpreting the large-scale circulation pattern in the Nordic Seas and Arctic Ocean.