Numerical Simulation of the Gulf Stream and Mid-Ocean Eddies

The circulation of the western North Atlantic is simulated with a primitive equation model that has 5 levels and a horizontal grid size of 37 km. The idealized model domain is a rectangular basin, 3000 km long, 2000 km wide and 4 km deep, which is oriented so that the long axis of the basin is parallel to the east coast of the United States. The nearshore side of the basin has a simple continental shelf and slope, whereas the other sides are bounded by vertical wills. The model ocean is driven by a 2½ gyre pattern of steady zonal wind stress and by a Newtonian-type surface heating. After initialization from a 15-year spin-up with a coarser grid, two experiments are carried out, each of several years duration: the first uses a Laplacian formulation for the subgrid-scale lateral diffusions of heat and momentum, the second uses a highly scale-selective biharmonic formulation for these diffusions. Bottom friction is present in each case. In both experiments, a western boundary current forms which separates from the coast and continues eastward as an intense free jet, with surface velocities >1 m s−1 for almost 1000 km downstream. In the experiment with biharmonic closure, this simulated Gulf Stream develops large-amplitude transient meanders, some of which become cold-core cyclonic rings and warm-core anticyclonic rings that drift westward. In both experiments, transient mesoscale eddies also form in the broad westward-moving North Equatorial Current, where the simulated thermocline in the model ocean slopes downward toward the north. The remaining regions of the model ocean also contain transient mesoscale eddies, but they are of weaker intensity. The dominant process of eddy kinetic energy production, in both experiments, is a baroclinic-barotropic instability which is concentrated in the part of the Gulf Stream that is over the continental slope. But where the Gulf Stream lies over the abyssal plains, there is a large reconversion of eddy kinetic energy into the kinetic energy of the time-averaged flow. Eddy kinetic energy is also produced by baroclinic instability in the North Equatorial Current, but at a much smaller rate. In the biharmonic experiment, the eddies transfer considerable kinetic energy downward, and bottom friction is the dominant process of eddy kinetic energy dissipation. An analysis of the heat transports in the biharmonic experiment, shows that the horizontal transport of heat by eddies is much larger than the subgrid-scale horizontal heat diffusion. In the Gulf Stream region, the eddy heat transport is comparable to the effect of a lateral diffusion coefficient of 107 cm2 s−1.