A Numerical Study of the Variability and the Separation of the Gulf Stream, Induced by Surface Atmospheric Forcing and Lateral Boundary Flows

Abstract

A primitive equation regional model is used to study the effects of surface and lateral forcing on the variability and the climatology of the Gulf Stream system. The model is an eddy-resolving, coastal ocean model that includes thermohaline dynamics and a second-order turbulence closure scheme to provide vertical mixing. The surface forcing consists of wind stroll and beat fluxes obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS). Sensitivity studies am performed by driving the model with different forcing (e.g., annual versus zero surface forcing or monthly versus annual forcing). The model climatology, obtained from a five-year simulation of each case, is then compared to observed climatologies obtained from satellite-derived SST and hydrocast data. The experiments in which surface best flux and wind stress were neglected show less realistic Gulf Stream separation and variability, compared with experiments in which annual or seasonal forcing are used. A similar unrealistic Gulf Stream separation is also obtained when the slope-water inflow at the northeast boundary is neglected. The experiments suggest that maintaining the density structure and the concomitant geostrophic flow in the northern recirculation gyre plays an important role in the separation of the Gulf Stream. The maintenance of the recirculation gyre is affected by beat transfer, wind stress and slope-water inflow. The beat transfer involves several processes: lateral eddy monster, surface heat gm and vertical mixing. Further improvement of the Gulf Stream separation and climatology are obtained when seasonal changes in the lateral temperature and salinity boundary conditions are included. The seasonal climatology of the model calculations compare reasonably well with the observed climatology. Although total transports on open boundaries are maintained at climatological values, there are, nevertheless, large seasonal and spatial variations of Gulf Stream transport between Cape Hatteras and 62°W. These changes are accompanied by transport changes in the northern recirculation gyre.