DYNAMICS OF THE GULF-STREAM DEEP WESTERN BOUNDARY CURRENT CROSSOVER .1. ENTRAINMENT AND RECIRCULATION

A regional primitive equation model is applied to the study of the int eraction between the Gulf Stream and the deep western boundary current (DWBC) where they cross at Cape Hatteras. It is found that fur a wide range of forcing parameters the upper core of the DWBC is split into two mean paths at the crossover, one flowing toward the south dong the western boundary and the other flowing toward the east under the Gulf Stream. The eastward branch is entrained into the southern recirculat ion gyre acid. after bring diverted into the interior for up to 1500 k m, eventually returns to the western boundary current and continues to flow southward. This recirculation and mixing is shown to have a sign ificant impact on the separation point and mean path of the Gulf Strea m, the basin-scale stratification, and the properties of the DWBC sout h of the crossover. Fur most configurations, the lower DWBC remains la rgely on the western boundary and interacts only weakly with the inter ior. The entrainment of the upper core is shown to be driven by a baro clinic, time-dependent process of DWBC water eddy formations into the recirculation pq res under the Gulf Stream. Potential vorticity consid erations are key to understanding the entrainment mechanism and its se nsitivity to variations in the model forcing and configuration. A scal ing estimate of the amount of entrained DWBC water as a function of th e eddy field is derived. The mean paths of the upper and loa er DWBCs and strength of the eddy fluxes compare well with various observationa l estimates. The importance of such entrainment and mixing processes o n large-scale ocean modeling and climate studies is discussed.