Dynamics of the Gulf Stream/Deep Western Boundary Current Crossover. Part I: Entrainment and Recirculation

Abstract

A regional primitive equation model is applied to the study of the interaction between the Gulf Stream and the deep western boundary current (DWBC) where they cross at Cape Hatteras. It is found that for 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 along the western boundary and the other flowing toward the cast under the Gulf Stream. The eastward branch is entrained Into the southern recirculation gyre and, after being diverted into the interior for up to 1500 km, eventually returns to the western boundary current and continues to flow southward. This recirculation and mixing is shown to have a significant impact on the separation point and mean path of the Gulf Stream, the basin-scale stratification, and the properties of the DWBC south of the crossover. For most configurations, the lower DWBC remains largely on the western boundary and interacts only weakly with the interior. The entrainment of the upper core is shown to be driven by a baroclinic, time-dependent process of DWBC water eddy formations into the recirculation gyres under the Gulf Stream. Potential vorticity considerations are key to understanding the entrainment mechanism and its sensitivity to variations in the model forcing and configuration. A scaling estimate of the amount of entrained DWBC water as a function of the eddy field is derived. The mean paths of the upper and lower DWBCs and strength of the eddy fluxes compare well with various observational estimates. The importance of such entrainment and mixing processes on large-scale ocean modeling and climate studies is discussed.