The effect of density stratification and a cape in a baroclinic western boundary current separation experiment

Abstract

Western boundary current separation has long been a mystery. For the Gulf Stream, different factors such as the coastal shape, inflow and outflow location, wind stresses, continental shelf slope, shallow underwater plateaus, and interaction with the deep circulation potentially play unique and important roles in separating the Gulf Stream from the coast. To study these effects, a model consisting of a circular tank of water rotating on a spinning table was set up. Sloping planes form the upper and lower boundaries of the enclosed tank, approximating the Coriolis force. Then, water was pumped through gaps in the tank, producing a western boundary current and an artificial cape having the geometry of Cape Hatteras at the Gulf Stream’s point of separation was introduced into the system. Finally, to study the effects of stratification on the point of separation, a 2-layer system was used. The results of varying different parameters, such as flow rate or density difference between layers, were compared with observations of the Gulf Stream and output from a numerical model. Ultimately, the experimental results showed that density differences alone do not affect the separation point to a meaningful degree, but rather that it is the position of inflow and outflow gaps that are much more significant. Density differences alone do not significantly affect the separation point. The relationship between high flow rates tending to create more modes of oscillation and a moving separation point was also observed. More so than any other setup, a cape at high density difference and low flow rate deflects the western boundary current flow. The results suggest that the interplay between the 1-layer and 2-layer modes is relevant to the oceanic case.