The Role of Wind Stress Curl in Jet Separation at a Cape

Abstract

A high-resolution numerical model is used to study the importance of spatial variability in the wind forcing to the separation of a coastal upwelling jet at a cape. An idealized topography and wind field based on observations from the Cape Blanco (Oregon) region are used. Several simulations are investigated, with both the intensity and the spatial structure of the wind forcing varied to isolate the importance of the observed intensification in the wind stress and wind stress curl magnitudes to the separation process. A simulation using a straight coast confirms that the presence of the cape is crucial for separation. Wind stress intensification by itself, with zero curl, does not aid separation. The wind stress curl intensification south of the cape, on the other hand, is important for controlling details of the process. Because the positive wind stress curl drives upwelling, isotherms in the offshore region tilt upward, creating a pressure gradient that sustains an intensification of the southward velocities via the thermal wind balance. This aids jet separation via continuity and by creating potential vorticity contours that track far offshore of the cape. The timing of the separation is dependent on the intensity of the wind stress curl (stronger curl leads to earlier separation), while how far offshore the jet is deflected depends on the offshore extent of the region of positive curl close to the coast (increasing the extent increases the deflection).