Modeling the generation of the Juan de Fuca Eddy

Abstract

Numerical simulations with the Regional Ocean Modeling System are used to study the generation of the cyclonic Juan de Fuca Eddy located off the entrance of Juan de Fuca Strait in summer. An initial simulation forced with average summer upwelling favorable winds, tides, and buoyancy boundary conditions that maintain an estuarine flow in Juan de Fuca Strait produces an eddy and currents that are in reasonable agreement with observations. Sensitivity studies are then carried out to explore the importance of these three forcing mechanisms. The relative proximity of dense water in the bottom estuarine flow entering the strait is shown to lead to enhanced upwelling off Cape Flattery when either wind or tidal forcing is applied. This upwelled water then mixes with the estuarine outflow and is advected offshore. The tidal upwelling arises through three mechanisms: M2 vertical excursions of nearly 20 m at 50 m depth west of the cape on the flood tide; strong tidally rectified vertical velocities west of the cape; and the spilling of denser bottom water over the western wall of Juan de Fuca Canyon on the ebb tide. The cyclonic eddy is a consequence of geostrophic adjustment to the doming isopycnals that arise from the upwelling. These model simulations refute an earlier hypothesis that the eddy is generated when California Undercurrent water is drawn up Tully Canyon and onto the Vancouver Island shelf, suggesting instead that these canyon dynamics play only a secondary role in maintaining the eddy once it is formed.