Modeling the formation of eddy dipoles at Cape St. James




Callendar, Wendy

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We present here a theory for the generation of mesoscale eddies, in the context of describing the generation of dipoles seen near the Queen Charlotte Islands in British Columbia. The Regional Ocean Modeling System (ROMS) is used to show dipoles forming from the coalescence of small headland eddies at Cape St. James. These headland eddies are formed by frictional generation of potential vorticity (PV) when the tide oscillates across the cape. Only 20% of the PV generated at the cape ends up in the headland eddies, with the remainder lost due to mixing of waters with PV of opposite signs. Coalescence of the headland eddies is achieved with a much higher efficiency - the PV contained in the final eddy is near 80% of the sum of that contained in the small eddies. Not all headland eddies coalesce. Coalescence of a positive PV eddy occurs only when the eddy is formed on a strong tidal flood followed by a weak ebb. Thus, a diurnal inequality in the tides is a requirement for coalescence. The eddies in the final dipole contain roughly equal amounts of PV; each has a radius of approximately 15 km and extends to nearly 100-m depth.



Haida Gwaii, British Columbia, oceanography, eddies, ocean mixing