Modelling the generation of Haida Eddies

A numerical model forced with average annual cycles of climatological winds, surface heat flux, and temperature and salinity along the open boundaries is used to demonstrate that Haida Eddies are typically generated each winter off Cape St. James, at the southern tip of the Queen Charlotte Islands of western Canada. Annual cycles of sea-surface elevation measured at coastal tide gauges and TOPEX/POSEIDON crossover locations are reproduced with reasonable accuracy. Model sensitivity studies show that Haida Eddies are baroclinic in nature and are generated by the merging of several smaller eddies that have been formed to the west of Cape St. James. The generation mechanism does not require the existence of instability processes and is associated with the mean advection of warmer and fresher water masses around the cape from Hecate Strait and from the southeast. These advected water masses generate plumes of buoyant flow, which intensify and sustain small patches of anticyclonic circulation immediately to the northwest of the cape. When the flow is stronger, several of these smaller eddies can merge to generate a larger eddy, the Haida Eddy. Similar to observations, a typical generation-shedding cycle for larger Haida Eddies in the model is 3-4 months. Consistent with previous in situ water property measurements, these experiments show that the eddies are generally comprised of mixed-layer water from Hecate Strait, Queen Charlotte Sound, and the continental shelves off northern Vancouver Island. Their vertical extent during the mature stage is roughly 1000 m.