Abstract
Abstract. Numerical simulations of tidal flow past Cape St. James on the south tip of Haida Gwaii (Queen Charlotte Islands) are presented that indicate mesoscale dipoles are formed from coalescing tidal eddies. Observations in this region demonstrate robust eddy generation at the Cape, with the primary process being flow separation of buoyant or wind driven outflows forming large anti-cyclonic, negative potential vorticity, Haida Eddies. However, there are other times where dipoles are observed in satellites, indicating a source of positive potential vorticity must also be present. The simulations here build on previous work that implicates oscillating tidal flow past the cape in creating the positive vorticity. Small headland eddies of alternating vorticity are created each tide. During certain tidal cycles, the headland eddies coalesce and self organize in such a way as to create large >20-km diameter eddies that then self-advect into deep water. The self advection speed is faster than the beta drift of anti-cyclones, and the propagation direction appears to be more southerly than typical Haida Eddies, though the model contains no mean wind-driven flows. These eddies are smaller than Haida Eddies, but given their tidal origin, may represent a more consistent source of coastal water that is injected into the interior of the subpolar gyre.
Highlights
Eddies are an important mechanism for stirring the ocean and transporting nutrients and other tracers long distances
James to form an eddy as seen in the laboratory studies of Cenedese and Whitehead (2000). This idea was refined by Di Lorenzo et al (2005) who reproduced Haida eddies in a Regional Ocean Modeling System (ROMS) model but, rather than a single separation from the cape producing a Haida eddy, it was found that a succession of smaller (≈80 km diameter) eddies generated at the cape would merge together to form a large (≈200 km diameter, extending to 1000 m depth) negative vorticity eddy
In our model runs below we find evidence of dipoles of negative and positive potential vorticity (PV) eddies
Summary
Eddies are an important mechanism for stirring the ocean and transporting nutrients and other tracers long distances. The southward return flow was concentrated to the west side of the strait due to the Coriolis force This buoyant current would separate from the shore when it passed Cape St. James to form an eddy as seen in the laboratory studies of Cenedese and Whitehead (2000). James to form an eddy as seen in the laboratory studies of Cenedese and Whitehead (2000) This idea was refined by Di Lorenzo et al (2005) who reproduced Haida eddies in a ROMS model but, rather than a single separation from the cape producing a Haida eddy, it was found that a succession of smaller (≈80 km diameter) eddies generated at the cape would merge together to form a large (≈200 km diameter, extending to 1000 m depth) negative vorticity eddy. 2) of the Cape St. James region that has a mean stratification and is only forced with tides and find a fortnightly generation of large dipoles via the coalescence of small frictionally generated headland eddies The character of the small and large eddies are discussed (Sect. 4) and the implications of our findings summarized (Sect. 5)
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