Physical characteristics of Haida Eddies

Haida Eddies are anti-cyclonic features that form in winter along the eastern continental margin of the Gulf of Alaska, west of the Canadian Queen Charlotte Islands. These eddies have been observed in many oceanographic programs, but their full life cycle has only been known for the past few years. Core waters of eddies are warmer below 100 m depth, and normally fresher at all depths than surrounding waters. Eddies formed in the ENSO winters of 1982/83 and 1997/98 were the highest observed, and were warmer than observed in other years. A study of historical variability of dynamic heights in regions where five eddies were observed shows that Haida Eddies are rare events in each region; and their impact cannot be simulated in any one year without directly modelling each individual eddy. The very large Haida Eddy formed in 1998 is the best sampled to date. Satellite-based measurements six months after its formation show the eddy to rise between 0.33 and 0.37 m above surrounding waters, with a Gaussian radius between 60 and 75 km. Dynamic height anomalies at this time, computed from ship-based observations along two lines through the eddy to 1500 metres depth, reveal a height of 0.32 and 0.35 m respectively, relative to surrounding waters, and Gaussian radius of 60 km. It was almost totally baroclinic, with one-half the baroclinic structure in the upper 375 in or so; however, its anti-cyclonic rotation depressed isotherms and isohalines at depths below 1000 m.