Circulation in the Juan de Fuca eddy region - observational and modeling results

Amy MacFadyen¹, Michael Foreman², Barbara Hickey¹ and Vera Trainer³

1 School of Oceanography, University of Washington, Seattle, 98195-07940, USA
EMAIL: amoreena@ocean.washington.edu
2 Institute of Ocean Sciences, Sidney B.C., V8L 4B2, Canada
3 Northwest Marine Fisheries Service, NOAA, Seattle, 98122-2013, USA

A quasi-stationary counterclockwise cold eddy off the Strait of Juan de Fuca has been implicated as an initiation site for toxic Pseudo-nitzschia cells that impact shellfish on the Washington coast. Recent surveys, conducted during the first field season of the ECOHAB PNW project, illustrate both the robust nature of this feature, and the complex interaction between winds, shelf currents, strait outflow and underlying topography in determining the circulation in this region.

In the late summer of 1998, a widespread toxic event resulted in record levels of toxin in razor clams on the Washington coast and long term harvesting closures. A diagnostic finite element circulation model was developed for the eddy region and the northern Washington shelf and forced with 1998 summertime temperature and salinity measurements, tides and typical summer winds. The model circulation shows strong retention in the eddy region and model drifter trajectories compare well with true drifters released in 2001 through 2003. Both model and true drifter trajectories show that surface waters tend to leave the eddy to flow southeastward along the Washington shelf. During storms onshore Ekman surface flow moves drifter pathways closer to the coast suggesting that phytoplankton from the surface waters of the Juan de Fuca eddy could impact the Washington coast. This result is consistent with time series data that shows toxic episodes at the beach preceded by downwelling-favorable wind events, and associated onshore transport.