Surface currents in British Columbia coastal waters: Comparison of observations and model predictions

Observations of the motion of ocean surface drifters ale used to evaluate n umerical simulations of surface currents in the region of Queen Charlotte S ound on the West Coast of Canada. More than 30 surface Argos drifters were deployed in the spring and summer of 1995, revealing daily average currents of 10 to 40 cm s(-1) near the coast of Vancouver Island in summer; and les s than 10 cm s(-1) in mid-sound. Wind observations in this region are provi ded by a network of weather buoys. Comparison of daily average drifter velo cities and winds shows that the drifters moved at 2 to 3% of the wind speed , and at about 30 degrees to the right of the wind. A complex transfer function is computed between daily wind and drifter vect ors using least squares techniques. The ratio of variance in the least squa res residual currents to the variance of observed drifter currents is denot ed gamma(2). A percent goodness-of-fit is defined as g(gamma(2)) = 100(1 - gamma(2)), and is 42% for the case of daily winds and drifter currents. Dri fter-measured currents are compared with two numerical simulations of surfa ce currents: Fundy5, a stendystate baroclinic model based on historical wat er property measurements in summer and the Princeton Ocean Model (POM), a p rognostic, baroclinic model forced by the measured 1 inds. Fundy5 by itself provides a goodness-of-fit of only 3%, whereas whereas 42%. The combinatio n of Fundy5 plus daily wind gives g(gamma(2)) = 43%. Although the prognosti c model performs only as well as the winds by themselves, it simulates the near shore currents more accurately and reproduces the speeds and veering i n the surface Ekman layer on average without bias. Residual currents unexpl ained by POM are likely due to advection of water masses into this region a nd horizontal inhomogeneities in the density field that are not input to th e model, as well as to Stokes di ift of wind waves and to net Lagrangian ti dal motion not represented by the model.