Summer circulation of the waters in Queen Charlotte sound

Abstract

Abstract An extensive set of measurements of currents, winds, subsurface pressures and water properties was undertaken in the summer of 1982 in Queen Charlotte Sound on the west coast of Canada. At most observation sites the summer‐averaged currents are found to be about 10 cm s−1, smaller than the tidal currents but comparable to the standard deviation of the non‐tidal currents. The strongest average flow was the outflow of surface water past Cape St James at the northwestern corner of the Sound. During strong winds from the north or northwest a strong outflow of near‐surface fresher water was also observed over Cook Bank in the south. Eddies dominate the motion in the interior of the Sound, as shown by the behaviour of a near‐surface drifter that remained in mid‐Sound for 40 days before a storm pushed it into Hecate Strait. The disorganized, weak currents in the central Sound will likely allow surface waters or floating material to remain there for periods of several weeks in summer. Empirical orthogonal function analyses of fluctuating currents, subsurface pressures and winds reveal that a single mode explains most of the wind and pressure variance but not the current variance. The first two pressure modes represent two distinct physical processes. The first mode is a nearly uniform, up‐and‐down pumping of the surface, while the second mode tilts across the basin from east to west, likely due to geostrophic adjustment of wind‐driven currents. This mode also tilts from south to north, owing to along‐strait wind stress. Most contributions to the first mode currents come from meters near shore or the edge of a trough. Coherence is high between these second mode pressures and first mode currents and winds, and lower but still significant between first mode pressures and first mode currents and winds. It is therefore difficult to predict the behaviour of currents in Queen Charlotte Sound in summer from pressure measurements at a single site, but the difference in sea‐level across Hecate Strait is a more reliable indicator.