Surface circulation in a Caribbean island wake

Flow disturbances caused by islands can have important biological effects on coastal marine ecosystems. In order to measure the surface current circulation in the lee of the Caribbean island of St. Croix, we deployed a high-frequency radar system for approximately 3 months which provided hourly current vectors, obtained over approximately 600 km2 with 1 km resolution. Here, we report two of the principal physical observations. First, contrary to expectations from computed Reynolds numbers, no wake eddies were formed. Using a horizontal eddy viscosity of 100 m2 s−1, Reynolds numbers often exceeded 80, large enough to produce wake eddies in rotating laboratory flow studies and in numerical modelling experiments. Although no wake eddy formation was observed, a persistent nearshore convergence region of weak currents off the northwest coast was present. This suggests that slightly larger values of horizontal eddy viscosity may be appropriate for circulation models of this region. Second, anomalously strong eastward flows were occasionally observed that were not driven by any local wind change. We present evidence from satellite altimetry which indicates that this type of event is caused by an anticyclonic mesoscale eddy centered south of the island. Such nearshore convergence regions and anticyclonic eddies may have strong effects on the retention and dispersal of larvae of coral reef fishes among islands within the Caribbean region.