Observations of bipolar residual circulation in two equatorward-facing semiarid bays

Two equatorward-facing semiarid bays in Chile (around 30°S) were sampled between March 23 and 27, 1997 with the purpose of determining whether the similar coastline morphology of the bays caused residual circulation patterns that were alike. A towed acoustic Doppler current profiler and a conductivity-temperature-depth recorder were used to measure water current and density profiles along a triangular trajectory during two semidiurnal tidal cycles at each of the bays. The bays were influenced by semidiurnal tides. The bay to the north, Coquimbo Bay, featured a bipolar residual circulation consisting of a well-defined surface counter-clockwise gyre that occupied the northern two-thirds of the bay. The southern third of the bay was not completely resolved by the sampling but measurements suggested the presence of a clockwise circulation. The bay to the south, Guanaquero Bay, showed a well-defined clockwise gyre that occupied the southern half of the bay. Counter-clockwise circulation was apparent over the northern half. Near-surface circulations changed little with depth in both bays. The similarity in surface residual circulation patterns between bays, namely the presence of two counter-rotating gyres, was attributed to flow separation at the upstream headland of the embayment. In this context, “upstream” was relative to the direction of wind forcing and prevalent coastal currents. This direction was different during sampling of Coquimbo Bay (northerly winds and coastal currents) than in Guanaquero Bay (southwesterly winds and coastal currents). It was proposed that, in general, two counter-rotating gyres will appear in bays where the entrance width exceeds twice the radius of curvature of the headlands that limit them.