Flow induced by upwelling winds in an equatorward facing bay: Gulf of Arauco, Chile

Shipborne observations of hydrographic and flow velocity profiles were combined with wind velocity measurements to describe the characteristics of the wind‐induced flow in an equatorward facing bay of central Chile in South America. The measurements, which were taken from two transects and one anchor station, were made during late austral spring, between 4 and 10 December 2000. Most observations concentrated on Boca Grande, a transect that crossed the deep, northern (equatorward) entrance to the bay. The other transect crossed the smaller and shallower, westward entrance to the bay, Boca Chica. The anchor station was located inside the bay, close to Boca Chica. The period of observations was characterized by persistent upwelling winds of up to 17 m/s. The direction of the wind showed very little variability, but its magnitude exhibited well‐defined diurnal fluctuations. Wind speeds peaked at around sunset (2100 local time), which coincided with the deepest pycnocline location. The shallowest pycnocline depth appeared a couple of hours after sunrise. These pycnocline oscillations had amplitudes of ∼5 m and were believed to propagate poleward along the coast and dissipate at the shallow entrance in Boca Chica. This mechanism could have mixed low‐oxygen, nutrient‐rich waters into the photic zone and enhanced the primary productivity. Observed flows at the Boca Grande transect consisted of downwind flow over the shallow ends and upwind flow in the deepest part. This wind‐induced pattern is believed to cause a pair of counterrotating gyres that allow near‐surface divergence and ventilation of upwelled subsurface waters at the southern end of the bay. This is the region where the highest concentrations of chlorophyll are identified in color imagery.