Seasonal variability of coastal upwelling and the upwelling front off central Chile

Abstract

We analyze sea surface temperature (SST) and chlorophyll a (Chl a) satellite data to study the seasonal variability of the upwelling off central Chile. Data from an oceanographic cruise are used to illustrate the vertical characteristics of the upwelling front and the geostrophic flow. The mean offshore extension of the SST front in summer is ∼110 km. There, SST changes from ∼13.5°C near the coast to 14.5°–15°C offshore. Hydrographic data show that the thermocline becomes progressively shallower toward the coast but that at a certain distance it abruptly bends to form a horizontal front that extends until ∼50 m depth. Satellite data show that the Chl a decays exponentially offshore, and a concentration of ∼1 mg m−3 indicates the offshore limit of the frontal region. The SST front observed most of the year is absent in winter, when the coastal water is only slightly colder than the oceanic one; however, relatively higher Chl a values remain present near the coast during winter. Northwest of Punta Lavapie (37°15′S), the frontal region is located progressively farther offshore, suggesting that a branch of an equatorward surface jet separates from the coast to form a meander, which is associated with a filament of cold water and an anticyclonic eddy observed frequently in this zone during summer. Hydrographic data show that isotherm and current disturbances related to mesoscale eddies can extend several hundreds of meters below the surface. In contrast, the coastal jet is quite shallow, extending until ∼150 m depth.