[1] The three-dimensional structure and the origin of mesoscale anticyclonic intrathermocline eddies (ITEs) in the coastal transition zone (CTZ) off central Chile (31-41 S) were analyzed through the combination of data from oceanographic cruises and satellite altimetry, and the application of an eddy-resolving primitive equation ocean model coupled with a numerical experiment using a passive tracer. In this region, ITEs are represented by subsurface lenses ($100 km diameter; 500 m thickness or vertical extension) of nearly homogeneous salinity (>34.5) and oxygen-deficient (<1.0 mL L À1 ) waters, properties which are linked to the equatorial subsurface water mass (ESSW) transported poleward by the Peru-Chile undercurrent (PCUC) in the coastal band. At least five to seven ITEs were observed simultaneously in the area between 31 and 38 S during winter cruises in 1997 and 2009. Satellite data indicated that the ITEs identified from in situ data moved westward, each at a mean speed of $2 km d À1 and transported a total volume of $1 Â 10 6 m 3 s À1 (¼1 Sv); the lifespan of each ITE ranged from a few months to 1 year. Model results indicate that ITEs become detached from the PCUC under summer upwelling conditions in the coastal zone.

[1] 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 À3 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 0 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.

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