[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.

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