Interannual variability of the subsurface eddy field in the Southeast Pacific

Abstract

The Southeast Pacific, which encompasses the coasts of Peru and Chile, is one of the world's most productive regions resulting principally from the upwelling of subsurface nutrient-rich waters. Over the satellite altimetry era, there have been numerous evidence that surface mesoscale eddies play an important role in the offshore transport of rich coastal waters, but it has been only recently that few observational/numerical studies have highlighted the importance of the subsurface eddies. The eddy field variability is explored using the results of a high-resolution model experiment from 1979 to 2012. The model results indicate an asymmetry of the surface and subsurface eddy fields. While surface-intensified cyclones are slightly more frequent than anticyclones, the subsurface field is dominated by anticyclones (IntrathermoclineEddies; ITEs), triggered by the instability of the subsurface Peru Chile undercurrent (PCUC). Composite maps are consistent with in situ observations. ITEs are associated with maximum vorticity around 150–200 m depth, warmer and more saline core, characteristic of the equatorial subsurface water from the PCUC. We find that the variability of the ITEs is significantly correlated with the ENSO equatorial signal. During strong El Nino events (e.g., 1982; 1998), we find that while the PCUC transport increases, the volume of coastal waters transported by ITEs however decreases during those periods. We find that the relaxation of the isopycnals along the coast during El Nino events leads to weakened baroclinic instability and to a decrease of the ITEs transport.