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Equatorial Pacific 13°C Water Eddies in the Eastern Subtropical South Pacific Ocean

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  • 1 NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington
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Abstract

Argo float profile data are used to analyze warm, salty, weakly stratified, subthermocline eddies of tropical origin in the eastern subtropical South Pacific Ocean. These eddies contain anomalous signatures of the equatorial Pacific “13°C Water” that is carried poleward within the Peru–Chile Undercurrent (PCU) as it flows along the west coast of South America. From their source along the Chilean coast between ∼29° and 39°S, the eddies spread westward and slightly northward, likely at least partly advected by the subtropical gyre. The eddy water properties contrast strongly with the colder, fresher, more strongly stratified waters of subantarctic origin being carried northward then westward by the gyre. Near the eddy source, about 6% of Argo profiles sample eddies that are above selected thresholds for both salinity and potential vorticity anomalies relative to maps of the mean distributions of these properties on and around the core isopycnal for the eddies. The proportion of such profiles diminishes to about 1% near the northwestern limit of the eddy range, near 15°S and 115°W. These eddies are anticyclonic, with a subsurface radial velocity maximum near the core isopycnal for water property anomalies, hence a reduced surface expression. Their geostrophic signature sometimes extends below 1000 dbar, suggesting the eddies may influence float subsurface trajectories. Radial transports around the eddy centers are estimated to be on the order of 2 × 106 m3 s−1 for the potential density layer 26.0 < σθ < 27.0 kg m−3, about the same magnitude as the mean poleward transport of the PCU.

Corresponding author address: G. C. Johnson, NOAA/Pacific Marine Environmental Laboratory, 7600 S and Point Way N.E., Bldg. 3, Seattle, WA 98115-6349. Email: gregory.c.johnson@noaa.gov

Abstract

Argo float profile data are used to analyze warm, salty, weakly stratified, subthermocline eddies of tropical origin in the eastern subtropical South Pacific Ocean. These eddies contain anomalous signatures of the equatorial Pacific “13°C Water” that is carried poleward within the Peru–Chile Undercurrent (PCU) as it flows along the west coast of South America. From their source along the Chilean coast between ∼29° and 39°S, the eddies spread westward and slightly northward, likely at least partly advected by the subtropical gyre. The eddy water properties contrast strongly with the colder, fresher, more strongly stratified waters of subantarctic origin being carried northward then westward by the gyre. Near the eddy source, about 6% of Argo profiles sample eddies that are above selected thresholds for both salinity and potential vorticity anomalies relative to maps of the mean distributions of these properties on and around the core isopycnal for the eddies. The proportion of such profiles diminishes to about 1% near the northwestern limit of the eddy range, near 15°S and 115°W. These eddies are anticyclonic, with a subsurface radial velocity maximum near the core isopycnal for water property anomalies, hence a reduced surface expression. Their geostrophic signature sometimes extends below 1000 dbar, suggesting the eddies may influence float subsurface trajectories. Radial transports around the eddy centers are estimated to be on the order of 2 × 106 m3 s−1 for the potential density layer 26.0 < σθ < 27.0 kg m−3, about the same magnitude as the mean poleward transport of the PCU.

Corresponding author address: G. C. Johnson, NOAA/Pacific Marine Environmental Laboratory, 7600 S and Point Way N.E., Bldg. 3, Seattle, WA 98115-6349. Email: gregory.c.johnson@noaa.gov

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