Abstract

The velocity structure of an anticyclonic lens in the southeast Pacific centered at 300–400 m depth was scanned in detail with a 75 kHz acoustic Doppler current profiler on the RV Atlantis during a February 2010 transit along 20° S. Embedded in the main thermocline with a vertical span of ∼500 m, the lens had an overall diameter of ∼150 km with a peak speed of 0.33 m s–1 at ∼35 km radius. Knowledge of the velocity field permitted determination of energetics and vorticity properties in detail. The ratio of potential to kinetic energy was ∼14, and the relative vorticity of the lens core was –0.7 times the local Coriolis parameter. Using a dynamically estimated density field, the potential vorticity of the lens's core was a factor of 6 less than the surrounding waters, the only source of which could be equatorial subsurface water. The lens had an estimated age of ∼4 years depending on where it was formed off the Peru-Chile coast. A search for similar lenses in the Argo database showed that they could be found throughout the southeast Pacific. Emanating from the southern Chile continental slope, their thickness-to-width aspect ratio decreased as they drifted west and equatorward. The inferred very slow rate of decay suggests that these rapidly spinning disks must be quite stable, but the thinning population suggests that they rarely get to live their full life cycle; more likely, they are torn apart by collision with other lenses or energetic mesoscale events.

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