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Eddies of newly formed upper Labrador Sea water

Authors

  • Robert S. Pickart,

  • William M. Smethie Jr.,

  • John R. N. Lazier,

  • E. Peter Jones,

  • William J. Jenkins


Abstract

A new type of submesoscale eddy has been observed south of the Labrador Sea during late winter, embedded in equatorward flow along the western boundary. The eddy radius is 20 km, with a weak dynamic signature (swirl speed of 0.5 cm/s). The center of the eddy is characterized by weak stratification, elevated concentrations of oxygen and anthropogenic tracers, and low tracer ages, all indicative of newly ventilated water. Strong lateral intrusions distort the shape of the feature. The water mass contained in the eddy is not classical Labrador Sea water (from the central Labrador Sea) but is significantly fresher and hence lighter. It is of the correct density to be the source of the high-chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper Labrador Sea water. Using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model, it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the Grand Banks of Newfoundland. This is in contrast to deeper lenses of classical Labrador Sea water which persist further equatorward. Tracer-derived ages of the upper Labrador Sea water eddy range from 3 to 5 years, much older than the lifetime deduced from their lateral diffusion. A simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation.

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