Small Water-Property Transporting Eddies: Statistical Outliers in the Hydrographic Data of the POLYMODE Local Dynamics Experiment

Abstract

Hydrographic measurements from the POLYMODE Local Dynamics Experiment (LDE; 31°N, 69.5°W; May?July 1978) are examined for highly anomalous, outlying observations of salinity, oxygen and vortex stretching on isopycnal surfaces. The methodology which defines outliers in the observations is based on examination of gaps in histograms of these water properties on each isopycnal. These outliers are indicative of vertically confined (over 20?2000 m), sub-mesoscale eddies (diameters of 25?50 km) at the LDE site. Evidence for 31 eddies is found at the LDE site over the two months of the experiment. These features are described individually and characterized by their core water-properly signal and dynamical structure. Eighteen features are found to be anticyclonic lenslike structure, eight have no detectable dynamic signal and only four are characterized by cyclonic circulation. The work establishes that sub-mesoscale eddies in a small portion of the western North Atlantic exhibit a variety of distinct water types at their core. Based on the characteristics described here, McDowell has deduced origins for some of the eddies in the eastern basin of the North Atlantic, the southern North Atlantic and Labrador Sea. The distant sources suggest that they may have lifetimes of several years. The dynamic characteristics of the eddies as described here are used in an assessment of theoretical implications of sub-mesoscale vortices by McWilliams.