Cyclonic eddies and upper thermocline fine-scale structures in the Antarctic Circumpolar Current

Harry Leach,Volker Strass

doi:10.1007/s10236-018-1241-x

Abstract

Mesoscale eddies in the open ocean are mostly formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is converted into the kinetic energy of the flow around the eddy. As a permissible form of motion within a rapidly rotating and stratified fluid eddies driven by baroclinic instability are important for the poleward and vertical transport, not only of physical properties, but also biogeochemical ones. In this paper, we present observations from four cyclonic eddies in the Antarctic Circumpolar Current. We have sorted them by apparent age, based on altimeter data and consideration of the degree of homogenisation of the potential temperature-salinity(\U0001d703S) relationship, and then looked at the spatial distribution of measures of fine-scale variability in the upper thermocline. The youngest eddy shows isopycnals which are domed upwards and it contains a variety of waters with differing temperature-salinity characteristics. The fine-scale variability is higher in the core of the eddy. The older eddies show a core which is more homogeneous in potential temperature and salinity. The isopycnals are flatter in the centre of the eddy, and in cross-section, they can be M-shaped, so that the steepest gradients are concentrated around the edge. The fine-scale variability is more concentrated around the edges where the density gradients are stronger. We hypothesise that lateral stirring and mixing processes within the eddy homogenise the water so that the temperature-salinity relationship becomes tighter. When the eddy eventually collapses, this modified water can be released back into the flow. Thus, we see how the interplay of mesoscale and small-scale processes are modifying water mass properties and, potentially, regulate biogeochemical processes.

Highlights

Mesoscale eddies in the open ocean are generally formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is Responsible Editor: Pierre De Mey-Fremaux
Edmon et al (1980) described how, as a baroclinic disturbance grows, heat is transported polewards and the available potential energy of the mean flow is converted to eddy kinetic energy
Work on the way in which eddies decay was done by Methven (1998) and Methven and Hoskins 1998, 1999); their calculations showed that, as an eddy forms, it winds in anomalies of potential vorticity, which eventually leads to an unstable situation and the eddy collapses releasing anomalies back into the mean flow

Summary

Introduction

Mesoscale eddies in the open ocean are generally formed by baroclinic instability, in which the available potential energy from the large-scale slope of the isopycnals is Responsible Editor: Pierre De Mey-Fremaux. Edmon et al (1980) described how, as a baroclinic disturbance grows, heat is transported polewards and the available potential energy of the mean flow is converted to eddy kinetic energy. Work on the way in which eddies decay was done by Methven (1998) and Methven and Hoskins 1998, 1999); their calculations showed that, as an eddy forms, it winds in anomalies of potential vorticity, which eventually leads to an unstable situation and the eddy collapses releasing anomalies back into the mean flow. The interesting point is that once formed eddies do not decay by friction running them down, but rather collapse quickly. Chelton et al (2011) have looked at the statistical properties of eddies based on the AVISO altimeter data and show that about 10% last 16 weeks or more, which corresponds to a half-life of about 5 weeks

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