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

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(𝜃S) 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.