Description: 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.

Description: The Weddell Gyre plays a role in the climate system by advecting heat poleward to the Antarctic ice shelves and by regulating the density of water masses that feed the lowest limb of the global ocean overturning circulation. Warm Deep Water is the water mass that delivers heat to the Weddell Gyre, which is subject to complex modification processes to form Weddell Sea Deep and Bottom Water, providing an important sink for atmospheric heat and carbon dioxide. A fleet of Argo floats drifting in the currents of the upper 2000 m of the water column have recently provided data covering more or less the entire Weddell Gyre region in the horizontal, and Warm Deep Water in the vertical. Argo float data thus provide an opportunity to investigate surface to mid-depth ocean dynamics by providing both hydrographic and trajectory data. Here, Argo float profile and trajectory data are exploited in order to produce a full gyre scale view of the Weddell Gyre’s circulation from a purely observation-based dataset, whose pertinent features include the double-cell structure, with a stronger eastern core that intensifies with depth, and a weaker western core that remains invariant with depth. Considerable recirculation occurs within the gyre interior about the eastern core, before the water is able to fully traverse the zonal extent of the gyre, which causes considerable uncertainty when determining the gyre strength using single transects as is typical with ship-based surveys. Further investigation of the causes, temporal stability and the physical consequences of the recirculation cells along the central gyre axis, is required if we are to fully identify and understand the ways in which the Weddell Gyre may be changing in a rapidly changing climate.

Description: The Weddell Gyre plays a fundamental role in the climate system by advecting heat poleward to the Antarctic ice shelves and by regulating the density of water masses that feed the lowest limb of the global overturning circulation. Profile and trajectory data between 2002 and 2016 from a fleet of Argo floats between 50 and 2000 m are exploited in order to produce a full gyre scale view of the Weddell Gyre's circulation. The data exhibit a gradual cooling of the Warm Deep Water (WDW) as it circulates cyclonically around the gyre. A double-cell structure of the Weddell Gyre is revealed, with a stronger eastern core that intensifies with depth, and a weaker western core that remains invariant with depth. The deep outflow of Weddell Sea Deep and Bottom Water (WSDW, WSBW) at the western boundary of the gyre, formed from WDW by complex modification processes involving sea ice formation, and basal ice shelf melting, is not covered by the float observations. Using mooring array-based observations near the tip of the Antarctic Peninsula between 1989 and 1998, and between 2005 and 2014, a cooling of the WSBW plume is revealed over the observational period. This is in striking contrast to the WSDW in the interior Weddell Sea which has been undergoing a decadal warming. While the cause of the cooling of the WSBW plume is currently unclear, the mooring-based velocity observations indicate that the volume transport of the WSBW plume of 2.5 Sv has been stable over time.