Aspects of the Antarctic Circumpolar Current dynamics investigated with drifter data

Interactions between eddies and mean flow are essential to close the momentum budget of the Southern Ocean, as well as to determine the structure of the global meridional overturning circulation. Both the structure of the Antarctic Circumpolar Current and the eddy dynamics arising at its heart are importantfor water-mass circulation and global ocean ventilation, and therefore for the global climate. However, the characterization of the eddy fields of heat and momentum are still poorly sampled by direct observations, and are difficult to accurately resolve or parameterize in numerical models. Here we present observation-based eddy statistics and eddy heat flux (EHF) from a consistent dataset, and estimate Southern Ocean surface EHF from direct measurements without any parameterization assumption. Observations obtained from near-surface drifters represent a very useful dataset to analyze the eddy field because of their ability to catch a large number of scales of motion while providing a quasi-synoptic coverage of the investigated area. Differences between winter and summer distributions of mean velocities, mean kinetic energy and eddy kinetic energy are shown to be strongly constrained by bottom topography and suggest a winter shift of the polar front in specific areas around the circumpolar belt. Overall, the flow follows the contours of constant planetary vorticity set by the depth of the ocean, and creates anisotropy in eddy dispersion with maximum dispersion along these contours. Furthermore, eddy–mean flow interactions are strongly influenced by topography, with eddies having a tendency to accelerate jets upstream of ridges, and decelerate them further downstream. These strong regional variabilities introduced by topography affect the flux of heat induced by eddies, and therefore affect formation and subduction of the main surface and intermediate Southern Ocean water masses. Integrated over the Southern Ocean, subduction of water masses induced by EHF appears as an order on component for the Southern Ocean thermocline ventilation. Our results based on direct observations emphasizes, once more, the importance of continuing sustained effort to better represent the effect of mesoscale eddies in a coarse climate model, if we are to represent and predict future climate.

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