Mesoscale Eddies at the Antarctic Margins

Ocean circulation is like a jigsaw puzzle. In the ocean a myriad of physical processes with small scales contribute to the global flow, just as many small puzzle pieces must be assembled properly to produce the picture on the box. Improvements in ocean observing techniques and numerical models are just now allowing oceanographers to study ocean circulation at the ``puzzle piece'' level. Physical processes occurring at this level are known as the ocean mesoscale and have typical length scales of tens of kilometres. The ocean mesoscale is dominated by coherent structures, such as eddies and jets, which are the oceanic equivalent of atmospheric storms and storm tracks. These features can be thought of as the bumps and grooves on the puzzle pieces, as they are essential for linking different parts of the ocean circulation together. When studying the ocean circulation, it is often advantageous to consider parts or components of the global flow just as one might focus on a patch of sky or a piece of clothing when working on a large puzzle. Two key components of the ocean circulation located in the Southern Ocean are the Antarctic margins and the Antarctic Circumpolar Current (ACC). The former is the site where the coldest, densest water in the ocean is formed. This water sinks around Antarctica, enters the ACC and eventually spreads to cover most of the ocean floor. The ACC is the largest and strongest current system in the ocean and is responsible for exchange between ocean basins, for regulating heat transport between low latitudes and the pole, and for replenishing the waters around Antarctica. While it is appreciated that exchange between these two regions helps regulate the global circulation, the mechanisms that enable and control this exchange, and especially the role of eddies, is poorly understood. In other words, the bumps and grooves that link waters around Antarctica with the surrounding ACC are presently missing in the circulation jigsaw. The Southern Ocean is crucial for the evolution of the Earth's climate; it is the world's most biologically productive ocean and a major site of heat and carbon dioxide uptake. Eddy processes not only link the ACC and the Antarctic margins, but they also dictate how the region responds to changing climate conditions. Significant regional changes have occurred around Antarctica during the past few decades, which have resulted in more water from the ACC penetrating onto the continental shelf. This has led to a thinning of the West Antarctic Ice Sheet and has caused water exported to the global circulation to become less salty. Accurate predictions of future changes around Antarctica and their impact on sea level rise, for instance, will need to take account of eddies. This proposal aims to study the ocean circulation around Antarctica using an eddy-resolving numerical model, or in other words, at the puzzle piece level. Important questions to be addressed include: How do eddies determine the location and frequency of exchange between waters around the Antarctic margins and those within the ACC; and how do eddy processes respond to changing wind and temperature patterns that mimic observed changes around Antarctica? As individual features of the circulation change, the puzzle pieces will be modified to fit together in new and interesting ways. A complete understanding of the processes linking the ACC and the Antarctic margins can not be achieved by numerical simulations alone. Thus an important component of this work is integrating results from the models with exciting national and international initiatives to observe these processes directly in the field. Through this combined effort, a clearer picture of the processes that couple the ACC and the Antarctic margins will emerge, thus moving another step closer to achieving the global picture on the box.