What controls the influx and mixing of warm waters onto the polar ocean shelves?

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Two of the main environmental challenges of the coming century will be the response of global sea levels and marine ecosystems to rapid ocean warming, particularly in the polar regions. Recently, large increases in ice melt have been recorded at many locations around Antarctica and Greenland, raising fears that sudden catastrophic ice loss could occur in the coming decades, with significant impacts on global sea level. Despite this, there is still insufficient knowledge about the pathways and mechanisms by which warm offshore ocean waters are delivered to the ice shelves and calving glaciers. One particular region of concern is the West Antarctic Peninsula. Here, much of the increased ice loss appears to have been caused by an enhanced flow of warm offshore water (termed Circumpolar Deep Water or CDW) reaching cavities under the floating ice shelves. Furthermore, the increase in ocean temperature and reducing sea ice in this region are likely to have significant impacts on the operation of the Antarctic ecosystem, particularly for populations of both krill and penguins. Understanding the mechanisms that cause CDW to be mixed both laterally and vertically on the shelves and the role of shelf-edge processes in delivering this water mass is vital to interpreting the regional physical, chemical and biological changes that are occurring.The key project objectives are thus:1. To quantify, describe and understand the spatial and time-varying patterns of lateral and vertical mixing on the West Antarctic Peninsula shelf.2. To resolve the dominant mechanisms driving lateral and vertical heat fluxes, with a specific focus on understanding how and where heat from the deep ocean waters is transferred to the upper ocean.3. To understand the role of key shelf-edge processes in controlling these phenomena, in particular by understanding and quantifying the importance of these processes in causing intrusions of warm, saline deep-ocean waters onto polar shelves.The project will use data from both traditional and novel oceanographic platforms, with the aim of describing how warm waters move from the edge of the shelf to the coast, where melting of land-based ice can occur. Ocean gliders, which have been developed extensively over the last 10 years, will provide measurements of temperature, salinity and small-scale velocity, from which the controlling mechanisms can be determined.The fieldwork will focus on two regions. The first, at the shelf edge, will determine how CDW moves across the shelf break (e.g. through eddies, upwelling or steering by topography). The second region, close to the British Antarctic Survey base at Rothera, will examine the mechanisms by which heat in this layer is mixed both laterally and vertically. As one of the first projects to measure mixing on the West Antarctic Peninsula directly, we will determine whether these processes are driven by wind, tides or processes relating to changing ocean density. To help interpret the glider measurements, moored velocity observations will be made and existing meteorological and hydrographic data from the BAS Rothera Time Series will be incorporated into the analysis. In addition, a through-ice wintertime deployment of an instrument that directly measures turbulence will be carried out to test whether vertical mixing is weakened by the presence of sea ice. In synthesising the data, the project will use a new methodological framework to examine the relative importance of horizontal and lateral processes.Ultimately, we will use our new mechanistic understanding from objectives 2 and 3 with the quantification of heat flows from objective 1 to devise a conceptual model of how heat moves onto the shelf and either gets into the near-surface layer (and lost to the atmosphere) or moves to the ice shelf edge. Understanding these processes will help to predict future ice sheet stability, regional climate and ecosystem changes in the rapidly warming polar regions.