Genesis of the Antarctic Slope Current in West Antarctica

Andrew F Thompson1, Xiaozhou Ruan2, Kevin Speer3, Lena M. Schulze Chretien4, Karen J. Heywood5 and Ria Oelerich5, (1)California Institute of Technology, Pasadena, United States, (2)Massachusetts Institute of Technology, Cambridge, United States, (3)Florida State University, Geophysical Fluid Dynamics Institute(GFDI), Department of Scientific Computing(DSC), Tallahassee, United States, (4)Jacksonville University, Marine Science Research Institute, Jacksonville, FL, United States, (5)University of East Anglia, Norwich, United Kingdom
The transport of warm Circumpolar Deep Water (CDW) towards West Antarctica, extensively documented along the West Antarctic Peninsula (WAP) and the Amundsen Sea, is responsible for the rapid thinning of ice shelves in this region. The Bellingshausen Sea (BS) connects the WAP and the Amundsen Sea, but remains remarkably understudied compared to its neighboring regions. In the BS, the Antarctic Circumpolar Current and the Ross Gyre create a bifurcation in the slope circulation with eastward and westward flows along the WAP and Amundsen slopes, respectively. The dynamics that support this structure have not previously been discussed. Here, we offer a mechanism for the initiation of the westward Antarctic Slope Current (ASC) that relies not on traditional wind-forcing arguments, but rather on water mass transformation processes over the BS continental shelf.

We present CTD and lowered ADCP observations from the BS continental shelf collected in 2007 and 2019. We document the modification of poleward-flowing warm waters based on meltwater concentrations exiting along the western side of the Belgica and Latady Troughs. These changes to modified CDW are complemented by the formation of relatively salty Winter Water in polynyas. Both ice shelf melting and sea-ice formation support lateral cyclonic circulations over the shelf as well as an overturning in density space. From observations, we estimate the strength (~1 Sv) and structure of the shelf overturning. The outflow of modified waters are focused into a narrow western boundary current in the Belgica Trough that generates a front at the shelf break, which initiates a geostrophically-balanced, westward-flowing ASC. This structure is supported by analysis of potential vorticity distributions in observations and model output. Critically, the BS not only connects circulation features between the WAP and the Amundsen Sea, but also contributes to setting water properties that circulate under ice shelves throughout the West Antarctic continental shelf.