Dependence of Antarctic sea-ice extent on mesoscale ocean variability in the Southern Ocean

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Overview: Sea ice extent around Antarctica has slowly increased over the satellite era (1979-present), against the general decline of other global cryosphere components and contrary to modern negative sea ice extent trends in global coupled climate models (CCMs). Previous hypotheses have focused on large-scale mechanisms that impact the sea ice mass balance: changes in wind stress fields due to stratospheric ozone depletion; a more energetic Southern Ocean hydrologic cycle; increased downwelling long wave radiation; and enhanced freshwater production under ice shelves. An additional hypothesis will be tested: Mesoscale ocean processes that are not resolved in CCMs play a significant role in controlling ice edge position and sea-ice characteristics in the marginal ice zone and that inter-annual variability and trends in ocean mesoscale activity can drive changes in sea ice extent that are not closely correlated with the annual-averaged Southern Ocean state such as position of Antarctic Circumpolar Current fronts.Intellectual Merit: Daily satellite-derived sea-ice concentration fields will be analyzed to determine the effect of ocean mesoscale variability on sea-ice edge characteristics that influence true and satellite-measured sea-ice extent. Statistics will include the increase in ice-edge length due to mesoscale processes as a measure of increased exposure of the Marginal Ice Zone to "open-ocean" processes. Sea ice extent derived from different gridded datasets of sea-ice concentration will be compared to determine whether inter-annual variability and trends in sea ice extent are sensitive to measurement methodology and spatial resolution. If justified by prior work, near-contemporaneous Envisat ASAR high-resolution radar images will then be compared with sea-ice concentration fields from AMSR-E. A database of tracked eddies will also be analyzed to determine whether eddies generated by Antarctic Circumpolar Current frontal instabilities and interactions with topography contribute to ice-edge variability. These analyses will be performed on circum-Antarctic and regional scales, at annual resolution to resolve inter-annual variability and trends.The output from an eddy-permitting three-dimensional regional ocean model with coupled sea ice or a data-constrained ocean state model (Southern Ocean State Estimate) will be analyzed to investigate how well these models represent the ocean mesoscale contribution to ice-edge perturbations. The investigators will also develop their own idealized models of ocean/ice/atmosphere interactions at the ice edge to investigate mechanisms for generation of ice-edge fronts whose instabilities may be significant contributors to the ice-edge mesoscale field.Broader Impacts: This study will elucidate the mechanisms coupling the highly energetic Southern Ocean mesoscale field with sea ice. Antarctic sea ice has a profound effect on albedo, ocean/atmosphere heat and moisture fluxes and gas exchange, and so is an important component of the global coupled climate system. The concurrent analyses of existing data and eddy-permitting ocean/sea-ice models will inform interpretation of sea-ice variability in modern CCMs, and so identify critical developments needed to improve their representation of sea ice. Careful examination of the marginal ice zone will help inform the interpretation of satellite retrievals of ice-edge position. The investigators will continue their participation in local outreach activities including presentations at Pacific Science Center (Seattle) and the Robinson Center for Young Scholars at the University of Washington, public talks in Oregon and frequent contributions on Climate Science to the Corvallis newspaper Opinion Page, development of a project web page targeted at a general audience, and updating their outreach web site including polar photo server.