Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which creates transient melting anomalies with an approximately decadal period. No anthropogenic influence on this process has been established. Here, we combine observations and climate model simulations to suggest that increased greenhouse gas forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the near-zero mean zonal winds of the present day. Strong internal climate variability, primarily linked to the tropical Pacific, is superimposed on this forced trend. We infer that the Amundsen Sea experienced decadal ocean variability throughout the twentieth century, with warm anomalies gradually becoming more prevalent, offering a credible explanation for the ongoing ice loss. Existing climate model projections show that strong future greenhouse gas forcing creates persistent mean westerly shelf-break winds by 2100, suggesting a further enhancement of warm ocean anomalies. These wind changes are weaker under a scenario in which greenhouse gas concentrations are stabilized.
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Sea-ice concentration and drift data that support the findings of this study are available from the National Snow and Ice Data Center (https://doi.org/10.5067/8GQ8LZQVL0VL and https://doi.org/10.5067/O57VAIT2AYYY, respectively). ERA-Interim reanalysis data are available from the ECMWF (https://apps.ecmwf.int/datasets/data/interim-full-daily). Extended Reconstructed Sea Surface Temperature data are available from the National Oceanic and Atmospheric Administration National Climatic Data Center (https://www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst-v5). Seabed data are available from the British Antarctic Survey (https://secure.antarctica.ac.uk/data/bedmap2). Climate indices are available from the National Oceanic and Atmospheric Administration Earth System Research Laboratory (https://www.esrl.noaa.gov/psd/data/climateindices/list). CMIP5 simulation data are available from the Centre for Environmental Data Analysis (http://www.ceda.ac.uk). CESM simulation data are available from the National Center for Atmospheric Research Climate Data Gateway (https://www.earthsystemgrid.org).
The Matlab scripts used for the analyses described in this study can be obtained from the corresponding author on reasonable request.
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We are grateful to the originators of the many open-access datasets synthesized in this study, including remotely sensed sea-ice data, atmospheric reanalysis model results, sea surface temperature and bathymetry observations, derived climate indices, and many climate model simulations. P.D. was supported by NSF awards 1643285 and 1644159. E.J.S. was supported by NSF award 1602435.
The authors declare no competing interests.
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Holland, P.R., Bracegirdle, T.J., Dutrieux, P. et al. West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing. Nat. Geosci. 12, 718–724 (2019). https://doi.org/10.1038/s41561-019-0420-9
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