NOAA GFDL - Geophysical Fluid Dynamics LaboratoryGFDL - Geophysical Fluid Dynamics Laboratory

Importance of the Antarctic Slope Current in the Southern Ocean Response to Ice Sheet Melt and Wind Stress Change

September 10th, 2024

Key Findings

  • The authors used two coupled models, CM4 and ESM4, to investigate how the Southern Ocean’s circulation and properties respond to changes in surface wind stress, Antarctic meltwater addition, and the combined forcing of the two.
  • Both models showed that changes in wind stress caused the surface of the Weddell Sea to warm, while freshwater from the Antarctic Ice Sheet caused the rest of the Southern Ocean surface to cool.
  • The models differed, however, in their subsurface ocean response along the West Antarctic shelf: CM4 yields strong cooling, while ESM4 yields strong warming.
  • The difference in the subsurface response is due to the models’ differing baseline representation of the Antarctic Slope Current. CM4 simulates a stronger Antarctic Slope Current, which accelerates in response to Antarctic meltwater addition, while ESM4 assumes a weaker, more diffuse current.
  • The results show how important the Antarctic Slope Current is in governing changes along the continental shelf, and particularly the West Antarctic region, and highlight the importance of coupling interactive ice sheet models to ocean models for better estimates of sea level rise and other global changes related to the melting Antarctic ice sheet.

R. L. Beadling, J. P. Krasting, S.M. Griffies, W. J. Hurlin, B. Bronselear, J. L. Russell, G. A. MacGilchrist, J-E. Tesdal, and M. Winton. Journal of Geophysical Research: Oceans. DOI: 10.1029/2021JC017608

Despite being the second-smallest ocean on Earth, the Southern Ocean plays an outsized role in absorbing heat and carbon generated from human activities, accounting for over 40% of the planet’s oceanic carbon uptake and 67%–98% of the global ocean heat uptake. As the climate warms, however, changes to the Southern Ocean’s circulation – caused by stronger, more southerly winds and increased meltwater from the Antarctic ice sheet – could affect its heat and carbon absorption abilities, which in turn would have significant consequences for global temperatures and sea level rise.

The authors used two coupled models, CM4 and ESM4, to examine how an increase in and southward shift of the winds over the Southern Ocean, as well as the continued melting of the Antarctic ice sheet, would alter the physical properties and circulation of the Southern Ocean, using a preindustrial control background state to isolate the response. The two models have different representations of the Antarctic Slope Current (ASC), which moves water westward along the Antarctic continental slope and plays a major role in the exchange of ocean properties, including heat, between the continental shelf and the rest of the global ocean.

When changes in wind stress and Antarctic meltwater were imposed, both models showed that as meltwater from the Antarctic Ice Sheet enters the ocean, the surface of all parts of the Southern Ocean except for the Weddell Sea cools. The surface of the Weddell Sea, meanwhile, showed limited sensitivity to freshwater from the ice sheet, and instead warmed as a result of changes in wind stress.

The models differed in their results relating to the subsurface ocean response along the West Antarctic shelf, with CM4 showing ocean cooling and ESM4 showing warming. This difference is due to the models’ differing baseline representations of the ASC: CM4 simulates a stronger ASC which accelerates sharply in response to meltwater addition, acting to trap the freshwater while also protecting the shelf from warm offshore waters. ESM4, its weaker and more spread-out ASC allows more meltwater to escape from the shelf, preventing a strong ASC acceleration, and the subsurface shelf waters to warm as warm offshore waters can still reach this region.

The results highlight how important the ASC is in shaping how waters along the shelf change, and also emphasize the importance of coupling interactive ice sheet models to ocean models for better estimates of sea level rise and other global changes related to Antarctic ice sheet loss. Better understanding of how meltwater and wind changes could affect the Southern Ocean will help scientists improve their projections of regional and global sea level rise.

Schematic representation of the along-slope and along-shelf circulation patterns at the Antarctic margin. The circumpolar view highlights the characteristic flow of the Antarctic Slope Current along the continental slope and the Antarctic Coastal Current on the shallow continental shelf. Beadling et al., (2022) found that a model’s ability to capture the Antarctic Slope Current matters for how the waters along the continental shelf respond to meltwater input from the Antarctic Ice Sheet. Image from Beadling (2023).