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Rapid subsurface warming and circulation changes of Antarctic coastal waters by poleward shifting winds

Key Findings

  • A poleward wind shift at the latitudes of the Antarctic Peninsula can within a few years produce an intense near-circumpolar warming of subsurface coastal waters that exceeds 2°C at 200-700m depth.
  • Model-simulated warming results from a rapid current-induced heat flux response induced by weakened near-shore, wind-driven downwelling (Ekman pumping), and is associated with weakened coastal currents.
  • Anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around Antarctica, with potentially significant ramifications for global sea level rise.

Spence, Paul, Stephen M. Griffies, Matthew H. England,  Andrew McC. Hogg, Oleg A. Saenko and Nicolas C. Jourdain. Geophysical Research Letters. DOI: 010.1002/2014GL060613.

Summary

Projected changes in the winds circling the Antarctic may accelerate global sea level rise significantly more than previously estimated. Changes to Antarctic winds may have a profound impact on warming ocean temperatures under the ice shelves along the coastline of West and East Antarctic. Projected Antarctic wind shifts were included in a detailed global ocean model, and the authors found water up to 4°C warmer than current temperatures rose up to meet the base of the Antarctic ice shelves. The projected sub-surface warming is twice as large as previously estimated, on average, and it affects almost all of coastal Antarctica. This relatively warm water provides a huge reservoir of melt potential right near the grounding lines of ice shelves around Antarctica. It could lead to a massive increase in the rate of ice sheet melt, with direct consequences for global sea level rise.

The southern hemisphere mid-latitude westerly winds have been strengthening and shifting poleward since the 1950s, and this wind trend is projected to persist through the 21st Century under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Prior to this research, most sea level rise studies focused on the rate of ice shelf melting due to the general warming of the ocean over large areas.

Using a global ocean-sea ice model (GFDL-MOM025) based on GFDL’s CM2.5 coupled climate model, the researchers were able to examine the impacts of changing winds on currents down to 700m around the coastline in greater detail than ever before. Previous global models did not adequately capture these currents and the structure of water temperatures at these depths. This more detailed approach suggests changes in Antarctic coastal winds due to climate change and their impact on coastal currents could be even more important on melting of the ice shelves than the broader warming of the ocean.

With both West and East Antarctica affected by the change in currents, in the future abrupt rises in sea level become more likely. Recent estimates suggest the West Antarctic Ice Sheet alone could contribute 3.3 metres to long-term global sea level rise. This mechanism may help explain why the melt rate of some of the glaciers in Western Antarctic are accelerating more than scientists expected.

Schematic of Antarctic coastal ocean response to a poleward wind shift. Isotherms (lines), zonal winds, Ekman pumping (arrows), and coastal currents are shown in blue for the CNTRL case and in red for a poleward wind shift. The wind shift decreases dynamic sea surface height along the coast and flattens the isotherms, which are generally well aligned with isopycnals in this region. The speed of the coastal current decreases, and the boundary between the cold and fresh surface water near the coast and the warmer layer below moves upward.