The ocean’s overturning circulation transports warm waters towards Antarctica. For thousands of years, the Antarctic ice sheet has been in a delicate balance between ice growth from snowfall, and ice melt at the coastal margins where the ice comes into contact with the warmer ocean.
Observing the oceans around Antarctica is extremely difficult, due to the remoteness and recurring presence of sea ice and icebergs that hinder ship movement for most of the year. Therefore, despite the importance of the southward warm water transport, we know very little about how it varies around Antarctica and what mechanisms push it towards Antarctica. But we do know that there are some warm regions and some cold regions of ocean near Antarctica.
A common assumption is that the largest flow of warm water towards Antarctica occurs in warm regions, such as in the Amundsen and Bellingshausen Seas, which have relatively high temperatures (above 0°C). However, this assumption has not yet been tested.
Using a high-resolution ocean model, we find that 80% of the transport in the warm water layer, known as Circumpolar Deep Water, instead approaches Antarctica in the colder regions. These colder regions have strong atmospheric cooling and brine rejection from sea ice formation in winter, which form dense water that cascades down the continental slope and fills the abyss of the world’s ocean basins. We find that pulses of dense water descending down canyons mechanically force the warmer water above to flow towards Antarctica.
The most surprising result is that warm regions of the Antarctic continental shelf actually have very limited warm water flow onto the shelf, compared with dense water formation sites. Instead, these regions are warm because the waters have been on the shelf for a long time and are subjected to minimal cooling from the atmosphere. These new results are forcing oceanographers to re-examine their understanding of the mechanisms that warm the Antarctic oceans.
- Paper: A. K. Morrison, A. McC. Hogg, M. H. England, and P. Spence (2020), Warm Circumpolar Deep Water transport towards Antarctica driven by local dense water export in canyons, Sci. Adv., Accepted