The deep waters of the Antarctic sea are shrinking and warming

June 13 () –

Antarctic seafloor waters, crucial for carbon cycling, are warming and shrinking in the Weddell Sea, one of the largest producers of dense bottom water.

Antarctic bottom water is the coldest and densest body of water on the planet and plays a critical role in regulating the ocean’s ability to store heat and sequester carbon.

This discovery –published in Nature Climate Change by researchers from the British Antarctic Survey (BAS)–is the first observational evidence that long-term changes in winds and sea ice are influencing the production of bottom water in the Weddell Sea.

The observed decline in bottom water could have a major impact on the world’s oceans, which would lead to less heat and carbon being captured from the atmosphere and trapped in the deep ocean. The ocean has absorbed more than 90% of human-induced global warming and nearly a third of the extra carbon released since the start of the industrial revolution.

Using decades of observations from ships along with satellite data, the team found that these waters have been declining in volume over the past 30 years. The measurements are the result of many scientific trips to the Weddell Sea, measuring the temperature and salinity of the oceans from the surface to the seabed.

By reviewing these sections over three decades, the team concludes that the volume of these bottom waters has been reduced by more than 20%, while the rest of the waters deeper than 2,000 meters it has warmed at four times the rate of the rest of the global ocean.

Antarctic Bottom Water is a mixture of warmer waters, drawn into the Weddell Sea from the Antarctic Circumpolar Current which flows clockwise (as viewed from the South Pole) from west to east around Antarctica, and very cold salt water created when these northernmost waters are exposed to the frigid surface air around the continent.

The new study found that the reduction in bottom waters is the result of changes in sea ice formation rates, which form a critical stage in bottom water production.

Weakening winds near the Filchner-Ronne Ice Shelf in the southern Weddell Sea have slowed the rate of sea ice production. Normally, winds act to push new ice away from the shelf, leaving areas of open water for even more ice to form.

This weakening is reducing the size of these gaps in the sea ice sheet, causing a slowdown in new ice formation. The creation of new ice is a key process in creating the cold, salty Antarctic bottom water, as new, relatively fresh sea ice leaves salt in the ocean as a result of the freezing process. The dwindling supplies of these briny shelf waters has led to the reduction of the bottom waters observed by the team.

Delving into wind-driven changes, the team discovered links to large-scale weather patterns connecting responses in the tropical Pacific to the Southern Ocean. Changes in these large-scale patterns have caused winds across the Southern Ocean to fluctuate and shift, reducing northerly winds across the Weddell Sea that cause severe freezing of sea ice, explains the BAS in a statement.

The current changes observed are a consequence of the natural variability of the system, although potentially stronger changes are anticipated in the future. They also line up with other recent high-profile studies this year showing similar reductions in bottom water coming from the Ross Sea, as well as modeled future collapse in bottom waters; both due to the accelerated melting of the Antarctic ice shelves.