Antarctic Bottom Water

Antarctic Bottom Water
Fred the Oyster · CC BY-SA 4.0 · source
Name	Antarctic Bottom Water
Formation	Weddell Sea, Ross Sea, Adélie Coast
Temperature	−0.8 to 0 °C
Salinity	34.6 to 34.7 psu
Density	~27.88 kg/m³

Antarctic Bottom Water. It is the coldest, densest, and most voluminous water mass on the planet, forming a critical component of the global thermohaline circulation. This frigid water originates on the continental shelves of Antarctica and cascades down the continental slope to fill the abyssal basins of the world's oceans. Its formation and movement are fundamental to the Earth's climate system, influencing global heat distribution and carbon sequestration over millennia.

Formation and properties

Antarctic Bottom Water forms primarily through intense air-sea-ice interactions in specific coastal polynya regions, notably within the Weddell Sea and the Ross Sea. The process begins with the formation of sea ice, which releases salt and increases the salinity and density of the underlying seawater, creating High Salinity Shelf Water. This dense shelf water mixes with slightly warmer, modified Circumpolar Deep Water as it flows off the shelf, further cooling through contact with the atmosphere and ice shelves like the Filchner-Ronne Ice Shelf. The resulting water mass has temperatures near the surface freezing point, typically between −0.8 and 0 °C, and a salinity range of 34.6 to 34.7 practical salinity units, giving it the highest density of any oceanic water. Key formation sites also include areas along the Adélie Coast and near the Amery Ice Shelf.

Circulation and distribution

Upon formation, this dense water flows northward along the seafloor, guided by the topography of the Southern Ocean and major ocean basins. It is steered by deep-reaching currents like the Antarctic Circumpolar Current and enters the Atlantic Ocean through passages such as the Vema Channel and the Romanche Fracture Zone. In the Pacific Ocean, it moves through the Samoa Passage and the Eltanin Fracture Zone, while in the Indian Ocean, it enters via the Crozet Basin and the Mozambique Channel. As it spreads, it mixes with overlying water masses, including North Atlantic Deep Water and Antarctic Intermediate Water, gradually losing its distinctive properties but remaining identifiable by its characteristic potential temperature and salinity signatures as far north as the Equator in the Atlantic.

Role in global climate

This abyssal water mass is a primary driver of the lower limb of the Atlantic meridional overturning circulation, playing a crucial role in the global conveyor belt that redistributes heat. By transporting cold water northward, it helps regulate Earth's thermal budget, influencing patterns from the Southern Hemisphere to the North Atlantic. Furthermore, it acts as a significant long-term reservoir for carbon dioxide and other anthropogenic gases, sequestering them away from the atmosphere for centuries. Its formation processes are intimately linked with the stability of Antarctic ice sheets and the rate of basal melt, making it a key factor in past climate transitions recorded in paleoceanographic proxies from the Last Glacial Maximum.

Measurement and research

The study of this water mass relies on an array of oceanographic instruments and international programs. Key tools include Conductivity Temperature Depth profilers, moored arrays like those deployed by the NOAA, and deep-diving Autonomous Underwater Vehicles such as those from the Woods Hole Oceanographic Institution. Major research initiatives, including the World Ocean Circulation Experiment, the Climate and Ocean: Variability, Predictability and Change project, and ongoing work by the British Antarctic Survey, have mapped its properties and pathways. Seminal contributions to understanding its dynamics have come from research vessels like the RRS Discovery and institutions like the Scripps Institution of Oceanography.

Changes and future outlook

Observations over recent decades, particularly from programs like Argo, indicate a warming and freshening trend in the abyssal layers influenced by this water mass, linked to changes in Antarctic sea ice extent and glacial meltwater input from the West Antarctic Ice Sheet. Climate models from the Intergovernmental Panel on Climate Change project a potential slowdown in its formation rate due to increased surface freshening, which could weaken the overall thermohaline circulation with profound climatic repercussions. Continued monitoring by satellites like CryoSat-2 and missions such as NASA's Operation IceBridge is essential to predict its future behavior and impacts on global sea level rise and oceanic heat uptake.

Category:Oceanography Category:Climate Category:Antarctica