Antarctic Bottom Water
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| Antarctic Bottom Water | |
|---|---|
 Fred the Oyster · CC BY-SA 4.0 · source | |
| Name | Antarctic Bottom Water |
| Caption | Schematics of global thermohaline circulation including Antarctic Bottom Water |
| Depth | abyssal |
| Region | Southern Ocean, global ocean basins |
| Formation | Antarctic continental margins, continental shelves, polynyas |
| Temperature | ≈ -0.5 to 2 °C |
| Salinity | high |
Antarctic Bottom Water Antarctic Bottom Water (AABW) is the densest water mass in the modern World Ocean formed around the Antarctic continental margins and exported into the global ocean basins. It influences deep ocean stratification, the long-term uptake of heat and carbon, and links processes on the Antarctic Peninsula, Ross Sea, and Weddell Sea to basins across the Atlantic Ocean, Indian Ocean, and Pacific Ocean. AABW interacts with major features such as the Antarctic Circumpolar Current, Mid-Atlantic Ridge, and abyssal plains to shape global thermohaline circulation.
Introduction
AABW is produced near the Antarctic coasts where intense cooling, sea-ice formation, and interactions with ice shelves and glaciers create very cold, saline water that sinks to fill the lowest layers of the Southern Ocean and spreads northward into other ocean basins. Recognition of AABW follows from classic surveying by research programs including the International Geophysical Year and later basin-scale expeditions by institutions such as the Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, and national Antarctic programs. AABW is central to ideas developed by pioneers like Walter Munk and Henry Stommel about deep circulation and abyssal waters.
Formation and Properties
AABW forms where coastal polynyas and shelf processes near the Antarctic ice sheet and floating ice shelves—for example adjacent to the Ross Ice Shelf and in the Weddell Sea—produce brines during sea-ice formation and mixing with glacial melt. Dense water masses result from combinations of very low temperature, high salinity, and pressure: processes studied by researchers from the British Antarctic Survey, Alfred Wegener Institute, and Lamont–Doherty Earth Observatory. Properties include potential temperature near the freezing point of seawater and high dissolved oxygen inherited from surface contact; AABW occupies the abyssal layer beneath North Atlantic Deep Water and Circumpolar Deep Water in many regions. Formation events are modulated by large-scale climate modes such as the Southern Annular Mode and teleconnections to the El Niño–Southern Oscillation.
Distribution and Circulation
After formation, AABW flows northward as bottom-intensified currents, filling deep basins and tracing pathways constrained by bathymetric features like the Mid-Atlantic Ridge, Kerguelen Plateau, and East Pacific Rise. It ventilates the deepest parts of the North Atlantic, South Atlantic Ocean, Southern Indian Ocean, and South Pacific Ocean, connecting to deep-water formation sites including the historic surveys of the Challenger expedition and later programs such as the World Ocean Circulation Experiment. AABW pathways interact with abyssal currents, boundary currents along continental slopes, and deep western boundary currents studied in programs led by agencies like the National Oceanic and Atmospheric Administration.
Role in Global Climate and Oceanography
AABW plays a key role in global heat and carbon storage, sequestering anthropogenic heat and CO2 on multi-decadal to centennial timescales and affecting global climate realized in assessments by the Intergovernmental Panel on Climate Change and observational syntheses by the Global Climate Observing System. By ventilating the abyss, AABW influences deep-ocean oxygenation, nutrient distributions observed by projects like the Joint Global Ocean Flux Study, and the transient response of the Atlantic Meridional Overturning Circulation. Changes in AABW properties feed back on sea level through steric effects measured in satellite missions such as TOPEX/Poseidon and Jason-3.
Observational Methods and Measurement
Observations of AABW combine hydrographic surveys using conductivity-temperature-depth (CTD) profilers deployed from research vessels of institutions like the Commonwealth Scientific and Industrial Research Organisation and automated platforms including Argo floats adapted for deep profiling. Ship-based tracer studies using chlorofluorocarbon inventories and noble gases have been performed by groups at Scripps Institution of Oceanography and Lamont–Doherty Earth Observatory to estimate formation rates and ages. Seafloor moorings, bottom landers, and autonomous underwater vehicles from organizations such as the Monterey Bay Aquarium Research Institute provide time series of velocity, temperature, and salinity, while paleoceanographic reconstructions employ isotopic records from the Integrated Ocean Drilling Program and sediment cores analyzed by university laboratories.
Variability, Trends, and Climate Change Impacts
Recent studies report regional variability and long-term trends in AABW properties linked to warming, freshening from increased glacial melt, and changes in sea-ice production. Observations and model projections by centers like the National Center for Atmospheric Research and Met Office indicate shoaling and reduced density in parts of AABW, with implications for abyssal ventilation and global heat uptake. These changes are assessed in international syntheses produced by the Intergovernmental Panel on Climate Change and monitored by sustained observing networks coordinated through bodies such as the Scientific Committee on Antarctic Research.
Ecological and Biogeochemical Significance
By transporting oxygen-rich water into the abyss and exporting nutrients, AABW supports deep-sea ecosystems investigated by programs like the Deep Sea Ecology and Technology (DEEP-ET) initiatives and connects surface productivity regions such as the Southern Ocean to the deep biosphere. Biogeochemical cycles influenced by AABW include carbon sequestration processes studied by the Global Carbon Project and benthic community dynamics explored in research by the International Seabed Authority and academic consortia. Changes in AABW can alter habitats for cold-water species documented in taxonomic surveys by museums and research institutes, with consequences for deep-sea biodiversity and biogeography.