North Atlantic Deep Water

North Atlantic Deep Water. It is a cold, dense water mass that forms in the northern Atlantic Ocean and constitutes a major component of the global thermohaline circulation. This deep-water formation is a critical driver of the Atlantic meridional overturning circulation, which redistributes heat and influences climate patterns worldwide. Its formation and southward flow represent a key process in the Earth's ocean current and carbon cycle systems.

Formation and sources

Primary formation occurs through open ocean convection in the Greenland Sea and the Labrador Sea, where intense winter cooling increases water density. Additional contributions come from the Iceland Sea and the overflow of dense water across the Greenland-Scotland Ridge. This process involves the cooling and sinking of surface waters, particularly the Norwegian Current and the East Greenland Current, which are influenced by the Gulf Stream. The resulting dense water spills over sills like the Denmark Strait and the Faroe Bank Channel, entraining surrounding water masses as it descends into the deep North Atlantic.

Physical and chemical properties

This water mass is characterized by relatively low temperatures, typically between 2 and 4 degrees Celsius, and high salinity values around 34.9 to 35.0 psu. It is identified by distinct signatures in potential temperature and salinity when plotted on a temperature-salinity diagram. Its chemical properties include high concentrations of oxygen and anthropogenic tracers like chlorofluorocarbons, which serve as markers of its recent formation and ventilation. These properties contrast sharply with those of older deep waters like Antarctic Bottom Water or Pacific Deep Water.

Circulation and global impact

After formation, it flows southward as a deep western boundary current along the continental rise of the Americas, traceable from the Grand Banks to the Antarctic Circumpolar Current. It constitutes the primary northward return flow of the global conveyor belt, upwelling in the Indian Ocean and Pacific Ocean after centuries-long transit. This circulation sequesters heat and carbon dioxide in the deep ocean for long periods, impacting global geochemical cycles. The export of this water from the Atlantic Basin is balanced by the inflow of warmer surface waters like the Brazil Current and the Agulhas Current.

Role in the climate system

It is a fundamental component of the Earth's energy budget, transporting vast amounts of heat poleward and helping to moderate the climate of Northern Europe. Changes in its formation rate are hypothesized to have played a role in past climate change events, such as the Younger Dryas and Heinrich events. Modern concerns focus on its potential slowdown due to global warming and increased freshwater input from melting Greenland ice sheet and Arctic sea ice, which could significantly alter weather patterns in regions like the Sahel and Amazon rainforest.

Measurement and research

Key insights have come from major international programs like the World Ocean Circulation Experiment, the RAPID array at 26th parallel north, and ongoing deployments of Argo floats. Sustained observations are also conducted by institutions such as the Woods Hole Oceanographic Institution and the National Oceanic and Atmospheric Administration. Research vessels like the RRS Discovery and RV Knorr regularly sample its properties, while paleoceanography studies using sediment cores from the Blake Ridge reconstruct its historical behavior. Satellite missions like GRACE and Jason-1 provide complementary data on related sea surface height and gravity anomalies.

Category:Oceanography Category:Physical oceanography Category:Climate