North Atlantic Deep Water

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation of the world's oceans involves the flow of warm surface waters from the southern hemisphere into the North Atlantic. Water flowing northward becomes modified through evaporation and mixing with other water masses, leading to increased salinity. When this water reaches the North Atlantic it cools and sinks through convection, due to its decreased temperature and increased salinity resulting in increased density. NADW is the outflow of this thick deep layer, which can be detected by its high salinity, high oxygen content, nutrient minima, and chlorofluorocarbons (CFCs). CFCs are anthropogenic substances that enter the surface of the ocean from gas exchange with the atmosphere. This distinct composition allows its path to be traced as it mixes with Circumpolar Deep Water (CDW), which in turn fills the deep Indian Ocean and part of the South Pacific. NADW and its formation is essential to the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for transporting large amounts of water, heat, salt, carbon, nutrients and other substances around the globe. In the conveyor belt model of thermohaline circulation of the world's oceans, the sinking of NADW pulls the waters of the North Atlantic drift northward; however, this is almost certainly an oversimplification of the actual relationship between NADW formation and the strength of the Gulf Stream/North Atlantic drift.

NADW has a temperature of 2-4 °C with a salinity of 34.9-35.0 psu found at a depth between 1500 and 4000m.

The NADW is a complex of several water masses formed by deep convection and also by overflow of dense water across the Greenland-Iceland-Scotland Ridge.

The upper layers are formed by deep open ocean convection during winter. Labrador Sea Water (LSW), formed in the Labrador Sea can reach depths of 2000 m as dense water sinks downward. Classical Labrador Sea Water (CLSW) production is dependent on preconditioning of water in the Labrador Sea from the previous year, and the strength of the North Atlantic Oscillation. During a positive NAO phase, conditions exist for strong winter storms to develop. These storms freshen the surface water, and their winds increase cyclonic flow, which allows denser waters to sink. As a result, the temperature, salinity, and density vary yearly. In some years these conditions do not exist and CLSW is not formed. CLSW has characteristic potential temperature of 3 °C, salinity of 34.88 psu, and density of 34.66. Another component of LSW is the Upper Labrador Sea Water (ULSW). ULSW forms at a density lower than CLSW and has a CFC maximum between 1200 and 1500 m in the subtropical North Atlantic. Eddies of cold less saline ULSW have similar densities of warmer saltier water and flow along the DWBC, but maintain their high CFCs. The ULSW eddies erode rapidly as they mix laterally with this warmer saltier water.

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