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 from the Tropical Atlantic to the Mid and High Latitude Atlantic. 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.
Contents
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.
Spreading pathways
The southward spread of NADW along the Deep Western Boundary current (DWBC) can be traced by its high oxygen content, high CFCs, and density.
ULSW is the major source of upper NADW. ULSW advects southward from the Labrador Sea in small eddies that mix into the DWBC. A CFC maxima associated with ULSW has been observed along 24°N in the DWBC at 1500 m. Some of the upper ULSW recirculates into the Gulf Stream, while some remains in the DWBC. High CFCs in the subtropics indicate recirculation in the subtropics. ULSW that remains in the DWBC dilutes as it moves equatorward. Deep convection in the Labrador Sea during the late 1980s and early 1990s resulted in CLSW with a lower CFC concentration due to downward mixing. However, convection allowed the CFCs to penetrate further downward to 2000m. These minimum could be tracked, and were first observed in the subtropics in the early 1990s.
ISOW and DSOW flow around the Irminger Basin and DSOW entering the DWBC. These are the two lower portions of the NADW. Another CFC maximum is seen at 3500 m in the subtropics from the DSOW contribution to NADW. Some of the NADW recirculates with the northern gyre. To the south of the gyre NADW flows under the Gulf Stream where it continues along the DWBC until it reaches another gyre in the subtropics.
Lower North Atlantic Deep Water (LNADW), originating in the Greenland and Norwegian Seas, brings high salinity, oxygen, and freon concentrations towards to the Romanche Trench, an equatorial fracture zone in the Mid-Atlantic Ridge (MAR). Found at depths around 3,600–4,000 m (11,800–13,100 ft), LNADW flow east through the trench over AABW, the trench being the only opening in the MAR where inter-basin exchange is possible for these two water masses.
Variability
It is believed that North Atlantic Deep Water formation has been dramatically reduced at times during the past (such as during the Younger Dryas or during Heinrich events), and that this might correlate with a decrease in the strength of the Gulf Stream and the North Atlantic drift, in turn cooling the climate of northwestern Europe. There is concern that global warming might cause this to happen again. It is also hypothesized that during the Last Glacial Maximum (LGM), NADW was replaced with an analogous watermass that occupied a shallower depth known as Glacial North Atlantic Intermediate Water (GNAIW).