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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, high 14C/12C, and chlorofluorocarbons (CFCs).

Formation and sources
The NADW is a complex of several water masses formed by deep convection and 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 (NAO). The formation of both of these waters involves the conversion of warm, salty, northward-flowing surface waters to cold, dense, deep waters behind the Greenland-Iceland-Scotland Ridge. Water flow from the North Atlantic current enters the Arctic Ocean through the Norwegian Current, which splits into the Fram Strait and Barents Sea Branch. Water from the Fram Strait recirculates, reaching a density of DSOW, sinks, and flows towards the Denmark Strait. Water flowing into the Barents Sea feeds ISOW. ISOW enters the eastern North Atlantic over the Iceland-Scotland Ridge through the Faeroe Bank Channel at a depth of 850 m, with some water flowing over the shallower Iceland-Faeroe Rise. ISOW has a low CFC concentrations and it has been estimated from these concentrations that ISOW resides behind the ridge for 45 years. Winter cooling and convection allow AIW to sink and pool behind the Denmark Strait. Upper AIW has a high amount of anthropogenic tracers due its exposure to the atmosphere. AIW's tritium and CFC signature is observed in DSOW at the base of the Greenland continental slope. This also showed that the DSOW flowing 450 km to the south was no older than 2 years. Both the DSOW and ISOW flow around the Irminger Basin and Labrador Sea in a deep boundary current. Leaving the Greenland Sea with 2.5 Sv, its flow increases to 10 Sv south of Greenland. It is cold and relatively fresh, flowing below 3500 m in the DWBC and spreading inward the deep Atlantic basins. ==Spreading pathways==
Spreading pathways
past the Mid-Atlantic Ridge. 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 maximum associated with ULSW has been observed along 24°N in the DWBC at 1500 m. ==Variability==
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, NADW was replaced with an analogous watermass that occupied a shallower depth known as Glacial North Atlantic Intermediate Water. ==See also==
Source: Wikipedia ↗
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