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North Atlantic Deep Water (NADW) is a water mass that forms in the North Atlantic Ocean. It is largely formed in the Labrador Sea and in the Greenland Sea by the sinking of highly saline, dense overflow water from the Greenland Sea. The watermass can be traced around the southern end of Greenland and then, at a depth of 2000-4000 meters, down the coast of Canada and the United States where it turns a bit east. It continues southeast, past the eastern tip of South America and across the South Atlantic. Its path can ultimately be traced into the Southern Ocean and around the tip of Africa as it mixes with Circumpolar Deep Water.

In the "conveyor belt" model of the thermohaline circulation of the world ocean, the sinking of NADW pulls the waters of the North Atlantic drift northward; however, this is almost certanly an oversimplification of the actual relationship between NADW formation and the strength of the Gulf Stream/N. Atlantic drift ^[1]. NADW's distinctive composition (particularly high-salinity) compared to other watermasses allows its path to be traced as it mixes with Circumpolar Deep Water, which in turn fills the deep Indian Ocean and part of the South Pacific.

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 a analgous watermass that occupied a shallower depth known as Glacial North Atlantic Intermediate Water (GNAIW).

The NADW comprises three different water masses, each of which is distinguished by a specific formation region (LSW, see below) or entry point (DSOW, see below) and has unique production and transformation history (I Yashayaev).

The upper one is the Labrador Sea Water (LSW), formed due to deep winter convection in the Labrador Sea. This water partwise recirculates in the Labrador Sea while sinking. (Other parts of Labrador Sea Water spreads over the whole northern atlantic.) After recirculation it enters the Deep Western Boundary Current (DWBC) between 1600 and 2500 m depth. The Formation of LSW is seasonal and occurs not every year. There seems to be a correlation to the state of the North Atlantic Oscillation.

The lower water mass is the Denmark Strait Overflow Water (DSOW). This water overflows the Denmark Strait, entraining water from its surrounding. 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.

The third water mass origins from the Greenland Sea, too, but it leaves the basin between Iceland and Scotland. While flowing southward it entrains warm and saline Atlantic waters (which are much saltier than the overflow itself, this is where this water acquires its salty signature) and the Labrador Sea Water. It passes the Gibbs Fracture Zone to the Irminger Sea, and arranges in the DWBC between 2500 and 3500 m. Because of its complex formation/transformation history this water has many names: Iceland-Scotland Overflow Water, Northeast Atlantic Deep Water and Gibbs Fracture Zone Water

1. ^ diagram at http://seis.natsci.csulb.edu/rbehl/NADW.htm