North Atlantic Deep Water

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The North Atlantic Deep Water is a water mass, built in the Atlantic Ocean. It is largely formed in the Labrador Sea and in the Greenland Sea, where the North Atlantic drift ends in the northeast Atlantic (northeast of Iceland) by the sinking of dense overflow water from the Greenland Sea. It runs around the southern end of Greenland and then follows the coast of Canada down to the coast of the United States where it turns a bit east, out from the coast and then continues southeast, past the eastern tip of South America.

The North Atlantic Deep Water forms a cold stream that runs about 2-4 kilometers below the surface along the western continental shelf (Deep Western Boundary Current). Driven by the ocean's conveyor belt (thermohaline circulation), the motion of it pulls the waters of the North Atlantic drift northward. (diagram [1]) From this current it spreads over wide areas of North and South Atlantic. NADW is the biggest water mass in the World Ocean, one can identify it in the Indian Ocean and still in the Southern Pacific.

It has been shown that the North Atlantic Deep Water has shut down in the past (such as during the Younger Dryas), and that this decreases 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.

[edit] Its formation more in detail

The NADW is distinguishable into three different subtypes of water masses.

The upper one is Labrador Sea Water, formed due to preconditioning 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 NAO.

The lower is the INADW (Inner NADW) or DSOW (Denmark Strait Overflow Water). 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.

The third water origins from the Greenland Sea, too, but it leaves the basin between Iceland and Scotland. While flowing southward it entrains Mediterranean and (upper) Labrador Sea Water, thus it gets more warm but salty. It passes the Gibbs Fracture Zone to the Irminger sea, therefore it is called Gibbs Fracture Zone Water GFZW, and arranges in the DWBC between 2500 and 3500 m.