North Atlantic Deep Water (NADW) is a
water massAn oceanographic water mass is an identifiable body of water which has physical properties distinct from surrounding water. Properties include temperature, salinity, chemical - isotopic ratios, and other physical quantities....
that forms in the North
Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
. It is largely formed in the
Labrador SeaLabrador Sea is an arm of the North Atlantic Ocean between Labrador and Greenland.Water depths in the center of Labrador Sea are around and it is flanked by continental shelves to the southwest, northwest, and northeast...
and in the
Greenland SeaThe Greenland Sea is the northernmost part of the North Atlantic Ocean immediately south of the Arctic Ocean. It encompasses some 1,205,000 square km . The average depth of the Greenland Sea is close to 1,450 m...
by the sinking of highly saline, dense overflow water from the
Greenland SeaThe Greenland Sea is the northernmost part of the North Atlantic Ocean immediately south of the Arctic Ocean. It encompasses some 1,205,000 square km . The average depth of the Greenland Sea is close to 1,450 m...
. The watermass can be traced around the southern end of
GreenlandGreenland is an autonomous constituent country within the Kingdom of Denmark located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago...
and then, at a depth of 2000-4000 meters, down the coast of
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
and the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
where it turns slightly east. It continues southeast, past the eastern tip of
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
and across the South Atlantic.
North Atlantic Deep Water (NADW) is a
water massAn oceanographic water mass is an identifiable body of water which has physical properties distinct from surrounding water. Properties include temperature, salinity, chemical - isotopic ratios, and other physical quantities....
that forms in the North
Atlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
. It is largely formed in the
Labrador SeaLabrador Sea is an arm of the North Atlantic Ocean between Labrador and Greenland.Water depths in the center of Labrador Sea are around and it is flanked by continental shelves to the southwest, northwest, and northeast...
and in the
Greenland SeaThe Greenland Sea is the northernmost part of the North Atlantic Ocean immediately south of the Arctic Ocean. It encompasses some 1,205,000 square km . The average depth of the Greenland Sea is close to 1,450 m...
by the sinking of highly saline, dense overflow water from the
Greenland SeaThe Greenland Sea is the northernmost part of the North Atlantic Ocean immediately south of the Arctic Ocean. It encompasses some 1,205,000 square km . The average depth of the Greenland Sea is close to 1,450 m...
. The watermass can be traced around the southern end of
GreenlandGreenland is an autonomous constituent country within the Kingdom of Denmark located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago...
and then, at a depth of 2000-4000 meters, down the coast of
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
and the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
where it turns slightly east. It continues southeast, past the eastern tip of
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
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 circulationThe term thermohaline circulation refers to the part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
of the world ocean, the sinking of NADW pulls the waters of the
North Atlantic driftNorth Atlantic Drift is:* An ocean current that continues from the North Atlantic Current* An album by Ocean Colour Scene: 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/N. Atlantic drift . 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 DryasThe Younger Dryas stadial, named after the alpine/tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, was a geologically brief cold climate period following the Bølling/Allerød interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years ago, and...
or during
Heinrich eventHeinrich events, first described by marine geologist Hartmut Heinrich, occurred during the last glacial period, or "ice age". During such events, armadas of icebergs broke off from glaciers and traversed the North Atlantic. The icebergs contained rock mass eroded by the glaciers, and as they...
s), and that this might correlate with a decrease in the strength of the
Gulf StreamThe Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland...
and the
North Atlantic driftNorth Atlantic Drift is:* An ocean current that continues from the North Atlantic Current* An album by Ocean Colour Scene: North Atlantic Drift...
, in turn cooling the climate of northwestern
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
. There is concern that
global warmingGlobal warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C during the last century...
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).
Formation details
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.
The lower water mass is the Denmark Strait Overflow Water (DSOW). This water overflows the
Denmark Straitthumb|250px|Location mapThe Denmark Strait is a strait between Greenland and Iceland . The Norwegian island of Jan Mayen is located northeast of the strait....
, entraining water from its surrounding. Leaving the Greenland Sea with 2.5
SvThe sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
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 originates 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