North Atlantic Deep Water
Encyclopedia
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 certainly an oversimplification of the actual relationship between NADW formation and the strength of the Gulf Stream
/North 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 Dryas
or during Heinrich event
s), 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).
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 does not occur 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 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
Water mass
An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water...
that forms in the North Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. It is largely formed in the Labrador Sea
Labrador Sea
The Labrador Sea is an arm of the North Atlantic Ocean between the Labrador Peninsula and Greenland. The sea is flanked by continental shelves to the southwest, northwest, and northeast. It connects to the north with Baffin Bay through the Davis Strait...
and in the Greenland Sea
Greenland Sea
The Greenland Sea is a body of water that borders Greenland to the west, the Svalbard archipelago to the east, Fram Strait and the Arctic Ocean to the north, and the Norwegian Sea and Iceland to the south. The Greenland Sea is often defined as part of the Arctic Ocean, sometimes as part of the...
by the sinking of highly saline, dense overflow water from the Greenland Sea
Greenland Sea
The Greenland Sea is a body of water that borders Greenland to the west, the Svalbard archipelago to the east, Fram Strait and the Arctic Ocean to the north, and the Norwegian Sea and Iceland to the south. The Greenland Sea is often defined as part of the Arctic Ocean, sometimes as part of the...
. The watermass can be traced around the southern end of Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
and then, at a depth of 2000–4000 meters, down the coast of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
where it turns a bit east. It continues southeast, past the eastern tip of South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
and across the South Atlantic. Its path can ultimately be traced into the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
and around the tip of Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
as it mixes with Circumpolar Deep Water.
In the "conveyor belt" model of the thermohaline circulation
Thermohaline circulation
The term thermohaline circulation refers to a 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 drift
North Atlantic Drift
North Atlantic Drift is:* An ocean current that continues from the North Atlantic Current* An album by Ocean Colour Scene: North Atlantic Drift this doesn't make any goddamn sense....
northward; however, this is almost certainly an oversimplification of the actual relationship between NADW formation and the strength of the Gulf Stream
Gulf Stream
The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean...
/North Atlantic drift. NADW's distinctive composition (particularly high-salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
) 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
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
and part of the South Pacific
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
.
It is believed that North Atlantic Deep Water formation has been dramatically reduced at times during the past (such as during the Younger Dryas
Younger Dryas
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...
or during Heinrich event
Heinrich event
Heinrich 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 Stream and the North Atlantic drift, in turn cooling the climate of northwestern Europe. There is concern that global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...
might cause this to happen again. It is also hypothesized that during the Last Glacial Maximum
Last Glacial Maximum
The Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
(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 does not occur every year. There seems to be a correlation to the state of the North Atlantic Oscillation
North Atlantic oscillation
The North Atlantic oscillation is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the...
.
The lower water mass is the Denmark Strait Overflow Water (DSOW). This water overflows the Denmark Strait
Denmark Strait
The Denmark Strait or Greenland Strait |Sound]]) is an oceanic strait between Greenland and Iceland...
, entraining water from its surrounding. Leaving the Greenland Sea with 2.5 Sv
Sverdrup
The 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
Irminger Sea
The Irminger Sea is a marginal sea of the North Atlantic Ocean. It is 480 km long and 290 km wide at its narrowest. The northern limit is the Greeland-Iceland Rise on the bottom of Denmark Strait between Iceland and East Greenland, which connects to Greenland Sea. To the southwest, it reaches to...
, 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