North Atlantic Deep Water

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The North Atlantic Deep Water is considered to be one of several possible tipping points in the climate system. Climate-tipping-points-en.svg
The North Atlantic Deep Water is considered to be one of several possible tipping points in the climate system.

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning 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, [1] and chlorofluorocarbons (CFCs). [2]

Contents

CFCs are anthropogenic substances that enter the surface of the ocean from gas exchange with the atmosphere. This distinct composition allows its path to be traced as it mixes with Circumpolar Deep Water (CDW), which in turn fills the deep Indian Ocean and part of the South Pacific. NADW and its formation is essential to the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for transporting large amounts of water, heat, salt, carbon, nutrients and other substances from the Tropical Atlantic to the Mid and High Latitude Atlantic. [3]

In the conveyor belt model of thermohaline circulation of the world's oceans, 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. [4]

NADW has a temperature of 2-4 °C with a salinity of 34.9-35.0 psu found at a depth between 1500 and 4000m.

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. [5]

The circulation patterns in the North Atlantic Ocean. Cold, dense water is shown in blue, flowing south from upper latitudes, while warm, less dense water, shown in red, flows north from low latitudes. North Atlantic Circulation.gif
The circulation patterns in the North Atlantic Ocean. Cold, dense water is shown in blue, flowing south from upper latitudes, while warm, less dense water, shown in red, flows north from low latitudes.

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). [5]

During a positive NAO phase, conditions exist for strong winter storms to develop. These storms freshen the surface water, and their winds increase cyclonic flow, which allows denser waters to sink. As a result, the temperature, salinity, and density vary yearly. In some years these conditions do not exist and CLSW is not formed. CLSW has characteristic potential temperature of 3 °C, salinity of 34.88 psu, and density of 34.66. [5]

Another component of LSW is the Upper Labrador Sea Water (ULSW). ULSW forms at a density lower than CLSW and has a CFC maximum between 1200 and 1500 m in the subtropical North Atlantic. Eddies of cold less saline ULSW have similar densities of warmer saltier water and flow along the DWBC, but maintain their high CFCs. The ULSW eddies erode rapidly as they mix laterally with this warmer saltier water. [5]

The lower waters mass of NADW form from overflow of the Greenland-Iceland-Scotland Ridge. They are Iceland-Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW). The overflows are a combination of dense Arctic Ocean water (18%), modified Atlantic water (32%), and intermediate water from the Nordic seas (20%), that entrain and mix with other water masses (contributing 30%) as they flow over the Greenland-Iceland-Scotland Ridge. [7]

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. [8] 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. [5] As the water flows southward at the bottom of the channel, it entrains surrounding water of the eastern North Atlantic, and flows to the western North Atlantic through the Charlie-Gibbs Fracture Zone, entraining with LSW. This water is less dense than DSOW and lays above it as it flows cyclonically in the Irminger Basin.

DSOW is the coldest, densest, and freshest water mass of NADW. DSOW formed behind the ridge flows over the Denmark Strait at a depth of 600m. The most significant water mass contributing to DSOW is Arctic Intermediate Water (AIW). [9] 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. [5] 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

The NADW flows southward through the Atlantic, approaching the Antarctic Bottom Water past the Mid-Atlantic Ridge. Antarctic bottom water.svg
The NADW flows southward through the Atlantic, approaching the Antarctic Bottom Water 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. [10]

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. [10] Some of the upper ULSW recirculates into the Gulf Stream, while some remains in the DWBC. High CFCs in the subtropics indicate recirculation in the subtropics. [5]

ULSW that remains in the DWBC dilutes as it moves equatorward. Deep convection in the Labrador Sea during the late 1980s and early 1990s resulted in CLSW with a lower CFC concentration due to downward mixing. Convection allowed the CFCs to penetrate further downward to 2000m. These minima could be tracked, and were first observed in the subtropics in the early 1990s. [5]

ISOW and DSOW flow around the Irminger Basin and DSOW entering the DWBC. These are the two lower portions of the NADW. Another CFC maximum is seen at 3500 m in the subtropics from the DSOW contribution to NADW. [10] Some of the NADW recirculates with the northern gyre. To the south of the gyre, NADW flows under the Gulf Stream, where it continues along the DWBC until it reaches another gyre in the subtropics.

Lower North Atlantic Deep Water (LNADW), originating in the Greenland and Norwegian seas, brings high salinity, oxygen, and freon concentrations towards to the Romanche Trench, an equatorial fracture zone in the Mid-Atlantic Ridge (MAR). Found at depths around 3,600–4,000 m (11,800–13,100 ft), LNADW flow east through the trench over Antarctic Bottom Water—the trench is the only opening in the MAR where inter-basin exchange is possible for these two water masses. [11]

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. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Atlantic Ocean</span> Ocean between Africa, Europe, and the Americas

The Atlantic Ocean is the second-largest of the world's five oceanic divisions, with an area of about 85,133,000 km2 (32,870,000 sq mi). It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for separating the New World of the Americas from the Old World of Afro-Eurasia.

<span class="mw-page-title-main">North Atlantic Current</span> Current of the Atlantic Ocean

The North Atlantic Current (NAC), also known as North Atlantic Drift and North Atlantic Sea Movement, is a powerful warm western boundary current within the Atlantic Ocean that extends the Gulf Stream northeastward.

<span class="mw-page-title-main">Physical oceanography</span> Study of physical conditions and processes within the ocean

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

<span class="mw-page-title-main">Labrador Sea</span> Arm of the North Atlantic Ocean

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. It is a marginal sea of the Atlantic.

<span class="mw-page-title-main">Water mass</span> Body of water with common formation history

An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water. Properties include temperature, salinity, chemical - isotopic ratios, and other physical quantities which are conservative flow tracers. Water mass is also identified by its non-conservative flow tracers such as silicate, nitrate, oxygen, and phosphate.

<span class="mw-page-title-main">East Greenland Current</span> Current from Fram Strait to Cape Farewell off the eastern coat of Greenland

The East Greenland Current (EGC) is a cold, low-salinity current that extends from Fram Strait (~80N) to Cape Farewell (~60N). The current is located off the eastern coast of Greenland along the Greenland continental margin. The current cuts through the Nordic Seas and through the Denmark Strait. The current is of major importance because it directly connects the Arctic to the Northern Atlantic, it is a major contributor to sea ice export out of the Arctic, and it is a major freshwater sink for the Arctic.

<span class="mw-page-title-main">North Atlantic Gyre</span> Major circular system of ocean currents

The North Atlantic Gyre of the Atlantic Ocean is one of five great oceanic gyres. It is a circular ocean current, with offshoot eddies and sub-gyres, across the North Atlantic from the Intertropical Convergence Zone to the part south of Iceland, and from the east coasts of North America to the west coasts of Europe and Africa.

<span class="mw-page-title-main">Fram Strait</span> Passage between Greenland and Svalbard

The Fram Strait is the passage between Greenland and Svalbard, located roughly between 77°N and 81°N latitudes and centered on the prime meridian. The Greenland and Norwegian Seas lie south of Fram Strait, while the Nansen Basin of the Arctic Ocean lies to the north. Fram Strait is noted for being the only deep connection between the Arctic Ocean and the World Oceans. The dominant oceanographic features of the region are the West Spitsbergen Current on the east side of the strait and the East Greenland Current on the west.

Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Antarctic Polar Front zone. This convergence zone is normally located between 50°S and 60°S, hence this is where almost all of the AAIW is formed.

<span class="mw-page-title-main">Irminger Sea</span> Marginal sea of the North Atlantic Ocean

The Irminger Sea is a marginal sea of the North Atlantic Ocean. It is bordered to the west by southern Greenland, to the north by Iceland and the Denmark Strait, to the east by the Reykjanes Ridge, and to the south by open waters of the North Atlantic.

The Great Salinity Anomaly (GSA) originally referred to an event in the late 1960s to early 1970s where a large influx of freshwater from the Arctic Ocean led to a salinity anomaly in the northern North Atlantic Ocean, which affected the Atlantic meridional overturning circulation. Since then, the term "Great Salinity Anomaly" has been applied to successive occurrences of the same phenomenon, including the Great Salinity Anomaly of the 1980s and the Great Salinity Anomaly of the 1990s. The Great Salinity Anomalies were advective events, propagating to different sea basins and areas of the North Atlantic, and is on the decadal-scale for the anomalies in the 1970s, 1980s, and 1990s.

<span class="mw-page-title-main">Denmark Strait overflow</span> Underwater waterfall in the Atlantic

The Denmark Strait overflow is an undersea overflow located in the Denmark Strait between Greenland and Iceland. The overflow transports around 3.2 million m3 (110 million cu ft) of water per second, greatly eclipsing the discharge of the Amazon River into the Atlantic Ocean and the flow rate of the former Guaíra Falls. The descending column of water is approximately 200 m (660 ft) wide and 200 m (660 ft) thick and descends over a length of around 1,000 km (620 mi). It is formed by the density difference of the water masses either side of the Denmark Strait; the southward-flowing water originating from the Nordic Seas is colder and consequently more dense than the Irminger Sea to the south of the strait. At the Greenland–Iceland Rise – an elevated ridge forming the overflow's apex – the colder water cascades along the seafloor to a depth of around 3,000 m (10,000 ft). Due to the Coriolis effect, the downward flow of water is deflected to the right, resulting in the descending water on the Greenland side of the channel being roughly 1 km (0.62 mi) higher than the opposite side of the channel.

<span class="mw-page-title-main">Labrador Sea Water</span> Water mass formed by convective processes in the Labrador Sea

Labrador Sea Water is an intermediate water mass characterized by cold water, relatively low salinity compared to other intermediate water masses, and high concentrations of both oxygen and anthropogenic tracers. It is formed by convective processes in the Labrador Sea located between Greenland and the northeast coast of the Labrador Peninsula. Deep convection in the Labrador Sea allows colder water to sink forming this water mass, which is a contributor to the upper layer of North Atlantic Deep Water. North Atlantic Deep Water flowing southward is integral to the Atlantic Meridional Overturning Circulation. The Labrador Sea experiences a net heat loss to the atmosphere annually.

<span class="mw-page-title-main">Overturning in the Subpolar North Atlantic Program</span> International research project

The Overturning in the Subpolar North Atlantic Program (OSNAP) is an international project designed to study the mechanistic link between water mass transformation at high latitudes and the meridional overturning circulation in the North Atlantic (AMOC) on interannual time scales. Though this linkage is evident in climate models on decadal time scales, to date there has been no clear demonstration of AMOC variability in response to changes in deep water formation on interannual and decadal time scales. OSNAP intends to fill that gap by providing a continuous record of the trans-basin fluxes of heat, mass and freshwater for a comparison to records of convective activity and water mass transformation at high latitudes in the North Atlantic.

<span class="mw-page-title-main">Nordic Seas</span>

The Nordic Seas are located north of Iceland and south of Svalbard. They have also been defined as the region located north of the Greenland-Scotland Ridge and south of the Fram Strait-Spitsbergen-Norway intersection. Known to connect the North Pacific and the North Atlantic waters, this region is also known as having some of the densest waters, creating the densest region found in the North Atlantic Deep Water. The deepest waters of the Arctic Ocean are connected to the worlds other oceans through Nordic Seas and Fram Strait. There are three seas within the Nordic Sea: Greenland Sea, Norwegian Sea, and Iceland Sea. The Nordic Seas only make up about 0.75% of the World's Oceans. This region is known as having diverse features in such a small topographic area, such as the mid oceanic ridge systems. Some locations have shallow shelves, while others have deep slopes and basins. This region, because of the atmosphere-ocean transfer of energy and gases, has varying seasonal climate. During the winter, sea ice is formed in the western and northern regions of the Nordic Seas, whereas during the summer months, the majority of the region remains free of ice.

<span class="mw-page-title-main">Iceland Sea</span> Small body of water in the North Atlantic

The Iceland Sea, a relatively small body of water, is bounded by Iceland. It is characterized by its proximity to the Mid-Atlantic Ridge, which transforms into the Kolbeinsey Ridge, and the Greenland-Scotland Ridge, and it lies just south of the Arctic Circle. This region is typically delineated by Greenland to the west, the Denmark Strait, and the continental shelf break south of Iceland to the south. Next in the boundary line are Jan Mayen, being a small Norwegian volcanic island, and the Jan Mayen Fracture Zone to the north, with the Jan Mayen Ridge to the east of the sea. This ridge serves as the northern boundary of the Iceland Sea, acting as the dividing line from the Greenland Sea. To the immediate south of Jan Mayen, the Iceland-Jan Mayen Ridge stretches towards the Iceland-Faroe Ridge, creating a boundary between the Iceland Sea and the Norwegian Sea to the east.

<span class="mw-page-title-main">Cecilie Mauritzen</span> Norwegian oceanographer

Cecilie Mauritzen is a Norwegian physical oceanographer who studies connections between ocean currents and climate change.

<span class="mw-page-title-main">Mediterranean outflow</span>

The Mediterranean Outflow is a current flowing from the Mediterranean Sea towards the Atlantic Ocean through the Strait of Gibraltar. Once it has reached the western side of the Strait of Gibraltar, it divides into two branches, one flowing westward following the Iberian continental slope, and another returning to the Strait of Gibraltar circulating cyclonically. In the Strait of Gibraltar and in the Gulf of Cádiz, the Mediterranean Outflow core has a width of a few tens of km. Through its nonlinear interactions with tides and topography, as it flows out of the Mediterranean basin it undergoes such strong mixing that the water masses composing this current become indistinguishable upon reaching the western side of the strait.

<span class="mw-page-title-main">Faroe-Bank Channel overflow</span> Overflow current from Nordic Seas towards North Atlantic Ocean

Cold and dense water from the Nordic Seas is transported southwards as Faroe-Bank Channel overflow. This water flows from the Arctic Ocean into the North Atlantic through the Faroe-Bank Channel between the Faroe Islands and Scotland. The overflow transport is estimated to contribute to one-third of the total overflow over the Greenland-Scotland Ridge. The remaining two-third of overflow water passes through Denmark Strait, the Wyville Thomson Ridge (0.3 Sv), and the Iceland-Faroe Ridge (1.1 Sv).

<span class="mw-page-title-main">Irminger Rings</span> Ocean eddies in Labrador Sea which influence deep convection

Irminger Rings (IRs) are mesoscale ocean eddies that are formed off the West coast of Greenland and travel southwestwards through the Labrador Sea. Most IRs are anti-cyclonic. There is considerable interest in researching IRs, because they have been hypothesized to influence deep convection in the Labrador sea, and therefore the formation of deep water.

References

  1. ↑ Broecker, Wallace (1991). "The great ocean conveyor" (PDF). Oceanography. 4 (2): 79–89. doi: 10.5670/oceanog.1991.07 .
  2. ↑ "North Atlantic circulation and thermohaline forcing". Sam.ucsd.edu. Retrieved 9 January 2015.
  3. ↑ Schmitner, Andreas; et al. (2007). "Introduction: The Ocean's Meridional Overturning Circulation" (PDF). People.oregonstate.edu. Retrieved 9 January 2015.
  4. ↑ "Atlantic Ocean water masses". seis.natsci.csulb.edu. Archived from the original on 25 September 2008. Retrieved 24 January 2024.
  5. 1 2 3 4 5 6 7 8 Smethie, William M.; Fine, Rana A.; Putzka, Alfred; Jones, E. Peter (2000). "Tracing the flow of North Atlantic Deep Water using chlorofluorocarbons". Journal of Geophysical Research: Oceans. 105 (C6): 14297–14323. Bibcode:2000JGR...10514297S. doi:10.1029/1999JC900274.
  6. ↑ "NASA GISS: Science Briefs: Modeling an Abrupt Climate Change". Giss.nasa.gov. Archived from the original on 18 February 2006. Retrieved 9 January 2015.
  7. ↑ Tanhua, Toste; Olsson, K. Anders; Jeansson, Emil (2005). "Formation of Denmark Strait overflow water and its hydro-chemical composition". Journal of Marine Systems. 57 (3–4): 264–288. Bibcode:2005JMS....57..264T. doi:10.1016/j.jmarsys.2005.05.003.
  8. ↑ Schauer, Ursula; Muench, Robin D.; Rudels, Bert; Timokhov, Leonid (1997). "Impact of eastern Arctic shelf waters on the Nansen Basin intermediate layers". Journal of Geophysical Research: Oceans. 102 (C2): 3371–3382. Bibcode:1997JGR...102.3371S. doi:10.1029/96JC03366.
  9. ↑ Swift, James H.; Aagaard, Knut; Malmberg, Svend-Aage (1980). "The contribution of the Denmark strait overflow to the deep North Atlantic". Deep Sea Research Part A. Oceanographic Research Papers. 27 (1): 29–42. Bibcode:1980DSRA...27...29S. doi:10.1016/0198-0149(80)90070-9.
  10. 1 2 3 Talley, Lynne D. (2011). Descriptive Physical Oceanography: An Introduction. Academic Press. doi:10.1016/C2009-0-24322-4. ISBN   9780750645522.
  11. ↑ Ferron, B.; Mercier, H.; Speer, K.; Gargett, A.; Polzin, K. (1998). "Mixing in the Romanche Fracture Zone". Journal of Physical Oceanography. 28 (10): 1929–1945. Bibcode:1998JPO....28.1929F. doi: 10.1175/1520-0485(1998)028<1929:MITRFZ>2.0.CO;2 . S2CID   140185816.
  12. ↑ Howe, Jacob N.W.; Piotrowski, Alexander M.; Noble, Taryn L.; Mulitza, Stefan; Chiessi, Cristiano M.; Bayon, Germain (2016). "North Atlantic Deep Water Production during the Last Glacial Maximum". Nature Communications. 7 (11765): 11765. Bibcode:2016NatCo...711765H. doi:10.1038/ncomms11765. PMC   4895795 . PMID   27256826.

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