Editing North Atlantic Deep Water (section)

==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.<ref name="Smethie">{{cite journal |doi=10.1029/1999JC900274 |title=Tracing the flow of North Atlantic Deep Water using chlorofluorocarbons |date=2000 |last1=Smethie |first1=William M. |last2=Fine |first2=Rana A. |last3=Putzka |first3=Alfred |last4=Jones |first4=E. Peter |journal=Journal of Geophysical Research: Oceans |volume=105 |issue=C6 |pages=14297–14323 |bibcode=2000JGR...10514297S }}</ref>

[[File:North Atlantic Circulation.gif|thumb|400px|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.<ref>{{cite web |url=http://www.giss.nasa.gov/research/briefs/legrande_01/|archive-url=https://web.archive.org/web/20060218120705/http://www.giss.nasa.gov/research/briefs/legrande_01/ |url-status=dead |archive-date=18 February 2006 |title=NASA GISS: Science Briefs: Modeling an Abrupt Climate Change |publisher=Giss.nasa.gov |access-date=9 January 2015}}</ref>]]

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).<ref name="Smethie"/>

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&nbsp;°C, salinity of 34.88 psu, and density of 34.66.<ref name="Smethie"/>

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.<ref name="Smethie"/>

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.<ref>{{cite journal |doi=10.1016/j.jmarsys.2005.05.003 |title=Formation of Denmark Strait overflow water and its hydro-chemical composition |date=2005 |last1=Tanhua |first1=Toste |last2=Olsson |first2=K. Anders |last3=Jeansson |first3=Emil |journal=Journal of Marine Systems |volume=57 |issue=3–4 |pages=264–288 |bibcode=2005JMS....57..264T }}</ref>

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.<ref>{{cite journal |doi=10.1029/96JC03366 |title=Impact of eastern Arctic shelf waters on the Nansen Basin intermediate layers |date=1997 |last1=Schauer |first1=Ursula |last2=Muench |first2=Robin D. |last3=Rudels |first3=Bert |last4=Timokhov |first4=Leonid |journal=Journal of Geophysical Research: Oceans |volume=102 |issue=C2 |pages=3371–3382 |bibcode=1997JGR...102.3371S }}</ref> 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.<ref name="Smethie"/> 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).<ref>{{cite journal |doi=10.1016/0198-0149(80)90070-9 |title=The contribution of the Denmark strait overflow to the deep North Atlantic |date=1980 |last1=Swift |first1=James H. |last2=Aagaard |first2=Knut |last3=Malmberg |first3=Svend-Aage |journal=Deep Sea Research Part A. Oceanographic Research Papers |volume=27 |issue=1 |pages=29–42 |bibcode=1980DSRA...27...29S }}</ref> 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&nbsp;km to the south was no older than 2 years.<ref name ="Smethie"/> 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 [[Sverdrup|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.