Editing Atlantic Ocean (section)

== Water characteristics ==
[[File:Gulf Stream Sea Surface Currents and Temperatures NASA SVS.jpg|thumb|upright=1.8|As the Gulf Stream meanders across the North Atlantic from the North American east coast to Western Europe its temperature drops by {{convert|20|C-change}}.|alt=Visualisation of the Gulf Stream stretching from the Gulf of Mexico to Western Europe]]
[[File:Circulacion termohalina.jpg|Path of the [[thermohaline circulation]]. Purple paths represent deep-water currents, while blue paths represent surface currents.|thumb|alt=Map displaying a looping line with arrows indicating that water flows eastward in the far Southern Ocean, angling northeast of Australia, turning sough-after passing Alaska, then crossing the mid-Pacific to flow north of Australia, continuing west below Africa, then turning northwest until reaching eastern Canada, then angling east to southern Europe, then finally turning south just below Greenland and flowing down the Americas' eastern coast, and resuming its flow eastward to complete the circle]]

Surface water temperatures, which vary with latitude, current systems, and season and reflect the latitudinal distribution of solar energy, range from below {{convert|-2|C|F}} to over {{convert|30|C|F}}. Maximum temperatures occur north of the equator, and minimum values are found in the polar regions. In the middle latitudes, the area of maximum temperature variations, values may vary by {{convert|7|–|8|C-change}}.<ref name="USN-2001" />

From October to June the surface is usually covered with sea ice in the [[Labrador Sea]], [[Denmark Strait]], and Baltic Sea.<ref name="USN-2001" />{{Failed verification|date=April 2023}}

The [[Coriolis effect]] circulates North Atlantic water in a clockwise direction, whereas South Atlantic water circulates counter-clockwise. The south [[tide]]s in the Atlantic Ocean are semi-diurnal; that is, two high tides occur every 24 lunar hours. In latitudes above [[40th parallel north|40° North]] some east–west oscillation, known as the [[North Atlantic oscillation]], occurs.<ref name="USN-2001" />

=== Salinity ===
On average, the Atlantic is the saltiest major ocean; surface water [[salinity]] in the open ocean ranges from 33 to 37 parts per thousand (3.3–3.7%) by mass and varies with latitude and season. Evaporation, precipitation, river inflow and [[sea ice]] melting influence surface salinity values. Although the lowest salinity values are just north of the equator (because of heavy tropical rainfall), in general, the lowest values are in the high latitudes and along coasts where large rivers enter. Maximum salinity values occur at about [[25th parallel north|25° north]] and [[25th parallel south|south]], in [[subtropical]] regions with low rainfall and high evaporation.<ref name="USN-2001">{{Harvnb|U.S. Navy|2001}}</ref>

The high surface salinity in the Atlantic, on which the Atlantic [[thermohaline circulation]] is dependent, is maintained by two processes: the [[Agulhas Current#Agulhas leakage and rings|Agulhas Leakage/Rings]], which brings salty Indian Ocean waters into the South Atlantic, and the "Atmospheric Bridge", which evaporates subtropical Atlantic waters and exports it to the Pacific.<ref>{{Harvnb|Marsh|Hazeleger|Yool|Rohling|2007|loc=Introduction, p. 1}}</ref>

=== Water masses ===
{| class="wikitable floatright" style="font-size: 0.9em; text-align: center;"
|+ Temperature-salinity characteristics for Atlantic water masses<ref>{{Harvnb|Emery|Meincke|1986|loc=Table, p. 385}}</ref>
|-
 ! Water mass !! Temperature !! Salinity
|-
 ! colspan="3" | Upper waters ({{cvt|0|-|500|m|ft|-2|disp=or}})
|-
 | align=left | Atlantic Subarctic<br />Upper Water (ASUW) || 0.0–4.0&nbsp;°C || 34.0–35.0
|-
 | align=left | Western North Atlantic<br />Central Water (WNACW) || 7.0–20&nbsp;°C || 35.0–36.7
|-
 | align=left | Eastern North Atlantic<br />Central Water (ENACW) || 8.0–18.0&nbsp;°C || 35.2–36.7
|-
 | align=left | South Atlantic<br />Central Water (SACW) || 5.0–18.0&nbsp;°C || 34.3–35.8
|-
 ! colspan="3" | Intermediate waters ({{cvt|500|-|1500|m|ft|-2|disp=or}})
|-
 | align=left | Western Atlantic Subarctic<br />Intermediate Water (WASIW) || 3.0–9.0&nbsp;°C || 34.0–35.1
|-
 | align=left | Eastern Atlantic Subarctic<br />Intermediate Water (EASIW) || 3.0–9.0&nbsp;°C || 34.4–35.3
|-
 | align=left | Mediterranean Water (MW) || 2.6–11.0&nbsp;°C || 35.0–36.2
|-
 | align=left | Arctic Intermediate Water (AIW) || −1.5–3.0&nbsp;°C || 34.7–34.9
|-
 ! colspan="3" | Deep and abyssal waters (1,500&nbsp;m–bottom or 4,900&nbsp;ft–bottom)
|-
 | align=left | North Atlantic<br />Deep Water (NADW) || 1.5–4.0&nbsp;°C || 34.8–35.0
|-
 | align=left | Antarctic Bottom Water (AABW) || −0.9–1.7&nbsp;°C || 34.6–34.7
|-
 | align=left | Arctic Bottom Water (ABW) || −1.8 to −0.5&nbsp;°C || 34.9–34.9
|}

The Atlantic Ocean consists of four major, upper [[water mass]]es with distinct temperature and salinity. The Atlantic subarctic upper water in the northernmost North Atlantic is the source for subarctic intermediate water and North Atlantic intermediate water. North Atlantic central water can be divided into the eastern and western North Atlantic central water since the western part is strongly affected by the Gulf Stream and therefore the upper layer is closer to underlying fresher subpolar intermediate water. The eastern water is saltier because of its proximity to Mediterranean water. North Atlantic central water flows into South Atlantic central water at [[15th parallel north|15°N]].<ref name="Emery-Atlantic">{{Harvnb|Emery|Meincke|1986|loc=Atlantic Ocean, pp. 384–386}}</ref>

There are five intermediate waters: four low-salinity waters formed at subpolar latitudes and one high-salinity formed through evaporation. Arctic intermediate water flows from the north to become the source for North Atlantic deep water, south of the Greenland-Scotland sill. These two intermediate waters have different salinity in the western and eastern basins. The wide range of salinities in the North Atlantic is caused by the asymmetry of the northern subtropical [[gyre]] and a large number of contributions from a wide range of sources: Labrador Sea, Norwegian-Greenland Sea, Mediterranean, and South Atlantic Intermediate Water.<ref name="Emery-Atlantic" />

The [[North Atlantic Deep Water|North Atlantic deep water]] (NADW) is a complex of four water masses, two that form by deep convection in the open ocean{{snd}}classical and upper Labrador sea water{{snd}}and two that form from the inflow of dense water across the Greenland-Iceland-Scotland sill{{snd}}Denmark Strait and Iceland-Scotland overflow water. Along its path across Earth the composition of the NADW is affected by other water masses, especially [[Antarctic Bottom Water|Antarctic bottom water]] and Mediterranean overflow water.<ref>{{Harvnb|Smethie|Fine|Putzka|Jones|2000|loc=Formation of NADW, pp. 14299–14300}}</ref>
The NADW is fed by a flow of warm shallow water into the northern North Atlantic which is responsible for the anomalous warm climate in Europe. Changes in the formation of NADW have been linked to global climate changes in the past. Since human-made substances were introduced into the environment, the path of the NADW can be traced throughout its course by measuring tritium and radiocarbon from [[nuclear weapon test]]s in the 1960s and [[chlorofluorocarbon|CFCs]].<ref>{{Harvnb|Smethie|Fine|Putzka|Jones|2000|loc=Introduction, p. 14297}}</ref>

=== Gyres ===
{{oceanic gyres}}
The clockwise warm-water [[North Atlantic Gyre]] occupies the northern Atlantic, and the counter-clockwise warm-water [[South Atlantic Gyre]] appears in the southern Atlantic.<ref name="USN-2001" />

In the North Atlantic, surface circulation is dominated by three inter-connected currents: the [[Gulf Stream]] which flows north-east from the North American coast at [[Cape Hatteras]]; the [[North Atlantic Current]], a branch of the Gulf Stream which flows northward from the [[Grand Banks of Newfoundland|Grand Banks]]; and the [[Subpolar Front]], an extension of the North Atlantic Current, a wide, vaguely defined region separating the subtropical gyre from the subpolar gyre. This system of currents transports warm water into the North Atlantic, without which temperatures in the North Atlantic and Europe would plunge dramatically.<ref>{{Harvnb|Marchal|Waelbroeck|Colin de Verdière|2016|loc=Introduction, pp. 1545–1547}}</ref>

[[File:North Atlantic Circulation.gif|thumb|In the subpolar gyre of the North Atlantic warm subtropical waters are transformed into colder subpolar and polar waters. In the Labrador Sea this water flows back to the subtropical gyre.]]
North of the North Atlantic Gyre, the cyclonic [[North Atlantic Subpolar Gyre]] plays a key role in climate variability. It is governed by ocean currents from marginal seas and regional topography, rather than being steered by wind, both in the deep ocean and at sea level.<ref>{{Harvnb|Tréguier|Theetten|Chassignet|Penduff|2005|loc=Introduction, p. 757}}</ref>
The subpolar gyre forms an important part of the global [[thermohaline circulation]]. Its eastern portion includes [[Eddy (fluid dynamics)|eddying]] branches of the [[North Atlantic Current]] which transport warm, saline waters from the subtropics to the northeastern Atlantic. There this water is cooled during winter and forms return currents that merge along the eastern continental slope of Greenland where they form an intense (40–50&nbsp;[[Sverdrup|Sv]]) current which flows around the continental margins of the [[Labrador Sea]]. A third of this water becomes part of the deep portion of the [[North Atlantic Deep Water]] (NADW). The NADW, in turn, feeds the [[meridional overturning circulation]] (MOC), the northward heat transport of which is threatened by anthropogenic climate change. Large variations in the subpolar gyre on a decade-century scale, associated with the [[North Atlantic oscillation]], are especially pronounced in [[Labrador Sea Water]], the upper layers of the MOC.<ref>{{Harvnb|Böning|Scheinert|Dengg|Biastoch|2006|loc=Introduction, p. 1; Fig. 2, p. 2}}</ref>

The South Atlantic is dominated by the anti-cyclonic southern subtropical gyre. The [[South Atlantic Central Water]] originates in this gyre, while [[Antarctic Intermediate Water]] originates in the upper layers of the circumpolar region, near the [[Drake Passage]] and the Falkland Islands. Both these currents receive some contribution from the Indian Ocean. On the African east coast, the small cyclonic [[Angola Gyre]] lies embedded in the large subtropical gyre.<ref>{{Harvnb|Stramma|England|1999|loc=Abstract}}</ref>
The southern subtropical gyre is partly masked by a wind-induced [[Ekman layer]]. The residence time of the gyre is 4.4–8.5&nbsp;years. [[North Atlantic Deep Water]] flows southward below the [[thermocline]] of the subtropical gyre.<ref>{{Harvnb|Gordon|Bosley|1991|loc=Abstract}}</ref>

=== Sargasso Sea ===
{{Main|Sargasso Sea}}
{{Multiple image
 | image1 = Sargasso.png | caption1 = Approximate extent of the Sargasso Sea
 | image2 = HanaOZ.jpg | caption2 = Sargassum fish (''Histrio histrio'')
}}
The Sargasso Sea in the western North Atlantic can be defined as the area where two species of ''[[Sargassum]]'' (''S. fluitans'' and ''natans'') float, an area {{cvt|4000|km}} wide and encircled by the [[Gulf Stream]], [[North Atlantic Drift]], and [[North Equatorial Current]]. This population of seaweed probably originated from Tertiary ancestors on the European shores of the former [[Tethys Ocean]] and has, if so, maintained itself by [[Vegetative reproduction|vegetative growth]], floating in the ocean for millions of years.<ref name="Lün-p223">{{Harvnb|Lüning|1990|pp=223–225}}</ref>

Other species endemic to the Sargasso Sea include the [[sargassum fish]], a predator with algae-like appendages which hovers motionless among the ''Sargassum''. Fossils of similar fishes have been found in fossil bays of the former Tethys Ocean, in what is now the [[Carpathian Mountains|Carpathian]] region, that were similar to the Sargasso Sea. It is possible that the population in the Sargasso Sea migrated to the Atlantic as the Tethys closed at the end of the Miocene around 17&nbsp;Ma.<ref name="Lün-p223" /> The origin of the Sargasso fauna and flora remained enigmatic for centuries. The fossils found in the Carpathians in the mid-20th century often called the "quasi-Sargasso assemblage", finally showed that this assemblage originated in the [[Carpathian Basin]] from where it migrated over [[Sicily]] to the central Atlantic where it evolved into modern species of the Sargasso Sea.<ref>{{Harvnb|Jerzmańska|Kotlarczyk|1976|loc=Abstract; Biogeographic Significance of the "Quasi-Sargasso" Assemblage, pp. 303–304}}</ref>

The location of the spawning ground for [[European eel]]s [[Eel life history#Search for the spawning grounds|remained unknown for decades]]. In the early 19th century it was discovered that the southern Sargasso Sea is the spawning ground for both the [[European eel|European]] and [[American eel]] and that the former migrate more than {{cvt|5000|km}} and the latter {{cvt|2000|km}}. Ocean currents such as the Gulf Stream transport eel larvae from the Sargasso Sea to foraging areas in North America, Europe, and northern Africa.<ref>{{Harvnb|Als|Hansen|Maes|Castonguay|2011|p=1334}}</ref> Recent but disputed research suggests that eels possibly use [[Earth's magnetic field]] to navigate through the ocean both as larvae and as adults.<ref>{{Cite web|url=https://www.scientificamerican.com/article/do-baby-eels-use-magnetic-maps-to-hitch-a-ride-on-the-gulf-stream/|first1=Andrea|last1=Marks|title=Do Baby Eels Use Magnetic Maps to Hitch a Ride on the Gulf Stream?|date=17 April 2017|website=Scientific American|url-status=live|archive-url=https://web.archive.org/web/20170419192009/https://www.scientificamerican.com/article/do-baby-eels-use-magnetic-maps-to-hitch-a-ride-on-the-gulf-stream/?WT.mc_id=SA_EVO_20170417|archive-date=19 April 2017|access-date=18 April 2017}}</ref>