Editing Norwegian Sea (section)

== Hydrology ==
[[File:Tides in Norway.png|thumb|300px|Tide ranges and tide times ([[North Sea#Selected tide ranges around North Sea|hours after Bergen]]) along the Norwegian coast]]
[[File:Thermohaline Circulation 2.png|thumb|left|Thermohaline circulation explains the formation of cold, dense deep water in the Norwegian Sea. The entire circulation pattern takes ~2000 years to complete.]]
[[File:Gulfstream.jpg|thumb|left|Surface currents in the North Atlantic]]
Four major water masses originating in the Atlantic and Arctic oceans meet in the Norwegian Sea, and the associated currents are of fundamental importance for the global climate. The warm, salty [[North Atlantic Current]] flows in from the Atlantic Ocean, and the colder and less saline [[Norwegian Current]] originates in the North Sea. The so-called East Iceland Current transports cold water south from the Norwegian Sea toward Iceland and then east, along the [[Arctic Circle]]; this current occurs in the middle water layer. Deep water flows into the Norwegian Sea from the Greenland Sea.<ref name=b366/> The tides in the sea are semi-diurnal; that is, they rise twice a day, to a height of about 3.3 metres.<ref name=bse/>

=== Surface currents ===
The hydrology of the upper water layers is largely determined by the flow from the North Atlantic. It reaches a speed of 10 [[Sverdrup|Sv]] (1 Sv = million m<sup>3</sup>/s) and its maximum depth is 700 metres at the Lofoten Islands, but normally it is within 500 metres.<ref name=b366/> Part of it comes through the Faroe-Shetland Channel and has a comparatively high [[salinity]] of 35.3‰ (parts per thousand). This current originates in the North Atlantic Current and passes along the European continental slope; increased evaporation due to the warm European climate results in the elevated salinity. Another part passes through the Greenland-Scotland trench between the Faroe Islands and [[Iceland]]; this water has a mean salinity between 35 and 35.2‰.<ref name="Aken119">[[#refAken|Aken, 2007]], pp. 119–124</ref> The flow shows strong seasonal variations and can be twice as high in winter as in summer.<ref name="Driving"/> While at the Faroe-Shetland Channel it has a temperature of about 9.5&nbsp;°C; it cools to about 5&nbsp;°C at Svalbard and releases this energy (about 250 terawatts) to the environment.<ref name="Driving44">Roald Sætre ''Driving forces'' in: [[#refSætre|Sætre, 2007]], pp. 44–58</ref><ref name=b366/>

The current flowing from the [[North Sea]] originates in the Baltic Sea and thus collects most of the drainage from northern Europe; this contribution is however relatively small.<ref name="Driving">[[#refSætre|Sætre, 2007]], pp. 44–58</ref> The temperature and salinity of this current show strong seasonal and annual fluctuations. Long-term measurements within the top 50 metres near the coast show a maximum temperature of 11.2&nbsp;°C at the [[63rd parallel north|63° N parallel]] in September and a minimum of 3.9&nbsp;°C at the North Cape in March. The salinity varies between 34.3 and 34.6‰ and is lowest in spring owing to the inflow of melted snow from rivers.<ref name=b366/> The largest rivers discharging into the sea are [[Namsen]], [[Ranelva]] and [[Vefsna]]. They are all relatively short, but have a high discharge rate owing to their steep mountainous nature.<ref name="Sepp"/>

A portion of the warm surface water flows directly, within the [[West Spitsbergen Current]], from the Atlantic Ocean, off the Greenland Sea, to the Arctic Ocean. This current has a speed of 3–5 Sv and has a large impact on the climate.<ref name="Tyler45">[[#refTyler|Tyler, 2003]], pp. 45–49</ref> Other surface water (~1 Sv) flows along the Norwegian coast in the direction of the [[Barents Sea]]. This water may cool enough in the Norwegian Sea to submerge into the deeper layers; there it displaces water that flows back into the North Atlantic.<ref name="Tyler">[[#refTyler|Tyler, 2003]], pp. 115–116</ref>

Arctic water from the East Iceland Current is mostly found in the southwestern part of the sea, near Greenland. Its properties also show significant annual fluctuations, with long-term average temperature being below 3&nbsp;°C and salinity between 34.7 and 34.9‰.<ref name=b366/> The fraction of this water on the sea surface depends on the strength of the current, which in turn depends on the pressure difference between the [[Icelandic Low]] and [[Azores High]]: the larger the difference, the stronger the current.<ref name="ICES2">[[#refICES|ICES, 2007]], pp. 2–4</ref>

=== Deep-sea currents ===
The Norwegian Sea is connected with the Greenland Sea and the Arctic Ocean by the 2,600-metre deep [[Fram Strait]].<ref name="Tyler240">[[#refTyler|Tyler, 2003]], pp. 240–260</ref> The Norwegian Sea Deep Water (NSDW) occurs at depths exceeding 2,000 metres; this homogeneous layer with a salinity of 34.91‰ experiences little exchange with the adjacent seas. Its temperature is below 0&nbsp;°C and drops to −1&nbsp;°C at the ocean floor.<ref name=b366/> Compared with the deep waters of the surrounding seas, NSDW has more nutrients but less oxygen and is relatively old.<ref name="Aken">[[#refAken|Aken, 2007]], pp. 131–138</ref>

The weak deep-water exchange with the Atlantic Ocean is due to the small depth of the relatively flat [[Greenland-Scotland Ridge]] between Scotland and Greenland, an offshoot of the [[Mid-Atlantic Ridge]]. Only four areas of the Greenland-Scotland Ridge are deeper than 500 metres: the [[Faroe-Bank Channel overflow#Faroe-Bank Channel|Faroe-Bank Channel]] (about 850 metres), some parts of the [[Iceland-Faroe Ridge]] (about 600 metres), the [[Wyville-Thomson Ridge]] (620 metres), and areas between [[Greenland]] and the [[Denmark Strait]] (850 metres) – this is much shallower than the Norwegian Sea.<ref name="Tyler"/><ref name="Aken"/> Cold deep water flows into the Atlantic through various channels: about 1.9 Sv through the Faroe Bank channel, 1.1 Sv through the Iceland-Faroe channel, and 0.1 Sv via the Wyville-Thomson Ridge.<ref name="Skreslet93">[[#refSkreslet|Skreslet & NATO, 2005]], p. 93</ref> The turbulence that occurs when the deep water falls behind the Greenland-Scotland Ridge into the deep Atlantic basin mixes the adjacent water layers and forms the [[North Atlantic Deep Water]], one of two major deep-sea currents providing the deep ocean with oxygen.<ref name="Threat">Ronald E. Hester, Roy M. Harrison ''Biodiversity Under Threat'', Royal Society of Chemistry, 2007 {{ISBN|0-85404-251-2}}, p. 96</ref>