Water characteristics

Surface water temperatures, which vary with latitude, current systems, and season and reflect the latitudinal distribution of solar energy, range from below −2 °C (28 °F) to over 30 °C (86 °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 7–8 °C (13–14 °F).^[5]
From October to June the surface is usually covered with sea ice in the Labrador Sea, Denmark Strait, and Baltic Sea.^[5]
The Coriolis effect circulates North Atlantic water in a clockwise direction, whereas South Atlantic water circulates counter-clockwise. The south tides in the Atlantic Ocean are semi-diurnal; that is, two high tides occur during each 24 lunar hours. In latitudes above 40° North some east-west oscillation, known as the North Atlantic Oscillation, occurs.^[5]

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 25° north and south, in subtropical regions with low rainfall and high evaporation.^[5]
The high surface salinity in the Atlantic, on which the Atlantic thermohaline circulation is dependent, is maintained by two processes: the 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.^[28]

Water masses

Temperature-salinity characteristics for Atlantic water masses^[29]

Water mass	Temperature	Salinity
Upper waters (0–500 m)
Atlantic Subarctic Upper Water (ASUW)	0.0–4.0 °C	34.0–35.0
Western North Atlantic Central Water(WNACW)	7.0–20 °C	35.0–36.7
Eastern North Atlantic Central Water (ENACW)	8.0–18.0 °C	35.2–36.7
South Atlantic Central Water (SACW)	5.0–18.0 °C	34.3–35.8
Intermediate waters (500–1500 m)
Western Atlantic Subarctic Intermediate Water (WASIW)	3.0–9.0 °C	34.0–35.1
Eastern Atlantic Subarctic Intermediate Water (EASIW)	3.0–9.0 °C	34.4–35.3
Mediterranean Water (MW)	2.6–11.0 °C	35.0–36.2
Arctic Intermediate Water (AIW)	-1.5–3.0 °C	34.7–34.9
Deep and abyssal waters (1500 m–bottom)
North Atlantic Deep Water (NADW)	1.5–4.0 °C	34.8–35.0
Antarctic Bottom Water (AABW)	-0.9–1.7 °C	34.64–34.72
Arctic Bottom Water (ABW)	-1.8–-0.5 °C	34.85–34.94

The Atlantic Ocean consists of four major, upper water masses with distinct temperature and salinity. The Atlantic Subarctic Upper Water in the northern-most 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 15°N.^[30]
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 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 the large number of contributions from a wide range of sources: Labrador Sea, Norwegian-Greenland Sea, Mediterranean, and South Atlantic Intermediate Water.^[30]
The North Atlantic Deep Water (NADW) is a complex of four water masses, two that form by deep convection in the open ocean — Classical and Upper Labrador Sea Water — and two that form from the inflow of dense water across the Greenland-Iceland-Scotland sill — 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 and Mediterranean Overflow Water.^[31] 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 man-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 tests in the 1960s and CFCs.^[32]