6
V. Barale
The Atlantic Ocean occupies about 16.2 % of the planetary surface, representing
an area of approximately 82,400,000 km
2 , with an average depth of 3,926 m, and a
volume of 323,600,000 km
3 (figures that rise to 20.8 %, 106,400,000 km
2 , 3,338 m
and 354,700,000 km
3 when including the major adjacent seas). Its deepest point,
still uncertain but possibly just over 8,600 m below the surface, is located in the
Puerto Rico Trench. The basin extends in latitude from the Arctic to the Antarctic
(where the boundary with the Southern Ocean has been set at 60
◦ S), and varies
in longitudinal width from 2,848 km, between South America and Africa, to about
4,830 km, between North America and Africa. Surface salinity ranges between 33
(just north of the equator, because of heavy tropical rainfall, at high latitudes and
along coasts impacted by large rivers) and 37 psu (at about 25
◦ N and S, in subtropical
regions with low rainfall and high evaporation). Average values between 35 and
36 psu make it the saltiest of the world’s oceans. Surface water temperatures vary
from −2
◦ C, in polar regions, to about 29
◦ C, near the equator.
In a schematic representation of the Atlantic Ocean water column (the hydrographic structure of which actually varies considerably with latitude), based on
combined temperature and salinity characteristics, Upper Water at the surface is
followed by Central Water, formed by subduction at the tropics (surface to 500 m).
Below this, Antarctic Intermediate Water from the south, formed by deep convection east and west of the southern tip of the American continent and spreading into
all oceans with the Circumpolar Current; Arctic Intermediate Water from the north,
formed at high latitudes by either convection or subduction; Atlantic Subarctic Water
and Mediterranean Sea Water sinking from the Strait of Gibraltar, in the northeast
(500–1500 m). Still deeper in the water colum, North Atlantic Deep Water, the dominant water mass in the deep Atlantic Ocean, formed by complex processes involving
deep convection in the Greenland Sea, Labrador Sea and Arctic Ocean; and the underlying Antarctic Bottom Water, formed by deep convection in the Weddell Sea and
Ross Sea, occupying the deepest basins (below 3000–4000 m and to the bottom).
The general thermohaline circulation of the Atlantic Ocean, part of the global
Conveyor Belt system, is composed by a prevalent northward flow in surface layers,
and by a prevalent southward flow in deeper layers (primarily in the western basins).
At the surface, the wind-driven circulation pattern consists of two main basin-wide
gyres, circulating in an anticyclonic direction, i.e. clockwise in the North Atlantic and
counter-clockwise in the South Atlantic. In between, as the trade winds drive waters
of the inter-tropical region from east to west, lies what is known as the equatorial
current system (see Fig. 1.2 for a sketch of these systems’ African components). In
the northern gyre, the Canary Current, flowing southward off northwest Africa, feeds
the westward North Equatorial Current. South of it, the North Equatorial Countercurrent moves eastward towards the Gulf of Guinea, to form (with the contribution
of the Canary Current) the Guinea Current, which is followed, along the coast, by the
southward flowing Angola Current. In the southern gyre, the Benguela Current, flowing northward off southwest Africa, feeds the westward South Equatorial Current,
bounded to the south by the South Equatorial Countercurrent.
The Canary Current, named after the Canary Islands archipelago – which partially block its flow – is the eastern boundary current within the North Atlantic Gyre.
V. Barale
The Atlantic Ocean occupies about 16.2 % of the planetary surface, representing
an area of approximately 82,400,000 km
2 , with an average depth of 3,926 m, and a
volume of 323,600,000 km
3 (figures that rise to 20.8 %, 106,400,000 km
2 , 3,338 m
and 354,700,000 km
3 when including the major adjacent seas). Its deepest point,
still uncertain but possibly just over 8,600 m below the surface, is located in the
Puerto Rico Trench. The basin extends in latitude from the Arctic to the Antarctic
(where the boundary with the Southern Ocean has been set at 60
◦ S), and varies
in longitudinal width from 2,848 km, between South America and Africa, to about
4,830 km, between North America and Africa. Surface salinity ranges between 33
(just north of the equator, because of heavy tropical rainfall, at high latitudes and
along coasts impacted by large rivers) and 37 psu (at about 25
◦ N and S, in subtropical
regions with low rainfall and high evaporation). Average values between 35 and
36 psu make it the saltiest of the world’s oceans. Surface water temperatures vary
from −2
◦ C, in polar regions, to about 29
◦ C, near the equator.
In a schematic representation of the Atlantic Ocean water column (the hydrographic structure of which actually varies considerably with latitude), based on
combined temperature and salinity characteristics, Upper Water at the surface is
followed by Central Water, formed by subduction at the tropics (surface to 500 m).
Below this, Antarctic Intermediate Water from the south, formed by deep convection east and west of the southern tip of the American continent and spreading into
all oceans with the Circumpolar Current; Arctic Intermediate Water from the north,
formed at high latitudes by either convection or subduction; Atlantic Subarctic Water
and Mediterranean Sea Water sinking from the Strait of Gibraltar, in the northeast
(500–1500 m). Still deeper in the water colum, North Atlantic Deep Water, the dominant water mass in the deep Atlantic Ocean, formed by complex processes involving
deep convection in the Greenland Sea, Labrador Sea and Arctic Ocean; and the underlying Antarctic Bottom Water, formed by deep convection in the Weddell Sea and
Ross Sea, occupying the deepest basins (below 3000–4000 m and to the bottom).
The general thermohaline circulation of the Atlantic Ocean, part of the global
Conveyor Belt system, is composed by a prevalent northward flow in surface layers,
and by a prevalent southward flow in deeper layers (primarily in the western basins).
At the surface, the wind-driven circulation pattern consists of two main basin-wide
gyres, circulating in an anticyclonic direction, i.e. clockwise in the North Atlantic and
counter-clockwise in the South Atlantic. In between, as the trade winds drive waters
of the inter-tropical region from east to west, lies what is known as the equatorial
current system (see Fig. 1.2 for a sketch of these systems’ African components). In
the northern gyre, the Canary Current, flowing southward off northwest Africa, feeds
the westward North Equatorial Current. South of it, the North Equatorial Countercurrent moves eastward towards the Gulf of Guinea, to form (with the contribution
of the Canary Current) the Guinea Current, which is followed, along the coast, by the
southward flowing Angola Current. In the southern gyre, the Benguela Current, flowing northward off southwest Africa, feeds the westward South Equatorial Current,
bounded to the south by the South Equatorial Countercurrent.
The Canary Current, named after the Canary Islands archipelago – which partially block its flow – is the eastern boundary current within the North Atlantic Gyre.