Australia: The Land Where Time Began

A biography of the Australian continent 

Atlantic Ocean

The Mid-Atlantic Ridge (MAR) is the spreading centre down the middle of the S-shapes Atlantic Ocean that dominates the topography of the ocean. A bit east of the Lesser Antilles in the eastern portion of the Caribbean, and east of the South Sandwich Islands, there are deep trenches. As the Atlantic Ocean is open at both its northern and southern extremities it connects the Arctic Ocean with the Southern Ocean. One of the 2 sources of the deep waters of the world oceans is located in the northern part of the North Atlantic. The Mediterranean Sea, one of the marginal seas of the Atlantic Ocean, adding highly saline, warm water to the mid-depth layers of the ocean as it is evaporative. The Weddell Sea at the southern margin is one of the main sites for the formation of bottom water of the oceans. There are other marginal seas of the Atlantic Ocean, The Norwegian Sea, Greenland Sea and Iceland Sea (aka collectively as the Nordic Seas), North Sea, Baltic Sea, Black Sea and the Caribbean Sea. The region southeast of Greenland is the Irminger Sea, between Labrador and Greenland the region is called the Labrador Sea, and the Sargasso Sea is the area of open ocean surrounding Bermuda. Marked tongues of low-salinity surface water are present where some rivers enter the Ocean, including the Amazon River, the Congo River and Orinoco River.

Transport in the upper Atlantic Ocean is dominated by gyres that are wind-driven and tropical circulation that is also wind-driven. Included among the gyres and their western boundary currents are:

• Anticyclonic gyres of the North Atlantic, the Gulf Stream and North Atlantic Current and the South Atlantic Brazil Current.
• The cyclonic subpolar gyre of the northern North Atlantic, East Greenland Current and the Labrador Current.
• The subtropical gyres include the eastern boundary current upwelling systems, the Canary Current System in the North Atlantic and Benguela Current System (BCS) in the South Atlantic.
• Tropical circulation, predominantly zonal (east-west), that includes the North Equatorial Countercurrent and the South Equatorial Current, as well as the North Brazil Current (NBC), a low-latitude western boundary current.

  In the northern part of the North Atlantic meridional overturning circulation (or thermohaline circulation) that converts upper ocean waters to deep waters and denser intermediate waters, is associated with deep circulation, that includes Deep Western Boundary Currents (DWBCs). The Labrador Sea and the Nordic Sea are sites where most of the final conversion of surface waters to deeper waters takes place. The upper circulation of the Atlantic is also affected by this conversion, increasing the northward transport in the Gulf Stream of the North Atlantic and North Atlantic Current by about 10 %, as well as providing a connection of tropical and subtropical waters to the subpolar region of the North Atlantic. The result of this overturning circulation is net northward heat transport through all latitudes in the North Atlantic, warm, saline surface waters being drawn to the north and sending dense, cold fresher water south at depth. This causes a reversal of the usual of subtropical heat transport towards the pole in the South Atlantic, the opposite of what is found in all other subtropical regions.

At the southern extremity of the South Atlantic the Southern Ocean connects the Atlantic to the other oceans. The Subantarctic Front (SAF) of the Antarctic Circumpolar Current (ACC) flows in an important excursion along the South American coast, where it enters the Atlantic from Drake Passage, as the Malvinas or Falkland Current, partially looping back to the south at the start of a long, slow drift, moving eastwards to the Indian Ocean and continuing on to the Pacific Ocean. Warm surface water from the Indian Ocean enters the South Atlantic where the Agulhas Current rounds the southern tip of Africa. Large eddies of Indian Ocean water move to the northwest into the Atlantic ocean as they are shed from the Agulhas Current and most of the Agulhas Current retroflects back to the Indian Ocean. The Benguela Current of the South Atlantic Ocean receives a small fraction of the Agulhas Current water.  From the Weddell Sea dense bottom water formed in Antarctica enters the Atlantic Ocean.

The Atlantic Ocean connects in the north to the Nordic Seas and the Arctic Ocean that are topographically separated from the North Atlantic Ocean by the ridge connecting Greenland to Iceland, than continues on to connect them to the Faroe Islands and the Shetland Islands. The Norwegian Atlantic Current that flows along the Norwegian coast is fed by water flowing from the Atlantic that flows northward to the Nordic Seas. In the East Greenland Current (EGC) water flows southwards into the Atlantic in the fresh surface layer, as well as through Davis Strait as dense subsurface overflows across all 3 channels in the Greenland-Shetland ridge. The dense deep water of the North Atlantic and the deep part of the Atlantic's branch of the global overturning circulation are formed from these overflows. Dense deep water is produced in the Antarctic that is associated with the other branch.

Sites that are important in the mass mixing and conversion of water are also present in the marginal seas of the North Atlantic. The subtropical western boundary current re-enters the North Atlantic after flowing through the Intra-American Seas, Caribbean Sea and the Gulf of Mexico. There is a series of almost separate sub-basins, each of which has a characteristic water mass formation and circulation. It also has its own marginal sea, the Black Sea. About 1/3 of the salinity difference that has been observed between the Atlantic and Pacific Oceans is contributed by Mediterranean Sea as a result of net evaporation in the Mediterranean. At the Strait of Gibraltar the dense Mediterranean re-enters the Atlantic. The Labrador Sea, that is suggested by the authors¹ to be more a large embayment than than a marginal sea, is the site of intermediate water formation, contributing to the meridional overturning circulation. To the north of the Labrador Sea, at Baffin Bay, the Atlantic connects to the Arctic Ocean to the west of Greenland, having an internal water mass formation process of its own. The shallow intra-European shelf seas such as the Nordic Seas and the Baltic Sea lie to the northeast.

In the Atlantic upper ocean water masses are similar to those present in the gyres that are driven by the wind of other oceans, including those with thermohaline ventilation - Central Water and Subtropical Underwater (STUW), as well as those that are associated with strong currents such as the Subtropical Mode Water (STMW). In the subpolar North Atlantic the North Atlantic Current has the Subpolar Mode Water (SPMW).

The North Atlantic Deep Water (NADW), new deep water for the global ocean is produced by the North Atlantic and its adjacent seas. The Labrador Sea, Mediterranean Sea and the Nordic Seas are the local convective sources. The deep water of the northern North Atlantic are relatively young, their age being measured in decades, as a result of the local deep water source, that contrasts with the age of the deep water in the North Pacific that have an age of hundreds of years. According to the authors they present the Atlantic Ocean first in their book¹ because its central position for the development of ideas about the circulation and the formation of water masses.

The authors suggest that in the Atlantic climatic variability is vigorous, a large portion of the quasi-decadal variability of the North Atlantic being associated with the North Atlantic Oscillation (NAO) which links to the Arctic Oscillation. Variability of the westerly winds, air-sea buoyancy fluxes, and ocean surface properties are effects of the NAO at the surface of the sea. The subtropical and subpolar circulations, and the rates of formation of water mass and properties are affects of the NAO. On centennial or millennial scales variation of long term Atlantic circulation, the Atlantic Multidecadal Oscillation (AMO) has been described. The authors¹ suggest this is important in the understanding of climate change that is possibly of anthropogenic origin in the Atlantic Ocean. In the Atlantic Ocean tropical climate modes have been observed that are intrinsic to the Atlantic, separate from the El Niño-Southern Oscillation  (ENSO) of the Pacific Ocean, the effects of which also intrude into the Atlantic sector. The Southern Annular Mode (SAM), the Weddell Sea and the southern sector of the South Atlantic Ocean are major action centres.

Archer²

The overturning circulation of the Atlantic Ocean makes the Atlantic different the other oceans in that none of the other oceans have circulation patterns that transports heat in the surface water to the north where it cools and sinks to depth, this Atlantic overturning circulation carries the warm water across the Equator, which doesn't happen in other oceans.

Sources & Further reading

Oceanography