Ocean’s water is constantly circulated by currents. Surface currents are influenced by the wind. However, other, much slower and deeper currents that occur from surface to seafloor are driven by changes in the saltiness and ocean temperature, a process called thermohaline circulation. These currents are carried in a large “global conveyor belt.”
Global overturning circulation are the equatorward transport of cold, deep waters and the poleward transport of warm, near-surface water.
It is thought that tectonic changes might have led to the formation of two separate water bodies — northern component water in the North Atlantic and Antarctic Bottom Water (AABW) in the Southern Ocean.
Consequently, it is also hypothesized that there would have been large-scale changes in the Deep-Water Circulation (DWC) in the oceans across the world, thus impacting global climate through ocean-atmosphere carbon dioxide and heat exchanges.
Global Overturning Circulation (GOC) is responsible for the transport of carbon and heat among the ocean basins and between the ocean and the atmosphere. The GOC can be conceptualized primarily as two connected overturning cells (Talley, 2013).
- Upper cell is linked to formation of North Atlantic Deep Water (NADW) and its shallower return flow to form Atlantic Meridional Overturning Circulation (AMOC), roughly in the depth range 1500–3500 m.
- Lower cell is associated with production of Antarctic Bottom Water (AABW) which occupies the deeper parts of the abyssal ocean, and its return flow mostly as Pacific Deep Water (PDW) and is referred to as the Southern Ocean Meridional Overturning Circulation (SOMOC).
These two circulation cells are interconnected via upwelling in the Southern Ocean where deep waters are mixed in the circumpolar circulation.
Now, the Indian Ocean does not have any major deep-water formations of its own. It acts only as a host for NCW and AABW.
CONCEPT OF WATER MASS
Water mass, body of ocean water with a distinctive narrow range of temperature and salinity and a particular density resulting from these two parameters. Water masses are formed as the result of climatic effects in specific regions.
Antarctic bottom water is an important water mass that forms on the Antarctic continental shelf as a cold, dense residual brine during the formation of sea ice. Its salinity of 34.62 parts per thousand and temperature of -1.9° C (28.6° F) result in a high density of 1.02789 grams per cubic centimeters, causing it to sink and flow northward along the bottom into the southern oceans.
Mediterranean water is another example of a water mass. Excessive evaporation, low rainfall, and high temperatures continually generate large volumes of warm (11.9° C), salty (36.5 parts per thousand) water. Its density of 1.02778 causes it to sink to the bottom of the Mediterranean and overflow across the submarine sill at the Strait of Gibraltar, whence it sinks and spreads at a depth of about 1,000 meters (3,300 feet) in the Atlantic.
WATER MASS AND THERMOHALINE CIRCULATION
Basic thermohaline circulation is one of sinking of cold water in the polar regions, chiefly in the northern North Atlantic and near Antarctica.
These dense water masses spread into the full extent of the ocean and gradually upwell to feed a slow return flow to the sinking regions.
In the Northern Hemisphere the primary region of deep-water formation is the North Atlantic. A variety of water types contribute to the so-called North Atlantic Deep Water.
Each one of them differs, though they share a common attribute of being relatively warm (greater than 2 °C) and salty (greater than 34.9 parts per thousand) compared with the other major producer of deep and bottom water, the Southern Ocean (0 °C and 34.7 parts per thousand).
Three types of deep waters in North Atlantic:
- Formed in Greenland and Norwegian seas, where cooling of the salty water introduced by the Norwegian Current induces sinking. This water spills over the rim of the ridge that stretches from Greenland to Scotland, extending to the seafloor to the south.
- Formed in Labrador sea but it is somewhat less dense as compared to the above one.
- Formed in Mediterranean sea: Derived from net evaporation within the Mediterranean Sea. This draws surface water into the Mediterranean through the Strait of Gibraltar. The mass of salty water formed within the Mediterranean exits as a deeper stream. It descends to depths of approximately 1,000 meters in the North Atlantic Ocean, forming the uppermost layer of North Atlantic Deep Water.
MOTION AND MOVEMENT
- Deep-water spreads away from its source along the western side of the Atlantic Ocean and, on reaching the Antarctic Circumpolar Current, spreads into the Indian and Pacific oceans.
- Sinking of North Atlantic Deep Water is compensated for by the slow upwelling of deep water, mainly in the Southern Ocean, to replenish the upper stratum of water that has descended as North Atlantic Deep Water.
North Atlantic Deep Water exported to the other oceans must be balanced by the inflow of upper-layer water into the Atlantic. Some water returns as cold, low-salinity Pacific water through the Drake Passage in the form of what is known as Antarctic Intermediate Water, and some returns as warm salty thermocline water from the Indian Ocean around the southern rim of Africa.
Considerable volumes of cold water generally of low salinity are formed in the Southern Ocean. Such water masses spread into the interior of the global ocean and to a large extent are responsible for the anomalous cold, low-salinity state of the modern oceans. The circumstances leading to this role for the Southern Ocean are related to the existence of a deep-ocean circumpolar belt around Antarctica that was established some 25 million years ago by the shifting lithospheric plates which make up Earth’s surface. This belt establishes the Antarctic Circumpolar Current, which isolates Antarctica from the warm surface waters of the subtropics.
The primary site of Antarctic Bottom Water formation is within the continental margins of the Weddell Sea, though some is produced in other coastal regions, such as the Ross Sea. Also, there is evidence of deep convective overturning farther offshore. Antarctic Bottom Water, formed at a rate of 30 million cubic metres per second, slips below the Antarctic Circumpolar Current and spreads to regions well north of the Equator. Slowly upwelling and modified by mixing with less dense water, it returns to the Southern Ocean as deep water.
