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Water mass can be defined as extensive homogeneous body of immense volume of ocean water in terms of temperature and salinity.
The characteristics of water masses can be seen as:
- Water masses are not confined to a single ocean. World oceans are open systems because they are inter-connected by water masses.
- Water masses move at a very slow pace.
- Water masses have definite temperature and salinity characteristics i.e. there is almost uniformity of temperature and salinity in a water mass
- Water mass is the result of downwelling of denser cold water and upwelling of less dense water. This process is called thermohaline circulation because density of seawater is the function of temperature and salinity.
- There is slight change in temperature and salinity of a water mass with time when there is mixing of seawater of adjoining water masses.
- Most of the water masses are cold water masses, which means the subsurface ocean water is cold. E.g., 75% of all ocean water is characterised by temperature ranging from 0-5 degree Celsius.
Role of water masses in circulation of water with different salinity and temperature gradients (thermohaline circulation) can be seen as:
- Water mass provide water with unique characteristics viz. salinity, temperature, which helps in thermohaline circulation. E.g., Arctic region is the source of cold-water mass for thermohaline circulation.
- Downwelling of denser water mass and upwelling of less dense water mass contribute towards thermohaline circulation.
- Well stratified water masses result in better thermohaline circulation.
- The movement of subsurface water mass through thermohaline circulation is closely linked with the circulation of surface water through conveyer belt circulation.
The thermohaline circulations are not as developed in the Pacific Ocean as they are in the Atlantic Ocean as:
- The mixing of the Arctic water mass with the water mass of the Pacific Ocean is not strong because the flow of cold deep-water mass and cold bottom water mass of the Arctic Ocean into the North Pacific Ocean is stopped by the shallow Bering strait.
- The Antarctic Deep Water (AADW) and the Antarctic Intermediate Water (AAIW) masses are not well developed in the extreme southern Pacific Ocean.
- Due to uniformity of temperature and salinity of ocean water below the depth of 2000 m in the Pacific Ocean, different layers of water masses in terms of varying combinations of temperature and salinity have not developed.
- Low salinity of surface water in the North Pacific Oceans does not encourage sinking of surface water.
Water masses, thus, play a significant role in the thermohaline circulations, however, the intensity of the same varies from ocean to ocean.
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