- The study of the temperature of the oceans is important for determining the
- movement of large volumes of water (vertical and horizontal ocean currents),
- type and distribution of marine organisms at various depths of oceans,
- Climate of coastal lands, etc.
A) Source of Heat in Oceans
- The sun is the principal source of energy (Insolation).
- The ocean is also heated by the inner heat of the ocean as the ocean crust is only about 5 to 30 km thick.
The ocean water is heated by three processes :
- Absorption of sun’s radiation.
- The conventional currents: Since the temperature of the earth increases with increasing depth, the ocean water at great depths is heated than the subsurface and intermediate water layers.
- Also, the temperate are high along mid-ocean ridges because of volcanism.
- So, convectional oceanic circulations develop causing circulation of heat in water.
- Heat is produced due to friction caused by the surface wind and the tidal currents.
The ocean water is cooled by :
- Back radiation(heat budget) or long wave terrestrial radiation from the seawater.
- Exchange of heat between the sea and the atmosphere if there is temperature difference.
- Evaporation: Heat is lost in the form of latent heat of evaporation (atmosphere gains this heat in the form of latent heat of condensation).
B) The deep-water marine organisms survive in spite of absence of sunlight.
- Photic zone (the zone that receives sunlight) is only about few hundred meters and that depends on factors like turbidity, presence of algae etc.
- There are no enough primary producers below few hundred meters till the ocean bottom.
- At the sea bottom, there are bacteria that make use of heat supplied by earth’s interior to prepare food. So, they are the primary producers at the depths.
- Other organisms feed on these primary producers and subsequent secondary producers.
- So, the heat from earth supports wide-ranging deep-water marine organisms.
But the productivity is too low compared to ocean surface.
- The process of heating and cooling of the oceanic water is slower than land due to vertical and horizontal mixing and high specific heat of water that is why diurnal range of ocean temperatures is insignificant.
C) Factors Affecting Temperature Distribution of Oceans.
- Insolation: duration of insolation and its intensity.
- Heat loss: loss of energy by reflection, scattering, evaporation and radiation.
- Albedo: albedo of the sea (depending on the angle of sun rays).
- The physical characteristics of the sea surface: Boiling point of the sea water is increased in the case of higher salinity and vice versa.
- The presence of submarine ridges and sills: Temperature is affected due to lesser mixing of waters on the opposite sides of the ridges or sills.
- The shape of the ocean (enclosed seas): enclosed seas in the low latitudes record relatively higher temperature than the open seas (due to less mixing and higher overall insolation); whereas the enclosed seas in the high latitudes have lower temperature than the open seas. Mediterranean Sea records higher temperature than the longitudinally extensive Gulf of California.
- Local weather conditions such as cyclones.
- Unequal distribution of land and water: The oceans in the northern hemisphere receive more heat due to their contact with larger extent of land than the oceans in the southern hemisphere.
- Prevalent winds generate horizontal and sometimes vertical ocean currents: The winds blowing from the land towards the oceans (off-shore winds: moving away from the shore) drive warm surface water away from the coast resulting in the upwelling of cold water from below.
- Contrary to this, the onshore winds (winds flowing from oceans into continents) pile up warm water near the coast, and this raises the temperature.
- Ocean currents: Warm ocean currents raise the temperature in cold areas while the cold currents decrease the temperature in warm ocean areas.
- Gulf stream (warm current) raises the temperature near the eastern coast of North America and the West Coast of Europe while the Labrador current (cold current) lowers the temperature near the north-east coast of North America (Near Newfoundland).
D) Vertical Temperature Distribution of Oceans
- Photic or euphotic zone extends from the upper surface to ~200 m. The photic zone receives adequate solar insolation.
- Aphotic zone extends from 200 m to the ocean bottom; this zone does not receive adequate sunrays.
- The profile shows a boundary region between the surface waters of the ocean and the deeper layersWhich begins around 500 m below the sea surface and extends several hundred of meters downward.
- This boundary region, from where there is a rapid decrease of temperature, is called the thermocline.
About 90 per cent of the total volume of water is found below the thermocline in the deep ocean.
E) Horizontal Temperature Distribution
- The average temperature of surface water of the oceans is about 27°C, and it gradually decreases from the equator towards the poles.
- The rate of decrease of temperature with increasing latitude is generally 0.5°C per latitude.