It may be pointed out that maximum temperature of the oceans is always at their surface because it directly receives the insolation and the heat is transmitted to the lower sections of the oceans through the mechanism of conduction. In fact, the solar rays very effectively penetrate upto 20m depth and they seldom go beyond 200m depth.
Consequently, the temperature decreases from the ocean surface with increasing depth but the rate of decrease of temperature with increasing depth is not uniform everywhere. The temperature falls very rapidly upto the depth of 200m and thereafter the rate of decrease of temperature is slowed down.
From this stand point the oceans are vertically divided into two zones
(1) Photic or euphotic zone represents the upper surface up to the depth of 200m and receives solar radiation.
(2) Aphotic zone extends from 200m depth to the bottom and does not receive solar rays.
The following are the salient features of vertical distribution of temperature of ocean water:
- Though the sea temperature decreases with increasing depth but the rate of decrease of temperature is not uniform. The change in sea temperature below the depth of 2000m is negligible.
- Diurnal and annual ranges of temperature cease after the depth of 5 fathoms (30 feet) and 100 fathoms (600 feet) respectively.
- The rate of decrease of temperature with increasing depth from equator towards the poles is not uniform.
- Though the surface temperature of the seas decreases from equator towards the poles but the temperature at the ocean bottoms is uniform from the equator towards the pole, which means that the rate of decrease of temperature with increasing depth is more rapid near the equator than towards the poles.
- The areas from where sea surface water is driven away by offshore winds resulting into upwelling of water from below record low temperature at sea surface and thus the rate of decrease of temperature with increasing depth becomes low.
- Contrary to this the areas where there is pilling of sea water because of onshore winds, record relatively high temperature at sea surface and thus the rate of decrease of temperature with increasing depth becomes rapid.
- In some areas high temperature is recorded at greater depths e.g., in Sargasso Sea, Red Sea, Mediterranean Sea, Sulu Sea etc. The Mediterranean Sea records 24.4°C at the depth of 1,829m whereas the Indian Ocean has only 1.1°C temperature at the same depth. Such anomalous conditions are noticed in the enclosed seas of low latitudes. The enclosed seas of high latitudes register inversion of temperature i.e., the temperature of sea surface is lower than the temperature below.
- There is clear-cut layered thermal structure of ocean water.
Vertically the oceans are divided into 3 layers from the stand point of thermal conditions of seawater, in the lower and middle latitudes as follows:
(1) The upper layer represents the top-layer of warm water mass with a thickness of 500 meters with average temperature ranging between 20°C to 25°C. This lighter ocean water mass floats over the thickest heavy water mass of the oceans extending up to the ocean bottoms. This layer is present within the tropics throughout the year but it develops in middle latitudes only during summer season.
(2) The lower layer extends beyond 1000m depth up to the ocean bottoms. This layer is very cold and represents denser ocean water mass.
(3) The upper and lower ocean water masses are separated by a transitional zone of rapid change of temperature with increasing depth. This zone of ocean water mass is called thermocline which extends between 300m-1000m depth.
Besides, there are seasonal thermoclines between the depth of 40m and 100m.
These seasonal thermoclines are formed due to heating of water surface through solar radiation during summer season. There are also diurnal thermoclines which form in shallow water depth usually less than 10-15m. The polar seas have only one layer of cold water mass from the ocean surface (sea level) to the deep ocean floor.