The temperature of the surface water of the oceans varies in much the same way as that of the land surface.
This is because insolation is responsible for the varying quantities of heat which are received at different latitudes and in different seasons.
Generally, the temperature is higher near the equator and gradually decreases towards the poles. .
The mean annual temperatures of about 27°C or higher, are common in tropical seas but there is a general decrease towards the poles where the mean temperatures of around 1.8°C are found.
However, the decrease of temperature of surface water towards the poles or increase towards the equator is not uniform because drifting warm water from the tropical seas may move into higher latitudes or vice versa and gives a local increase or decrease of temperature.
Upwellings of deep, cold water also reduce locally the surface temperature of tropical and subtropical sea waters.
The high temperatures of waters are found in enclosed seas in the tropics e.g. the Red Sea.
The Arctic and Antarctic waters are so cold that their surface remains permanently frozen down to a depth of several metres.
In the summer months, parts of the ice break off as icebergs which dilute the water and lower the surface temperature of surrounding ice free seas.
There is also variation in the vertical distribution of temperature.
Temperature decreases with increase in depth.
This is because the surface of the sea water receives the largest amount of insolation.
As the rays penetrate the water, their intensity is reduced by scattering, reflection and diffusion.
However, the rate of decrease in the temperature is not equal at all depth.
Upto a depth of about 100 metres, the temperature of water is about the same as that of the surface, while it falls from 15°C to about 2°C between the surface and a depth of 1,800 metres.
The decrease between 1,800 and 4,000 metres is from 2°C to about 1.6°C.
The main process of heating the ocean waters are
(1) by absorption of heat from the sun
(2) by convection of heat through the ocean bottom from the interior of the earth
The cooling processes are:
(1) by loss of heat to the atmosphere
(2) by evaporation