The temperature of the ocean surface affects the salinity, or salt level, of ocean water. Near the equator, warm water evaporates into the atmosphere more quickly than it can be replaced by waters from below.
Since evaporation only occurs when the water is warmer than the air, this results in high salinity in tropical regions. On the other hand, regions closer to the poles are cooler, so less water evaporates into the atmosphere.
The cooled air also means that there is not much strong convection currents strong enough to bring large amounts of warm water to the surface.
Thus, polar regions are notoriously cool with low salinity levels closer to freezing. At high latitudes, sea ice forms when temperatures drop below freezing for an extended period.
Factors Affecting Temperature Distribution
(i) Latitude: the temperature of surface water decreases from the equator towards the poles because the amount of insolation decreases poleward.
(ii) 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.
(iii) Prevailing wind: the winds blowing from the land towards the oceans drive warm surface water away from the coast resulting in the upwelling of cold water from below. It results into the longitudinal variation in the temperature. Contrary to this, the onshore winds pile up warm water near the coast and this raises the temperature.
(iv) 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.
