Ocean water receives heat from the sun and its temperature rises. e temperature of the ocean water varies bot h vertically and horizontally. Vertically, the temperature of the ocean water decreases with depth. There is a boundary region between surface waters and the deeper layers. The boundary usually begins about 100’400 metres below and extends several hundred metres downward. This is known as thermocline. There is a rapid decrease in temperature below this level. As much as 90 per cent of the total volume of water is found below thermocline. At greater depths the temperature falls considerably but never reaches freezing point.
Horizontally, temperature of the ocean water decreases from equator towards the poles. In the tropical zone, the temperature of the ocean water is about 27’C. The average temperature is about 22’C at 20′ latitudes, 14’C at 40′ C latitudes and 0′ near the poles. The oceans of the northern hemisphere record relatively higher temperature than in the southern hemisphere. This is due to comparative vastness of the oceans in the southern hemisphere. The highest temperature is not recorded at the equator but a little north of it. The average temperature of all the oceans is 17.2’C. The average annual temperature for the northern hemisphere is 19.4’C and for the southern hemisphere is 16.1′ C.
Sea water is brackish to taste because it contains a number of dissolved salts. This gives the property of salinity to the sea water. Salinity is the term used to define the total content of dissolved salts in sea water. e salinity is calculated as the number of grams of dissolved salts in 1,000 grams of sea water. It is usually expressed as parts per thousand (‘) or ppt. The average salinity of the sea water is about 35 per thousand. It means that in one kilogram of sea water there are 35 grams of dissolved salts.
Highest salinity is found near the Tropic of Cancer and the Tropic of Capricorn. is is due to active evaporation owing to clear skies, high temperature and steady trade winds. Rivers are few and bring little fresh water to the oceans. Salinity is about 37 per thousand in these areas. From the tropics, salinity decreases both towards the equator and the poles.
The average salinity of the Indian Ocean is 35%. The low salinity trend is observed in the Bay of Bengal due to inux of river water by the river Ganga. On the contrary, the Arabian Sea shows higher salinity due to high evaporation and low inux of fresh water.
Differences in salinity are well-marked in partially or wholly enclosed seas as is clear from the following description:
Enclosed seas and lakes have exceptionally high proportion of salinity. This is due to the regular supply of salt by the rivers owing into them. Evaporation makes their water progressively more saline. The salinity, for example, of the Great Salt Lake (Utah in the United States), the Dead Sea and the lake Van in Turkey is 220, 240 and 330 per thousand respectively. Caspian Sea is also enclosed but it gets sut supply of fresh water from Volga and Ural rivers. So, the salinity here is only 14 to 17 per thousand.
The ocean water is never static but keeps on moving horizontally as well as vertically. The movement of the ocean water takes place in the following three ways:
The horizontal motion of the ocean water refers to ocean waves and currents while the vertical motion of ocean water refers to tides.