Near the tropics of cancer and capricorn temperature is very high. Here the sky is usually clear and the oceans are exposed to the onshore trade winds. As a result of these factors, evaporation is very high near the tropics. This causes concentration of salts.

For this reason, areas of the highest salinity are seen near the tropics (about 38 gm per kg). But the salinity near the equatorial region is relatively low because of higher rainfall.

Apart from higher rainfall in this belt, many great rivers carrying fresh water enter the ocean. So, higher rainfall, higher relative humidity, and supply of fresh water do not allow active evaporation. As such, salinity is relatively low here.

Average salinity of this region is about 35 per cent. Away from the tropics, the salinity decreases poleward because of low evaporation and low temperature.

In addition to this, the melting of ice yielding fresh water leads to decrease in salinity in the polar region. Average salinity near the poles is 34 per cent.

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