阅读理解
If the salinity(盐分)of ocean waters is analyzed(分析), it is found to differ only slightly from place to place.However, some of these small changes are important.There are three basic processes(过程)that cause a change in oceanic salinity.One of these is the subtraction(减少)of water from the ocean by means of evaporation-conversion(转化)of liquid water to water vapour.In this manner, the salinity is increased, since the salts stay behind.If this is carried to the extreme(极度), of course, white crystals of salt would be left behind; this, by the way, is how much of the table salt we use is actually got.
The opposite of evaporation is precipitation(降水), such as rain, by which water is added to the ocean.Here the ocean is being diluted(冲淡)so that the salinity is decreased.This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean.Thus salinity may be increased by the subtraction of the water by evaporation, or decreased by the addition of fresh water by rainfall or runoff.
Normally, in hot areas where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation.Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be different is associated with(与……有关)the formation and melting of sea ice.When seawater is frozen, the dissolved(溶解的)materials are left behind.In this manner, seawater directly under freshly formed sea ice has a higher salinity than it did before the ice appeared.Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.
In the Weddell Sea of America, the densest(浓的)water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water.This heavy water sinks and is found in the deeper part of the oceans of the world.