Examine Figure 9.4 in your textbook. What do you think is the cause of the “dip” (i.e., decrease) in salinity around the equator? Could it not be argued that higher temperature causes more evaporation from the ocean surface, which, in turn, results in higher salinity content?
Figure 9.4 shows how the surface temperature of the ocean and the surface salinity change with latitudinal position. Salinity the measure of the concentration of dissolved salts in the ocean water. Salinity influences where plants and animals can thrive, and also affects aspects of water quality and treatment. The measure of salinity directly correlates with surface temperature. This means that at high temperatures, there is a large degree of evaporation surface. As a result, the ocean has high salinity. In addition to evaporation, however, many other factors may influence salinity. One such additional factor that affects ocean salinity is rainfall. With reference to the basic definition of salinity, a decrease in salinity results by either decreasing the amount of dissolved salts in the ocean, or increasing the volume of water in which the salts are dissolved. Natural influences such as rivers, volcanoes, and certain species of plants and animals help to maintain the relative concentration of seawater components at a constant level. This means that overall the concentration of dissolved sea salt to the ocean remains unchanged. Therefore, the most likely contributor to decrease salinity of the ocean at the equator is an increase water volume. At the equator, there is high rainfall and consequently, a large number of days during which the sky is cloudy. These two factors affect water volume and evaporation, to result in a decreased salinity.
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