¶ … 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?

This theorem can help:

Higher salinity = higher density

Higher temperature = lower density

Low salinity = lower density

Lower temperature = higher density

Salinity is the highest around 20 and 30 degrees. Many of the earth's deserts are located at these temperatures and evaporation of water in these deserts is high leading to high rates of salinity.

One would think that the equator too would have high levels of salinity since the dip of the equator is due to the large amounts of rainfall that happen there.

However, part of the reason for its reduction in salinity is that the dip is larger and more spread out than, let's say the smaller mass of water in the Atlantic Ocean that has the highest level of salinity. Part of the reason for this is its concentrated, compact size where it is smaller than the Equatorial dip so the evaporation process concentrates the salt. Another possible reason for higher salinity in the Atlantic Ocean is that the Atlantic Ocean has a midoceanic ridge traversing it. This adds volcanic gasses (since it is a volcanic feature) therefore salt to the water.

Density of water increases with temperature. So at the equator, we have high temperature and low density water (as see theorem). (At the poles, on the other hand, as temperature lowers, density increases). As the theorem shows, low density represents lower salinity since density is controlled both by temperature of water and by the amount of salts that have evaporated in the pool.

In short, for both these two reasons - spread-out mass of dip that leads to less evaporation and less density -- is there less salinity in Equator.

Source

GS106. Lab Key #3

http://www.wou.edu/las/physci/taylor/gs106/lab3key.pdf