Effects of temperature and evaporation on equatorial ocean salinity

¶ … Dip in Salinity at the Equator

The objective of this study is to examine the possible cause of the dip in salinity around the equator and to answer the question of whether it could be argued that higher temperature causes more evaporation from the ocean surface, which, in turn results in higher salinity content.

Salinity is defined as "the total amount of solid material dissolved in water" and it is reported as well that salinity is generally expressed in terms of percentage. The substances dissolved in seawater are small numbers and are expressed "in parts per thousand." (Lutgen and Tarbuck, 2011) The majority of the salt in seawater is reported as "sodium chloride" or table salt. (Lutgen and Tarbuck, 2011) Seawater salinity is stated at 35%. (Lutgen and Tarbuck, 2011) Sources of salts in the sea include rocks when they are chemically worn and the interior of the Earth occurring in volcanic eruptions as well as a process called outgassing. (Lutgen and Tarbuck, 2011)

The composition of seawater is reported as having remained "relatively stable for millions of years." (Lutgen and Tarbuck, 2011) The ocean contains three layers including the shallow surface mixed zone, the transition zone and the deep zone. When a layer undergoes temperature change rapidly below the zone or mixing it is referred to as the thermocline. (Lutgen and Tarbuck, 2011, paraphrased) Both the temperature and the salinity of the water change as the ocean depths change. The salinity is varied according to the depth and when a zone of salinity is changing rapidly, it is referred to as the halocline.

It is reported that the density of seawater is affected by two primary factors: (1) salinity; and (2) temperature. (Lutgen and Tarbuck, 2011) Salinity increases dissolved substances and increases in the density of the seawater. However, when the temperature increases the wate3r expands and seawater density is reduced. The relationship of one decreasing variable due to another increasing variable is termed an "inverse relationship" and is such that one variable "is inversely proportional with the other." (Lutgen and Tarbuck, 2011)

Temperature is the greatest influencer of the density of seawater due to the variations in the surface seawater temperature being larger than the variations in salinity. Therefore, it can be understood that as the seawater grows warmer and expands that the salinity of the seawater is reduced and when the seawater becomes colder and contracts, the salinity percentage of the seawater is increased and the seawater becomes denser.

In regards to the question of whether warmer seawater results in evaporation that results in a decrease in salinity, this does not appear plausible as it is acknowledged in this study that there is very little in way of variations in the salinity of seawater and that seawater salinity is most affected by variations in the temperature of the seawater. The evaporation process would not remove from the earth's oceans near enough water to actually increase the salinity of the waters of the oceans.