Sea Science: by Jordanna Sheermohamed
The origin of the word salt comes from salarium, Latin for salary, which refers to the amount of compensation a Roman soldier would receive in order to purchase the life-essential mineral. While we use salt to flavor foods, produce paper and keep livestock healthy, it also affects the density of seawater – and more importantly, the ocean currents.
The salinity of Earth’s water systems, or the amount of salt dissolved within a given volume of water, can be classified into four levels from least to greatest: fresh, brackish, saline and briny. Fresh water contains less than 0.5 percent salt, whereas seawater averages about 3.5 percent salt.
So how does this salt affect what happens in our oceans, and what happens when the salt content changes from higher to lower values, or vice versa? While surface ocean currents are mainly wind-driven, deeper water currents are a function of the water’s density, which is dependent on both temperature differences and salinity. Just as colder air is denser than warmer air, colder water is denser than warmer water, so it sinks.
When salt is dissolved in water, it increases the mass of the water, making it denser than it was before. The human body can essentially float in any ocean, but the higher the salt content, the easier it is to float. This basic concept can be experienced in the Dead Sea, where the salt content of the water is about 10 times higher than usual and the human body is able to float with no effort.
Saltier water will sink below fresh water, and this contributes to the global circulation of oceans. But just as the amount of salt in a diet can be altered, the amount of salt in a body of water can be affected by many external factors. The more fresh water added via a hydrological transport mechanism, such as rain or ice melt, the lower the salt content of the ocean because of dilution.
Conversely, the more fresh water removed via mechanisms such as freezing or lack of rain, the saltier the body of water. Both of these scenarios will alter the density of the seawater, and therefore can have a localized effect on both the strength and location of currents.
Modern methods of testing ocean salinity involve analyzing the electrical conductivity, the temperature and the depth of a collected sample, or the use of satellite remote sensing technology, which takes a weekly measurement of a given spot.
By knowing the temperature of the water through ground observations or additional satellite technology, salinity can be determined by the reflected brightness from the ocean surface.
A new study from the Universiti Malaysia Terengganu published in the “Pertanika Journal of Science & Technology” has proposed using the daily images of ocean colors from alternative satellites, as ocean color is dependent on the amount of organic matter dissolved within the water, which was found to have an inverse relationship with salinity.
While the results of the experiments were limited to the local region of Malaysia, the study indicated that further testing in alternative regions could certainly aid in solidifying a widespread accepted algorithm.
The ability to compute this information on a daily basis, rather than using the traditional once-a-week option, would allow for a more comprehensive understanding of global ocean circulations, which would ultimately help to better understand the earth’s dependence on ocean currents in an evolving climate.
Jordanna Sheermohamed is president and lead meteorologist of Weather Forecast Solutions, a weather-forecasting firm (WeatherForecastSolutions.com). Comments are welcome below.