Salinity

Two attributes of the oceans—temperature and salinity—determine the density of seawater, and the differences in density between the water masses in the world’s oceans causes the water to flow in thermohaline circulation, thereby producing the greatest oceanic current on the planet. Salinity is the distinct taste of seawater and is the result of the presence of dissolved salts (more than 85 dissolved constituents), of which chloride (Cl) and sodium (Na), the elements of common table salt, are the most abundant. The term salinity refers to the content of these dissolved salts, and has been defined as grams of dissolved salts per kg of seawater. Salinity has been expressed as parts per thousand (‰ or ppt), and more recently, by practical salinity units (psu). On average, 1 kg of seawater has 35 grams of dissolved salts, so its salinity content is 35‰, or 35 psu. The accuracy of most laboratory salinometers is about 0.001 psu. Thus, only those components with a concentration over 0.001‰ will contribute to such salinity estimates. Only 15 of the dissolved salts have concentrations above that limit.

A key observational result (known as the principle of Dittmar, after William Dittmar, a Scottish professor of chemistry; the principle of Maury, after Matthew Fontaine Maury, an American astronomer, oceanographer, and geologist; or the hypothesis of Forchhammer, after Johan Georg Forchhammer, a Danish mineralogist and geologist) is that the relative concentration between some of these most abundant salts is virtually constant over much of the world ocean. This finding indicates that the physical characterization of seawater is given by its temperature, pressure, and a single number reflecting the concentration of the most abundant components. Salinity is that number.

Salinity and Climate

The range of temperatures on Earth allows water to be present as a solid (ice) in ice caps and glaciers;

liquid (water) in oceans, groundwater, lakes, and rivers; and gas (water vapor) in the atmosphere. The idealized path of a water molecule from one phase to the other is known as the hydrological cycle. The residence times, that is, the average time that the molecules spend in each phase, range from a few days (water vapor in the atmosphere), to several months (seasonal snow cover, rivers), to the thousands of years (oceans and groundwater). 

Changes in the hydrological cycle affecting precipitation, evaporation, ice cap thawing, and river runoff have the potential to change the salinity of the oceans. The reverse is also true, and salinity changes may have an imprint in the hydrological cycle after thousands of years.

The mechanisms by which salinity affects the hydrological cycle are numerous. Because of its role in density variations, salinity gradients contribute to ocean currents transporting heat, salt, microorganisms, and nutrients across the oceans. In regions of strong precipitation, a layer of low salinity may isolate the uppermost surface of the ocean from the cold ocean below, forming the barrier layer, which blocks the wind-stirring effects that cool the surface by mixing heat downward.

This manifests as warmer sea surface temperatures, modifying the surface temperature gradients that drive surface winds. In the equatorial Pacific Ocean, such a phenomenon is of importance in the El Niño–La Niña cycles. Similar salinity effects also occur in the tropical Indian and Atlantic oceans and have potential feedbacks to the hydrological cycles in the region. Tropical surface anomalies may be advected to the deep convection regions, modulating the thermohaline circulation.

One of the largest ocean climate events recorded in the Atlantic Ocean is the Great Salinity Anomaly, which lasted from 1968 to 1982. A salinity anomaly propagated over thousands of kilometers reached the Labrador Sea and perturbed the thermohaline circulation intensity. The origin and evolution of these anomalies is still not fully understood because of the historical lack of salinity observations, and studies of the mechanisms by which these salinity anomalies evolve are usually based on ocean and climate models.