The Mediterranean Sea has a notably higher salt content than the average global ocean. Salinity refers to the amount of dissolved salts in water. These dissolved salts are primarily sodium chloride, common table salt, but also include elements like magnesium, calcium, and potassium. The concentration of these dissolved substances plays a significant role in the water’s physical characteristics, such as its density and heat capacity. Understanding its elevated salinity explains various oceanographic processes.
Measuring Mediterranean Salinity
Salinity is typically measured using Practical Salinity Units (PSU), a scale based on the electrical conductivity of seawater. One PSU is roughly equivalent to one gram of salt per thousand grams of water. While the average salinity of the world’s oceans is approximately 35 PSU, the Mediterranean Sea consistently exhibits higher values. Surface waters generally range from 38 to 39 PSU.
Salinity levels vary across the Mediterranean basin. The western parts, closer to the Atlantic Ocean, tend to have slightly lower surface salinities, around 36.2 PSU near Gibraltar. As water moves eastward, salinity progressively increases, reaching up to 40 PSU in the eastern Mediterranean, particularly during summer. Deeper waters also maintain high salinity, often around 38.4 PSU or slightly less. Variations occur near major river inputs, where freshwater inflow can locally reduce salinity. Over recent decades, the Mediterranean Sea has shown a trend of increasing salinity, with a more pronounced rise in the eastern basin.
What Makes the Mediterranean So Salty?
Several factors contribute to the Mediterranean Sea’s elevated salinity. The primary reason is the region’s warm, dry climate, which leads to a high rate of evaporation from the sea surface. This intense evaporation removes freshwater, increasing the concentration of dissolved salts. The Mediterranean Sea loses approximately one meter of water height each year due to evaporation.
Limited freshwater input from rivers, relative to the sea’s size and high evaporation, is another factor. Although major rivers like the Rhône and Po discharge into the Mediterranean, their collective freshwater contribution is insufficient to significantly offset the evaporative losses. For instance, the Nile River’s discharge, once substantial, has been significantly reduced by the Aswan Dam. This imbalance between water loss through evaporation and replenishment by freshwater sources results in a net accumulation of salt.
The Mediterranean Sea’s restricted exchange with the less saline Atlantic Ocean also plays a role. The connection occurs through the narrow and shallow Strait of Gibraltar. While Atlantic water flows into the Mediterranean at the surface, a counter-current of denser, saltier Mediterranean water flows out at deeper levels. This two-way exchange, driven by the density difference between the two water bodies, allows salt to concentrate within the Mediterranean basin over time. The inflowing Atlantic water, which is less salty, becomes progressively saltier as it moves eastward within the Mediterranean due to ongoing evaporation.