The Mediterranean Sea is saltwater, meaning its water contains a high concentration of dissolved salts, primarily sodium chloride. It is classified as a marginal, semi-enclosed body of water connected to the Atlantic Ocean. This unique geographical configuration and its climatic setting create a distinct environment where the seawater maintains a consistently high salt content.
Salinity Levels Compared to the Open Ocean
The salt content of water is measured in practical salinity units (PSU), which is essentially the same as parts per thousand (ppt). The average salinity of the world’s open oceans typically falls around 34.7 to 35 PSU.
The Mediterranean Sea maintains a significantly higher average salinity than the open ocean. While the Atlantic water entering the Mediterranean starts at approximately 36.2 PSU, the overall mean salinity across the entire basin averages about 38.5 to 38.6 PSU. This measurement increases steadily from the western to the eastern end of the sea, reaching values of 39 PSU or more in the Levantine basin near Turkey and Egypt. This elevated salt content gives Mediterranean water a greater density compared to the Atlantic.
Factors Driving High Salinity
The primary reason for the Mediterranean Sea’s high salt concentration is a substantial water deficit caused by a high rate of evaporation. Located in a warm, arid region, the sea experiences intense solar heating and dry air, which causes a large volume of pure water to evaporate from the surface. When water vaporizes, the dissolved salts are left behind, which increases the salt concentration in the remaining body of water.
This loss of freshwater through evaporation is not sufficiently replenished by precipitation or river runoff. The total evaporation rate is estimated to be approximately three times greater than the total freshwater input, including rain and the flow from all rivers. This constant removal of water concentrates the salt already present in the basin. The semi-enclosed nature of the sea prevents this highly saline water from easily mixing and diluting with the vast volume of the open Atlantic Ocean.
The Role of the Strait of Gibraltar
The Mediterranean Sea’s connection to the Atlantic Ocean is restricted to the Strait of Gibraltar, a narrow channel that acts as a geographic bottleneck. This restriction allows the high salinity, created by evaporation, to be maintained within the basin. The strait facilitates a unique, two-layer flow circulation pattern known as anti-estuarine circulation.
The less dense, lower-salinity Atlantic water flows eastward into the Mediterranean at the surface to compensate for the water lost through evaporation. As this water travels across the Mediterranean, it evaporates and becomes progressively saltier, warmer, and therefore denser. Once this dense, highly saline water mass is formed, it sinks and flows westward, out of the Mediterranean, moving along the bottom of the Strait of Gibraltar. This continuous exchange limits the flushing of the basin, amplifying the concentration effect of evaporation and keeping the Mediterranean significantly saltier than the neighboring Atlantic Ocean.