The Mediterranean Sea is measurably saltier than the global ocean average, a characteristic that shapes its ecosystem and internal dynamics. This elevated salt content results from a specific combination of geography, climate, and ocean physics. The Mediterranean functions as a large-scale, inverse estuary where the water balance is consistently negative, leading to salt concentration over time. High salinity fundamentally drives the sea’s circulation patterns and the composition of its deep-water masses.
Quantifying the Salinity
The average salinity of the world’s oceans hovers around 35 parts per thousand (ppt), the standard measure of salt content in seawater. In contrast, the Mediterranean Sea maintains a higher average salinity of approximately 38 ppt across the basin. This three-unit difference represents a significant physical change in the water’s properties.
Salinity levels vary geographically within the Mediterranean itself. The western basin, closer to the Atlantic Ocean, registers slightly lower surface salinity due to the continuous inflow of fresher Atlantic water. The salt concentration progressively increases eastward, reaching its maximum in the Levantine Basin (the far eastern end), where surface salinity can approach 40 ppt. This eastward increase establishes a gradient that influences the movement of water masses.
The Critical Role of the Strait of Gibraltar
The primary reason this saltiness persists, rather than being diluted by the Atlantic Ocean, is the geographical constraint imposed by the Strait of Gibraltar. This narrow, shallow channel is the only natural connection between the Mediterranean Sea and the Atlantic Ocean. The shallowest point in the strait, the Camarinal Sill, restricts the exchange of water masses between the two bodies.
This restriction creates a density-driven, two-layer flow known as inverse estuarine circulation. Less dense, fresher Atlantic water flows eastward into the Mediterranean as an upper layer, typically extending about 125 meters deep. Simultaneously, the highly saline, denser Mediterranean water flows westward beneath this layer and out into the Atlantic Ocean. This continuous, two-way exchange ensures that the salt concentrated within the Mediterranean is exported, preventing it from becoming saltier, while the restricted volume of the exchange prevents rapid dilution.
Evaporation and Freshwater Input
The two-layer exchange at Gibraltar compensates for the Mediterranean’s sustained freshwater deficit, which is driven by its climate. The region is characterized by hot, dry summers and experiences a high rate of surface evaporation. This atmospheric loss of freshwater is significantly greater than the water gained from precipitation and river runoff combined.
The high evaporation rate is intensified by strong, dry winds, such as the Mistral and Tramontane, which cool the surface and increase moisture loss, particularly in winter. This loss of water vapor leaves the salt behind, causing the concentration of dissolved solids to rise. Furthermore, the sea has few large rivers contributing freshwater. Flow from historically major rivers, like the Nile, has been reduced by damming and water management projects. This imbalance means that if the Strait of Gibraltar were closed, the sea level would drop by an estimated 0.5 to 1 meter per year.
How High Salinity Affects Circulation and Density
The concentration of salt directly increases the density of the seawater, the physical property that governs the sea’s internal movement. Saline water is heavier than fresher water, so the Mediterranean’s water masses are inherently denser than those in the Atlantic. This density difference explains why the two-layer flow at Gibraltar operates: the denser Mediterranean Outflow Water sinks beneath the lighter Atlantic inflow.
High salinity also plays a large part in deep water formation, which drives the sea’s internal thermohaline circulation. During winter, cold, dry winds cause intense cooling and evaporation in specific northern areas, such as the Gulf of Lions and the Adriatic Sea. This cooling, combined with the high salt content, increases the water’s density until it becomes heavy enough to sink rapidly. This sinking process replenishes the deep-water masses, moving oxygen and nutrients vertically and creating the distinct layers of water that characterize the Mediterranean Sea.