Page "Mediterranean Sea" Paragraph 3

Evaporation greatly exceeds precipitation and river runoff in the Mediterranean, a fact that is central to the water circulation within the basin.

Evaporation is especially high in its eastern half, causing the water level to decrease and salinity to increase eastward.

This pressure gradient pushes relatively cool, low-salinity water from the Atlantic across the basin ; it warms and becomes saltier as it travels east, then sinks in the region of the Levant and circulates westward, to spill over the Strait of Gibraltar.

Thus, seawater flow is eastward in the Strait's surface waters, and westward below ; once in the Atlantic, this chemically distinct Mediterranean Intermediate Water can persist thousands of kilometres away from its source.