One Sweet, One Salty

At the Skagerrak, the strait where the Baltic meets the North Sea, two bodies of water lie on top of each other like oil and water. The Baltic, fed by rivers and rain, is far less salty; the North Sea, open to the Atlantic, is far saltier and denser. The lighter water rides up over the heavier water in clearly defined layers, and the interface between them can persist for tens of kilometres. The German oceanographer Albert Defant described the layered structure in the 1920s, and the description has held for the past century: where the two seas meet, they do not so much mix as stack.

The mechanism is straightforward. Dissolved salt adds mass to water without changing its volume, so saltier water weighs more than fresher water at the same temperature. When two water masses of different salinity are brought into contact, the heavier sinks and the lighter rises until each finds its own depth. The boundary that forms is not a wall; it is a region across which density changes continuously but rapidly. The steeper the change, the more the two layers resist mixing. Without external energy — a storm, a tide, a strong wind — the layers will hold their positions.

Some of the most striking examples of the phenomenon occur at depth. At the Strait of Gibraltar, Mediterranean water spills over a sill and flows out into the Atlantic as a subsurface layer several hundred metres down, identifiable for years afterward by its temperature and salinity signature alone. The American oceanographer Henry Stommel described the exchange in the 1950s, and the Mediterranean layer has since been traced as it spread into the North Atlantic at depth. Where a river enters the sea, the same principle produces a wedge-shaped boundary that can extend kilometres offshore — freshwater flows seaward above, saltwater flows landward below, and the two meet at a sharp boundary that can be felt by anyone swimming across it.