In 1908, Johan Sandström laid the foundations of our modern understanding of ocean currents with a series of classic experiments carried out at Bornö oceanographic station in Sweden. He filled a small tank with water layers of different densities from an adjacent fjord and then blew air over the surface and/or heated and cooled the fluid at different levels. He thus elucidated the properties of 'wind-driven' and 'thermal' circulation. The latter term was amended by the 1920s to 'thermohaline circulation', because water density in the ocean is determined by both temperature and salinity.

Sandström found that thermal forcing can give rise to a steady circulation only if heating occurs at a greater depth than cooling — a fact that is familiar to oceanography students as 'Sandström's theorem'. But 'thermohaline forcing' — that is, fluxes of heat and freshwater — occurs only at the ocean's surface, except for a small contribution from geothermal heating.

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