The Global Conveyor Belt Winds drive ocean currents in the upper
100 meters of the ocean’s surface. However, ocean currents also
flow thousands of meters below the surface. These deep-ocean
currents are driven by differences in the water’s density, which
is controlled by temperature (thermo) and salinity (haline).
This process is known as thermohaline circulation. In the
Earth's polar regions ocean water gets very cold, forming sea
ice. As a consequence the surrounding seawater gets saltier,
because when sea ice forms, the salt is left behind. As the
seawater gets saltier, its density increases, and it starts to
sink. Surface water is pulled in to replace the sinking water,
which in turn eventually becomes cold and salty enough to sink.
This initiates the deep-ocean currents driving the global
conveyer belt.