The Global Conveyor Belt: How Cold, Salty Water Drives the Ocean's Deepest Current

Below the wind-driven currents of the ocean surface runs a slower, planet-wide system of currents known as the global conveyor belt. It is driven by thermohaline circulation: differences in the water's temperature (thermo) and saltiness (haline), which change how dense it is.

The conveyor belt. Blue arrows trace deep, cold, dense currents; red arrows trace warmer, lighter surface water. Image from NOAA.

Where it starts

The belt begins at the surface of the North Atlantic, near the pole. There the water is chilled by Arctic temperatures, and it also gets saltier: when sea ice forms, the salt does not freeze and is left behind in the surrounding water. Colder and saltier means denser, so this water sinks toward the ocean floor. Surface water flows in to take its place, and that flow is the start of a current.

Cold, salty, dense water sinks near the North Pole and heads south along the western Atlantic. Image from NOAA.

Around the world

The deep water flows south between the continents, across the equator and down past the tips of Africa and South America. It then travels round the edge of Antarctica, where the water is chilled and sinks again, just as in the North Atlantic, so the belt is "recharged".

Along Antarctica the current picks up more cold, dense water. Image from NOAA.

Two branches split off there and turn north, one into the Indian Ocean and the other into the Pacific.

The current divides. Image from NOAA.

As they travel north toward the equator the branches warm, become less dense and rise to the surface, a process called upwelling. They then loop back south and west to the South Atlantic and finally return to the North Atlantic, where the cycle begins again.

The branches warm, rise and loop back. Image from NOAA.

Warmed surface water returns to the North Atlantic. Image from NOAA.

Slow, but enormous

The conveyor moves at only a few centimeters per second, far slower than wind-driven or tidal currents, which run at tens to hundreds of centimeters per second. Any given cubic meter of water is estimated to take about 1,000 years to complete the circuit. Yet the volume it moves is immense: more than 100 times the flow of the Amazon River.

Why it matters for life

The belt is also a vital part of the ocean's nutrient and carbon dioxide cycles. Warm surface water is poor in both, and it is replenished as it travels the conveyor as deep or bottom water. That matters because the base of the world's food chain depends on cool, nutrient-rich water, which supports the growth of algae and seaweed.

Sources

• NOAA National Ocean Service, "Currents: The Global Conveyor Belt," Ocean Service Education tutorial; rewritten in hubnx's own words. Maps from NOAA.