Below the wind-driven surface currents, the deep ocean also moves, slowly. These deep currents are driven by density differences, like convection: cold, salty water is dense, so it sinks.
Key idea
Cold water and salty water are denser. Near the poles, surface water becomes cold and salty enough to sink, then creeps along the ocean floor. This drives a slow loop through every ocean, often called the global conveyor belt.
Why it works
Cooling packs water molecules closer together, raising density. Salt adds mass to the same volume, raising it too. Near Greenland and Antarctica, winter air chills the surface water, and when sea ice forms it leaves most of its salt behind in the water below. That water becomes the densest in the ocean and sinks to the bottom. It flows toward the equator along the sea floor and slowly rises in other oceans. One trip around the loop takes about 1,000 years.
Worked example: Which sinks?
Sample A is 2 °C and very salty. Sample B is 20 °C and less salty. Which sinks below the other?
- Colder water is denser, and saltier water is denser.
- Sample A is both colder and saltier, so it is denser and sinks.
Answer: Sample A
Worked example: Sea ice and salt
When seawater freezes into sea ice, most of the salt stays in the water. What happens to the water just under the ice?
- It gets saltier, and it is already near freezing.
- Cold and salty means very dense, so it sinks, feeding the deep current.
Answer: It becomes denser and sinks
Common mistake
Thinking salt makes water lighter, or that the deep ocean sits still. Dissolved salt adds mass, so salty water is denser than fresh water at the same temperature (an egg can float in very salty water). And the deep water moves, slowly, all around the globe. Check: deep water forms only where surface water is both cold and salty.