Rocket has filled the lab sink and is blowing across it through a straw. Tiny waves race to the far side.
"Wind pushes water," he announces. "That is the whole story of currents."
Raven drops a scrap of paper in. It drifts, then curls away from the straight line Rocket blew.
"What do you notice?" she asks. "It did not go straight."
Nova dims her lights and spins slowly in place. "Rocket, what else is moving while the wind blows?" she asks. "Something very large."
Rocket stares at the beach ball Earth on the shelf. "The whole planet is turning," he says.
"And the water is not all the same," Raven adds, remembering the salt flats. "Some of it is heavier."
NOAA's National Ocean Service lists three drivers of ocean currents: tides, winds and differences in water density.
Tides come from the pull of the Moon and Sun. They make currents near shore that rise and fall in a regular pattern.
Winds drive currents at or near the ocean's surface, in about the top 100 meters of water.
Density differences come from temperature and saltiness. Cold, salty water is denser, so it sinks and drives slow, deep currents.
| Driver | Where the current flows | Speed |
|---|---|---|
| Tides | Near shore, in bays and estuaries | Changes in a regular, predictable pattern |
| Winds | At or near the surface, top 100 meters | Tens to hundreds of centimeters per second |
| Density (cold, salty water) | Deep ocean, thousands of meters down | A few centimeters per second |
If Earth did not rotate, air would simply flow back and forth between the warm equator and the cold poles.
But Earth does rotate. Moving air is bent to the right north of the equator and to the left south of it.
This bending is called the Coriolis effect. It is why winds and surface currents travel in curved paths, not straight lines.
Near the equator, warm air rises and flows toward the poles. It cools and sinks near 30 degrees latitude.
The air that returns toward the equator becomes the trade winds. Air that keeps going toward the poles becomes the westerlies.
These steady winds push the surface of the ocean and set the great surface currents moving.
In the polar regions, ocean water gets very cold and sea ice forms. When sea ice freezes, the salt is left behind.
The water around the ice becomes saltier and colder, so it is denser. It sinks toward the ocean floor.
Surface water flows in to replace it. That sinking starts a slow, planet-wide loop that NOAA calls the global conveyor belt.
The loop begins near the pole in the North Atlantic. Deep water travels south, around Antarctica, and splits toward the Indian and Pacific Oceans.
There it warms, rises to the surface and loops back toward the North Atlantic, where the cycle begins again.
NOAA estimates one cubic meter of water takes about 1,000 years to make the full trip.
The conveyor moves more than 100 times the flow of the Amazon River, but only a few centimeters per second.
NOAA gives two very different speeds. Wind-driven surface currents move tens to hundreds of centimeters per second.
The deep conveyor belt creeps along at only a few centimeters per second.
A fast surface current can carry warm water a long way in a season. The deep loop takes about 1,000 years to finish one trip.
Both matter. The surface moves heat quickly to nearby coasts, and the deep loop stores heat for centuries.
| Statement | True or false? |
|---|---|
| Winds drive currents in about the top 100 meters of the ocean. | ? |
| When sea ice forms, the salt freezes into the ice. | ? |
| Cold, salty water is denser and sinks. | ? |
| The deep conveyor belt is faster than wind-driven surface currents. | ? |
| One cubic meter of water takes about 1,000 years to travel the whole conveyor belt. | ? |
Excellent. Tomorrow's Station Lab makes cold, salty water sink right in front of you.