Air moving from the equator to the south pole will turn which direction?

Ocean water is constantly moving, and not only in the form of waves and tides. Ocean currents flow like vast rivers, sweeping along predictable paths. Some ocean currents flow at the surface; others flow deep within water. Some currents flow for short distances; others cross entire ocean basins and even circle the globe.

By moving heat from the equator toward the poles, ocean currents play an important role in controlling the climate. Ocean currents are also critically important to sea life. They carry nutrients and food to organisms that live permanently attached in one place, and carry reproductive cells and ocean life to new places.

Rivers flow because of gravity. What makes ocean currents flow?

Tides contribute to coastal currents that travel short distances. Major surface ocean currents in the open ocean, however, are set in motion by the wind, which drags on the surface of the water as it blows. The water starts flowing in the same direction as the wind.

But currents do not simply track the wind. Other things, including the shape of the coastline and the seafloor, and most importantly the rotation of the Earth, influence the path of surface currents.

In the Northern Hemisphere, for example, predictable winds called trade winds blow from east to west just above the equator. The winds pull surface water with them, creating currents. As these currents flow westward, the Coriolis effect—a force that results from the rotation of the Earth—deflects them. The currents then bend to the right, heading north. At about 30 degrees north latitude, a different set of winds, the westerlies, push the currents back to the east, producing a closed clockwise loop.

The same thing happens below the equator, in the Southern Hemisphere, except that here the Coriolis effect bends surface currents to the left, producing a counter-clockwise loop.

Large rotating currents that start near the equator are called subtropical gyres. There are five main gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre.

These surface currents play an important role in moderating climate by transferring heat from the equator towards the poles. Subtropical gyres are also responsible for concentrating plastic trash in certain areas of the ocean.

In contrast to wind-driven surface currents, deep-ocean currents are caused by differences in water density. The process that creates deep currents is called thermohaline circulation—“thermo” referring to temperature and “haline” to saltiness.

It all starts with surface currents carrying warm water north from the equator. The water cools as it moves into higher northern latitudes, and the more it cools, the denser it becomes.

In the North Atlantic Ocean, near Iceland, the water becomes so cold that sea ice starts to form. The salt naturally present in seawater does not become part of the ice, however. It is left behind in the ocean water that lies just under the ice, making that water extra salty and dense. The denser water sinks, and as it does, more ocean water moves in to fill the space it once occupied. This water also cools and sinks, keeping a deep current in motion.

This is the start of what scientists call the “global conveyor belt,” a system of connected deep and surface currents that moves water around the globe. These currents circulate around the globe in a thousand-year cycle.


The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around Earth. The Coriolis effect is responsible for many large-scale weather patterns.

The key to the Coriolis effect lies in Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.

Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.

Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.

Everywhere you play global-scale "catch" in the Northern Hemisphere, the ball will deflect to the right.

This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents appear to bend to the left.

The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances.

Weather Patterns

The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.

Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.

In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.

Outside storm systems, the impact of the Coriolis effect helps define regular wind patterns around the globe.

As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.

Impact on Human Activity

The weather impacting fast-moving objects, such as airplanes and rockets, is influenced by the Coriolis effect. The directions of prevailing winds are largely determined by the Coriolis effect, and pilots must take that into account when charting flight paths over long distances.

Military snipers sometimes have to consider the Coriolis effect. Although the trajectory of bullets is too short to be greatly impacted by Earth’s rotation, sniper targeting is so precise that a deflection of several centimeters could injure innocent people or damage civilian infrastructure.

The Coriolis Effect on Other Planets

The Earth rotates fairly slowly, compared to other known planets. The slow rotation of Earth means the Coriolis effect is not strong enough to be seen at slow speeds over short distances, such as the draining of water in a bathtub.

Jupiter, on the other hand, has the fastest rotation in the solar system. On Jupiter, the Coriolis effect actually transforms north-south winds into east-west winds, some traveling more than 610 kilometers (380 miles) per hour.

The divisions between winds that blow mostly to the east and those that blow mostly to the west create clear horizontal divisions, called belts, among the planet’s clouds. The boundaries between these fast-moving belts are incredibly active storm regions. The 180-year-old Great Red Spot is perhaps the most famous of these storms.

The Coriolis Effect Closer to Home

Despite the popular urban legend, you cannot observe the Coriolis effect by watching a toilet flush or a swimming pool drain. The movement of fluids in these basins is dependent on manufacturer’s design (toilet) or outside forces such as a strong breeze or movement of swimmers (pool).

You can observe the Coriolis effect without access to satellite imagery of hurricanes, however. You could observe the Coriolis effect if you and some friends sat on a rotating merry-go-round and threw or rolled a ball back and forth.

When the merry-go-round is not rotating, rolling the ball back-and-forth is simple and straightforward. While the merry-go-round is rotating, however, the ball won’t make to your friend sitting across from you without significant force. Rolled with regular effort, the ball appears to curve, or deflect, to the right.

Actually, the ball is traveling in a straight line. Another friend, standing on the ground near the merry-go-round, will be able to tell you this. You and your friends on the merry-go-round are moving out of the path of the ball while it is in the air.