Global winds and currents | Middle school Earth and space science | Khan Academy

One of my favorite things to do is go camping. For me, there's nothing better than getting outside, breathing in some fresh air, and taking a swim in my favorite river. Have you ever jumped into a river and felt that the deeper, cooler water closer to your feet was moving faster than the shallow, warmer water at your knees? That's a current, which is the word we use to describe how water, or even air, flows within a larger body of water or air.

But what causes a current? Well, let's start with the sun. The sun actually heats Earth unevenly. We know that it's hotter near the equator and it gets colder as you go towards the poles. Near the equator, the sun's rays hit Earth's surface more directly, while near the poles, the sun's rays hit Earth's surface less directly. In both regions, the same amount of solar energy is hitting Earth, but near the equator, this energy is concentrated into a smaller area, and near the poles, it's spread out over a larger area.

So, the regions near the equator get more solar energy, which makes them warmer, and the regions near the poles get less solar energy, which makes them cooler. This uneven heating of Earth also affects air pressure. Where it's cooler near the poles, cool air will sink, making the air pressure high, but where it's warmer near the equator, warm air will rise, resulting in low pressure. This is where the terms low pressure cells and high pressure cells come from.

The low pressure warmer air at the equator rises into the upper atmosphere, where it cools and flows away towards higher latitudes, away from the equator. Because the air is now cooler, it starts to sink again and creates a high pressure band near these latitudes. This process repeats and creates a pattern of high and low pressure bands from the equator to the poles. We know that air flows from areas of high pressure to areas of low pressure. This creates air currents, or winds.

Now, you might think that these winds would blow in straight lines from high to low pressure areas, but the global wind patterns, which we call prevailing winds, look like they curve to the right in the northern hemisphere and to the left in the southern hemisphere. This curving has to do with the rotation of Earth and is called the Coriolis effect. As these prevailing winds blow across the surface of the land and water, they also push against the surface of the ocean and produce wind-driven surface currents, which help to move ocean water.

Here's what the global pattern of ocean surface currents looks like. Like wind currents, ocean surface currents are also curved due to the Coriolis effect. We can see that in these currents that are traveling north and south, which curve to the right in the northern hemisphere and to the left in the southern hemisphere. Together, as the surface currents of the ocean connect, they form giant rotating systems of ocean currents called gyres.

The currents that drive these gyres extend from the surface to about one kilometer down into the ocean and help to move water all around the globe. But these gyres aren't just moving water; they're moving heat energy as well. Water is pretty good at holding onto heat it absorbs from the sun. So as the water in our oceans moves around the world through this gyre circulation, the water also carries heat.

Here, warm water generally moves from the equator to the poles, and cold water moves from the poles to the equator. But the ocean has other deeper currents that are affected by differences in temperature and density. Remember how we talked about areas heated directly and less directly by the sun and how that results in low and high pressure areas? Same thing with water, except that water density is affected by both temperature and salinity, which is a measure of how salty the ocean is.

Cooler and saltier water is more dense, so it tends to sink just like cool air, whereas warmer and less salty water is less dense and tends to rise, just like warm air. So, with these deeper ocean currents, water actually moves vertically or up and down. For example, water near the poles gets very cold. It also gets very salty because when sea ice is formed, the salt can't go into the ice; instead, the salt stays behind in the water, and so the water gets saltier or more saline.

Together, the coldness and salinity make the water very dense, causing it to sink deep into the ocean. In other parts of the ocean, wind drags deep water up to the surface in a process called upwelling. These vertical currents are connected by horizontal currents at the surface and in the deep ocean. Collectively, this system of currents is known as the overturning circulation. You might also hear it called the global ocean conveyor belt.

So, here's a map showing the overturning circulation. This map might look a little bit strange, but here we're looking at Earth from the south pole, so Australia and the southern tips of Africa and South America are closest to the center of the map, while Europe, most of Asia, and North America are at the edges. Now, if you follow the currents in the overturning circulation, you can see that they flow all over the world's oceans, from the southern ocean around the south pole to the Pacific, the Indian, and all the way into the North Atlantic.

Like the currents in the gyre circulation, the currents in the overturning circulation also carry and disperse heat energy all around the world. Here I am back in my favorite river, wading around and enjoying currents of cool water flowing around my feet. Even though this river is small, the currents that flow through it are similar to the global wind and ocean currents that flow all around the world.

So, these currents connect in our atmosphere and oceans, which means that we are all connected. So, currents connect us all.