Ocean Currents

Tilt-A-Whirl rides at amusement parks spin in circles while swinging up and down at the same time. These movements slosh water (and other fluids) around really well, which you may have learned from getting sick after riding one. Ocean currents are kind of like a Tilt-A-Whirl. They move ocean water around the planet and up to the surface then down to the ocean floor. This dizzying water ride is made up of two systems: surface currents and deep ocean currents.

As the Earth spins on its axis, surface ocean water shifts to one side, creating currents. Click for more detail.

The first type of ocean current happens at the surface of the water. The spinning of the Earth and the trade winds causes these surface currents. At the equator, winds tend to blow toward the west, and close to the poles winds tend to blow toward the east. These winds move across the surface of the ocean, pulling the water along with it.

But when land gets in the way of these currents, they have to change course. In the Northern Hemisphere, the water turns to the right, and in the Southern Hemisphere it turns to the left. These currents form giant loop-de-loops, or gyres, spinning the ocean’s water around in circles.

Surface currents form 5 major ocean gyres between the continents. These gyres are named for the ocean basins, or sections of the ocean, that they form in: two each in the Pacific and Atlantic Oceans (one north and one south), and one in the Indian Ocean.

Some deep ocean currents are controlled by water temperature. Click for more detail.

The second type of ocean current moves water from the top of the ocean to the bottom. These currents move much slower compared to surface currents, and instead of starting at the equator they start at the poles. If you drop an ice cube into a very still glass of juice, you can watch from the side as the cold water near the ice grows heavy and sinks to the bottom of the glass.

The same thing happens in the ocean near the poles – cold water near sea ice becomes heavier and starts to drop to the bottom of the ocean. As this dense water sinks, it allows more water to move in, cool, and sink to the bottom behind it. When cold water plunges to the ocean floor, it starts a giant slow motion current along the bottom of the ocean.

Unlike the fast, isolated surface gyres, this deep ocean current travels all over the planet into every ocean basin. Eventually this cold water warms and is pushed to the surface where it starts the cycle again. Following this deep ocean current would be like riding a roller coaster that takes a thousand years from start to finish.

Two Worlds, One Biome

This roller coaster-like global conveyor belt connects the top and bottom of the ocean, which can be like two completely different worlds. Instead of thinking about the open ocean biome horizontally (from left to right), many scientists characterize this biome vertically (from top to bottom). This is because the open ocean is incredibly deep – over 2 miles deep in some areas. The environment changes quickly as you dive deeper and deeper into the open ocean.

The vertical zones of the open ocean biome. Click for more detail.

The top section of the open ocean only goes down as far as you can see. This isn’t because the water is too murky, but because deeper down there is no light. Most sunlight can only reach down to about 500 ft (200 m), and all light disappears below 3,280 ft (1,000 m). Unlike in deeper water, in this zone (the epipelagic zone), algae can use sunlight to grow and animals can use their eyes to hunt.

Many animals live deep in the ocean, like this foot-long giant isopod (related to the roly poly). Image by Lycaon.