13.6 Surface Currents

Why is so much trash so far from land?

The Great Pacific Garbage Patch is a region in the center of the north Pacific Ocean where plastic bits and chemicals are concentrated. Trash from the countries bordering the region enters the oceans and is transported into the center of the North Pacific Gyre, where it remains. Seabirds may get sick from ingesting so much plastic instead of food. More about the patch can be found in the chapter Human Impacts on Earth's Systems.

Surface Currents

Ocean water moves in predictable ways along the ocean surface. Surface currents can flow for thousands of kilometers and can reach depths of hundreds of meters. These surface currents do not depend on weather; they remain unchanged even in large storms because they depend on factors that do not change.

Surface currents are created by three things:

• global wind patterns
• the rotation of the Earth
• the shape of the ocean basins

Surface currents are extremely important because they distribute heat around the planet and are a major factor influencing climate around the globe.

Global Wind Patterns

Winds on Earth are either global or local. Global winds blow in the same directions all the time and are related to the unequal heating of Earth by the Sun — that is, more solar radiation strikes the Equator than the polar regions — and the rotation of the Earth — that is, the Coriolis effect. Coriolis was described in the chapter Earth as a Planet. The causes of the global wind patterns will be described in detail in the chapter Atmospheric Processes.

Water in the surface currents is pushed in the direction of the major wind belts:

• trade winds: east to west between the Equator and 30^oN and 30^oS
• westerlies: west to east in the middle latitudes
• polar easterlies: east to west between 50^o and 60^o north and south of the Equator and the north and south pole

Shape of the Ocean Basins

When a surface current collides with land, the current must change direction. In the Figure below, the Atlantic South Equatorial Current travels westward along the Equator until it reaches South America. At Brazil, some of it goes north and some goes south. Because of Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere.

Gyres

You can see on the map of the major surface ocean currents that the surface ocean currents create loops called gyres (Figure below). The Antarctic Circumpolar Current is unique because it travels uninhibited around the globe. Why is it the only current to go all the way around?

Local Surface Currents

The surface currents described above are all large and unchanging. Local surface currents are also found along shorelines (Figure below). Two are longshore currents and rip currents.

Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Each summer in the United States at least a few people die when they are caught in rip currents.

Summary

• Major surface ocean currents are the result of global wind patterns, Earth's rotation, and the shape of the ocean basins.
• Major surface currents circle the oceans in five gyres.
• Local surface currents, like longshore and rip currents, move near shorelines.

Review

1. Describe the motion of a water particle that is stuck in a gyre in the North Pacific.
2. What should you do if you get stuck in a rip current?
3. What would happen if a major surface current did not run into a continent? Note that this is what happens with the Antarctic Circumpolar Current.

Explore More

Use this resource to answer the questions that follow.

1. Which direction do currents move across the equator both north and south? What direction does the counter current flow?
2. What is unique about the Antarctic Circumpolar Current and why?
3. What are gyres?
4. Where are currents that move heat from the equator to the higher latitudes flowing relative to the continents?
5. Why is the Caribbean so hot and humid?
6. Why does the Gulf Stream cool before it reaches Europe?
7. Why is it warmer and more pleasant to swim off of the Eastern Seaboard than off of California?
8. What are the western boundary currents due to?