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 Concept Human Impacts on Earth's Systems.
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 "Concept Earth as a Planet." The causes of the global wind patterns will be described in detail in "Concept 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 30oN and 30oS
- westerlies: west to east in the middle latitudes
- polar easterlies: east to west between 50o and 60o 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 (Figure below). 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.
The major surface ocean currents.
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?
The ocean gyres. Why do the Northern Hemisphere gyres rotate clockwise and the Southern Hemisphere gyres rotate counterclockwise?
This video shows the surface ocean currents set by global wind belts (5a): http://www.youtube.com/watch?v=Hu_Ga0JYFNg (1:20).
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.
Longshore currents move water and sediment parallel to the shore in the direction of the prevailing local winds.
Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly. Look at the rip-current animation to determine what to do if you are caught in a rip current: http://www.onr.navy.mil/focus/ocean/motion/currents2.htm. Each summer in the United States at least a few people die when they are caught in rip currents.
This animation shows the surface currents in the Caribbean, the Gulf of Mexico, and the Atlantic Ocean off of the southeastern United States: http://polar.ncep.noaa.gov/ofs/viewer.shtml?-gulfmex-cur-0-large-rundate=latest.
Coriolis effect: The apparent deflection of a freely moving object like water or air because of Earth's rotation.
gyre: Five loops created by surface ocean currents.
longshore current: Local surface currents that move along a shoreline in the direction of prevailing winds.
rip current: A strong surface current that returns to the ocean from the shore.
surface current: A horizontal movement of ocean water, caused by surface winds.
- 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.
Use this resource to answer the questions that follow.
1. What is a surface current?
2. What is a thermocline?
3. Where is the thermocline?
4. How do surface currents form?
5. What factors determine the movement of surface currents?
6. Why are currents different temperatures?
7. How are the currents monitored?
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.