It's All Connected: Global CirculationThe lab activity described here was adapted by Erin Bardar of TERC for the EarthLabs project.
Summary and Learning Objectives
Students are introduced to the global circulation patterns of the atmosphere and the oceans, and investigate how those circulation patterns might influence their local region. Students use computer models to test predictions of ocean currents.
After completing this investigation, students should be able to:
- trace pathways of wind and water on a world map to and from their region, and across an ocean to other parts of the Earth;
- describe specifically how their region is connected as a system to others across the Earth by identifying what their wind and water carry and where the wind and water go; and
- write about what activities in their region might affect other regions, and what activities in other regions might affect theirs
Activity Overview and Teaching Materials
In Part A: Students are introduced to the global circulation patterns of the atmosphere and the oceans, and investigate how those circulation patterns might influence their local region.
In Part B: Students study surface ocean currents and then predict the pathway of a floating object dropped into the ocean at a particular point, maybe one closest to their own region. Then, using a computer model of ocean currents, they test their predictions.
Printable MaterialsTo download one of the PDF or Word files below, right-click (control-click on a Mac) the link and choose "Save File As" or "Save Link As."
- Work Sheet 1: Black Outline Map of the World (Acrobat (PDF) 94kB Jan7 10)
- World Map of Average Ocean Currents (Acrobat (PDF) 168kB Jan7 10)
- World Map of Average Winds in January (Acrobat (PDF) 127kB Jan7 10)
- World Map of Average Winds in July (Acrobat (PDF) 119kB Jan7 10)
- Stop and Think Questions (PDF (Acrobat (PDF) 38kB Sep13 10) and Word (Microsoft Word 2007 (.docx) 20kB Aug7 18))
For more information about circulation of the atmosphere and the oceans , read the section titled Background Information under Additional Resources below.
Teaching Notes and Tips
In Part A: Students move back and forth between learning about global circulation patterns of air and water, and looking for connections between the local area and those global circulations. Part of this work involves tracing the path of a local stream until it joins the global ocean circulation. Resources that allow you to investigate the local-to-global connections are not provided in this Lab. You'll need to gather those resources and explore the possibilities. What pathways do your local streams follow as they wind their way toward the ocean? (Or might a local stream actually terminate at an endorheic body of water, which has no outflow, such as the Great Salt Lake?)
In Part B: The Ship Drift model is an engaging tool that makes the topic of ocean currents become very concrete and alive; it's interesting and fun to use.
Here are some notes about using the model:
- To see a map of ocean currents in a particular month, select the month in the drop-down menu and then click the PopUp Map button. The map of ocean currents on the home page is interactive but it does not change when you change the month. The PopUp maps are the maps that students use to make their predictions about the path along which ocean currents may carry an object during a particular time of year. You can open multiple PopUp maps on your computer screen to compare ocean currents across the seasons.
- To see the probable paths that a set of five drifters (buoyant objects that remain near the ocean surface) might follow over the course of five years, select a month in the drop-down menu and click on the map of ocean currents on the home page. A new window will open showing the probable paths of the 5 drifters. You can open multiple drifter path maps on your computer screen to see how dropping a drifter in the same spot* can vary across the seasons.
* The latitude and longitude you select on the home page map may not be identical to the coordinates that appear at the bottom of the drifter path map that opens, but they will be very close. The home page map displays latitude and longitude in 5 degree increments.
For more information about the Ship Drift model, visit Navigating the Ocean on NASA's Ocean Motion Web page.
You can assess student understanding of topics addressed in this Investigation by grading their responses to the Stop and Think questions.
State and National Science Teaching Standards
The global circulation of air drives some of the Earth's ocean currents and helps to redistribute the solar energy that reaches Earth, moderating climate and impacting environments for all life on Earth. Without that circulation and redistribution of solar energy, the equatorial regions would be hotter and the polar regions colder.
What drives this atmospheric circulation? It's not simply solar energy; it's solar energy interacting with the rotating sphere that is our planet. The differential heating that occurs when the sun heats a sphere (the earth) plays a significant role. Another factor is the Coriolis effect, which results from the fact that the rotational speed of our spherical planet varies with latitude.
Earth's solar energy is most concentrated in the region of the equator. This heating causes warm air to rise, which creates a general area of low pressure at the equator. As the rising air cools and starts to fall back toward Earth it spreads into both the northern and southern hemispheres (see figure) and returns to the surface of Earth at approximately 30 degrees north and south of the equator. That cooled air is then drawn back towards the low-pressure area at the equator and repeats the circulation. This circulation creates what are known as the Hadley Cells. (See figure.)
Something similar but opposite happens in the polar regions. The solar energy falling on Earth's surface in the polar regions is spread over a much greater surface area. The resulting colder air creates high-pressure areas at Earth's surface, with cold air flowing away from the poles. As that air reaches lower latitudes it is subject to more intense solar energy, warms, rises, and is drawn toward the low-pressure areas that are high above the poles. This circulation creates two additional cells in the polar regions.
In between the Hadley and polar cells in each hemisphere is a third cell of air circulation.
What is the Coriolis Effect, and why does it occur? First think just about the solid Earth. As the Earth rotates, a point on the equator moves in a large circle, and travels more than 38,000 km during one full revolution, or in 24 hours. Its rotational speed is approximately 1600 km/hr. During that same 24 hours, a point close to one of the poles makes a much smaller circle in the course of one day, and travels just a tiny fraction of the distance, so it's rotational speed is significantly slower than a point on the equator. In fact, a point on Earth's equator rotates at a greater speed than any point north or south of the equator.
If Earth did not rotate, air flowing from a higher latitude toward the equator would move directly south or north (depending on the hemisphere). However, because of the rotation, air that flows toward the equator is moving from a latitude where its rotational speed is less to a latitude where the rotational speed is more. As a result, that air rotates more slowly than the land below it, so it "falls behind" and it is deflected toward the west (see figure below). This produces a wind at the equator that flows from east to west.
Likewise, air flowing away from the equator and towards a higher, more slowly rotating latitude will rotate more rapidly than the land below, and will deflect towards the east to produce a west-to-east wind.
Study the figure above to draw the connections between the cells that result from differential heating of the Earth and the major patterns of global atmospheric circulation that result from cell circulation and the Coriolis effect.
While the bands of circulating air cells and the Coriolis effect drive the general atmospheric circulation patterns, that circulation is also influenced by local conditions and is much more complex than the simplified illustrations suggest.
This image below shows twenty years of cyclone paths in the Atlantic. Can you see evidence of the general atmospheric circulation patterns?
Read the National Oceanic and Atmospheric Administration's concise, well-illustrated resources that explain the driving forces behind ocean circulation. Some of these are related to the winds described above; others have a very different driving mechanism (a combination of temperature and salinity). The sections that address Surface Ocean Currents and The Global Conveyer Belt are just two of the topics covered in the Currents section of this education Web site.