ENGAGE
Now that we've explored some of the physics of how our planet and the water in the ocean absorbs and releases heat, let's turn our attention to how on Earth that heat energy moves from where it's hot to where it's not -- from the sunny equator to the frosty poles.
Close that door! Were you born in a barn? Sound familiar? Even if your parents weren't rocket scientists, they understood the practicalities of thermodynamics -- that heat is restless and always on the move. But how does the heat actually, physically move?
On Earth, by far the greatest amount of thermal energy transfer from the equator toward the poles is through evaporation and condensation in the atmosphere. Really?
Yup. Evaporation is a cooling process. Remember the last time you were standing in a breeze dripping wet? But we're ahead of ourselves. More on that in the next module.
Helpful Hint: This cool concept is a clue to the Extend question from the Rate of Temperature Change of Water graph at the beginning of this module.
In addition to evaporative global heat transfer, an enormous amount of heat also moves poleward carried in the flow of enormous, warm ocean surface currents. At the same time, deep cold currents in the abyss drive cooling waters toward warmer regions.
In this section, we'll explore some of the physical mechanisms that move heat at the surface of the ocean.
Ocean Circulation - Motion in the Ocean
Prevailing winds blowing over the surface of thousands of miles of open sea sets the ocean in motion. Trade Winds blowing east to west (Easterlies) at the equator cause equatorial ocean currents to flow in the same direction. Likewise, Westerly winds prevailing at higher latitude push currents in the opposite direction.
But it's not easy for anything to take a strictly straight path over the surface of a curved planet. The same is true for ocean currents because there are other forces at work and obstacles in the way. Enter, the coriolis effect.
Coriolis Effect
Have you ever heard how water in toilets and sinks in the southern hemisphere drain in the opposite direction of those in the north? Most us have heard this bit of science trivia and pass it along as a matter of fact.
But, just for the record, IT'S NOT TRUE! However, the notion of large-scale, hemispherically counter-rotating gyres in the open oceans IS TRUE!
The Bad News: Under the best of circumstances, it is difficult to describe the complexities of the coriolis effect and how it affects Earth's oceans and atmosphere. Minds that think and visualize in terms of motion and spatial relationships will do better than the rest of us.
The Good News: There are some excellent diagrams and animations on the web that help demonstrate why something - like an ocean current - moving in a straight line over a curved surface takes a curved path. There is good one included in the Wikipedia link provided for the term coriolis effect.
EXPLORE
Though the production quality and writing are lacking in this next YouTube video, it does show a rather clever method for NOT demonstrating The Coriolis Effect in Bathrooms.
The Coriolis Effect in Bathrooms.
Here's another good one - Coriolis Effect
EXPLAIN
- What force causes ocean currents?
- Why do ocean currents rotate in large gyres?
EXTEND
- Try your own coriolis demonstration or experiment.
EVALUATE
- Describe the value of digital resources for demonstrating abstract concepts like the coriolis effect.
ENGAGE
Our Current Understanding
The planet's prevailing winds provide most of the energy that sets ocean currents in motion. But, because of the coriolis effect over the course of time and distance, the path of the current is deflected to the right (poleward) in the northern hemisphere and to the left (poleward) down under.
While these forces do not work in bathroom basins, they do work in concert to create very large ocean gyres. As these immense, slowly rotating currents flow from the equator toward the poles and back, they carry with them an enormous quantity of thermal energy stored in the water by virtue of its high heat capacity.
This heat will express itself a number of ways as it flows from where its hot to where its not, mostly through evaporation and condensation, as mentioned earlier. But for now, its safe to say that the great ocean currents can be compared to enormous rivers of warm water in the oceans, flowing from where it's hot to where it's not, while gradually surrendering their stores of thermal energy along the way.
EXPLORE
Take a look at this TD interactive resource showing the dynamics of the great ocean currents: Examine Global Surface Currents.
Examine Global Surface CurrentsEXPLORE SOME MORE
For those living on either coast of the Atlantic Ocean, the Gulf Stream has interesting history because of its affects on climate and transportation. Let's take a closer look at the natural and cultural history of this famous current with a TD video, What Causes the Gulf Stream? And explore this interesting YouTube tidbit relating to one of our greatest minds.
What Causes the Gulf Stream? Ben Franklin and the Gulf Stream.EXPLAIN
- What role do ocean surface currents play in distributing heat on Earth?
- Describe the role of prevailing winds associated with different currents.
- Describe how geography and the shape of the ocean basin affects currents.
EXTEND
- How do surface currents affect local climate?
- Explain why San Francisco and Washington DC, at roughly the same latitude, have such different climates.
EVALUATE
- Describe the value of these resources in enhancing concepts of ocean currents for yourself or your students.
ENGAGE
Google Earth Let's re-visit that question about climate differences between San Francisco and Washington, D.C. using Google Earth.
EXPLORE
Place your cursor on San Francisco and note the N. latitude coordinates for its location. Hint - they're located on the bottom of the screen...and they change whenever you move your cursor. Cool, huh? Now, do the same for Washington DC.
EXPLAIN
- Compare the latitude coordinates for each city.
- Considering what you have learned about ocean circulation patterns, describe the relative temperature of the currents off the coast of each city.
- What kind of climate differences would you expect for these two cities?
EXTEND
- Search the internet for climate data for San Franscisco and Washington DC.
- While you're still in Google Earth, visit the Add Content feature again and search for plug-in layers that demonstrate ocean currents and sea surface temperatures. One that is particularly powerful comes from the NOAA Science on a Sphere project.
EVALUATE
- How does Google Earth work for you?
