This lesson will help students understand the ocean’s role in the context of unequal heating of Earth’s surface by the Sun, and how the ocean redistributes that energy around the planet. Students learn about the large-scale movement of the ocean’s waters and the significance of the North Atlantic region in the transport of heat throughout the ocean. Students explore data maps and visualizations to make connections between the various factors that influence global ocean circulation.
Grade Level: 9–12
Standard: ESS2.A: Earth Materials and Systems
- The geological record shows that changes to global and regional climate can be caused by interactions among changes in the sun’s energy output or Earth’s orbit, tectonic events, ocean circulation, volcanic activity, glaciers, vegetation, and human activities. These changes can occur on a variety of time scales from sudden (e.g., volcanic ash clouds) to intermediate (ice ages) to very long-term tectonic cycles. (HS-ESS2-4)
Two 45-minute class periods
- Students will be able to relate the connections between unequal heating of Earth’s surface and patterns of sea surface temperature, surface winds, and ocean density to ocean currents.
- Students will be able to communicate how the ocean redistributes heat around the planet.
Prep for Teachers
Before the Lesson
- Note: The “meridional overturning circulation” (MOC) model replaces what has traditionally been thought of as the “ocean conveyor belt" model. For decades, the conveyor belt concept, which was often used synonymously with “thermohaline circulation,” described the large-scale sinking, rising, and flow of ocean water due to the influence of water temperature, salinity, and density. The term “thermohaline circulation” remained inconsistently defined and could not be described mathematically, however. With recent advances in ocean research techniques (including satellite observations, the deployment of fleets of thousands of drifting data-sensing floats, and fine-resolution computer ocean circulation models), it is recognized that the long-standing ocean conveyor belt model greatly oversimplifies the complexities of ocean circulation; it does not adequately account for factors such as winds and tides on the mixing of waters, and the crucial role of ocean eddies. While this lesson provides an overview of a number of aspects of the ocean’s circulation, it does not specifically explore, or address, the meridional overturning circulation model. (The MOC is sometimes referred to simply as overturning circulation.)
- Arrange to have computer access for students to work individually or in small groups.
- Download the An Early Observation of the Deep Ocean handout and have ready for distribution.
- Elicit students’ preconceptions of the ocean by asking, “How many oceans are there in the world? How are they connected?” (It may be helpful to show a global map.) After soliciting responses, tell students that the names, boundaries, and even number of oceans have evolved over time as as people’s world views have changed—geographical, cultural, scientific, and even political. Emphasize to students that while the five current named oceans (the Atlantic, Pacific, Indian, Arctic, and Southern Oceans) represent geographic subdivisions of our watery planet, there is really just one, big global ocean. Our one global ocean consists of several ocean basins (i.e. North Pacific basin, North Atlantic basin, etc.). The waters that flow through all the ocean basins—albeit slowly—are interconnected.
Survey how students think water in the ocean moves, or what they think of when they hear the terms “ocean currents” or “ocean circulation”. Then, as a class, watch the video from the Perpetual Ocean resource. (Note: This resource features a video and 4 images. You need only use the video, also titled Perpetual Ocean, for the purposes of this lesson.) Ask:
- What do you notice? Describe your observations.
- What do you think this visualization shows?
- How do you think the patterns of movement seen in the video impact the Earth?
- What do you think these patterns of movement in the ocean have to do with the distribution of energy around Earth?
- The constant motion of the ocean, and the multitude of flowing dots, lines, and swirls (eddies), should be prominent features for most students. Replay the video and define for students what these features look like, if needed. Point out that only surface flow data were used to produce this visualization.
- To launch students into thinking about energy distribution around the planet, ask students to share their thinking about what drives the ocean currents through writing, think-pair-share, or class discussion. Look for answers that allude to heat or the Sun; draw those answers out by encouraging students to elaborate. Do not correct them at this point, the point is for them to document their thinking.
- Spend some time discussing the radiation that Earth receives from the Sun. The Sun is the primary source of energy for Earth’s climate system, driving atmospheric and ocean circulation, and weather and climate patterns.
Share one image—for example, March (slide 4) or September (slide 10)—from the Insolation on Earth slideshow. Provide context so they can interpret it—identify continents, describe the key, etc. Students should be able to see that the heating of Earth is uneven. Share additional images from the slideshow. Ask:
- Why do you think that the amount of solar energy striking Earth varies by latitude?
Point out how the areas that receive the most or least insolation vary throughout the year. For example, the amount of solar energy reaching the Northern Hemisphere is higher during the month of June (summer in the Northern Hemisphere) and lower during December (winter in the Northern Hemisphere). This variation is due to the tilt of Earth's axis and changes in its orientation toward the Sun throughout the year.
- Now, have students step through the Ocean Circulation in the North Atlantic interactive lesson. This interactive lesson can be utilized as an individual activity or as a guided group experience, and should take approximately 45–60 minutes. If students do not complete the interactive lesson by the end of class, remind them to save their work.
- Page-specific notes:
- Page 2: Sample answer: Different bands of wind directions are visible. For example, winds blow west to east in certain latitudes, and east to west in others. Places where winds move in a northward direction include over the upper Atlantic and Pacific Oceans. Unequal heating of Earth’s surface leads to different patterns of surface wind flow at different latitudes.
It may be helpful for students to refer to a world map with latitude lines as they view the animation of surface winds.
- Page 3: Sample answer: The general pattern of wind currents and pattern of ocean surface currents correlate to one another at the various latitudinal bands.
Suggest students replay the videos several times to better observe the patterns. Note that the datasets used in these visualizations are not synchronized and that students are looking for general relationships only. (The dataset used in the Surface Winds visualization runs from May through August 1988; Surface Currents utilizes data observations from January 2010 through April 2011. Although the winds dataset is from 1988, the main wind patterns still apply.) Point out that although the interactions between winds and the ocean are complex, a general relationship exists between surface wind flow and surface ocean current flow.
Note: Students may observe that in some localized places, the ocean currents flow in directions opposite to the wind. Explain to students that although surface winds provide the initial push on surface currents, and that the surface currents generally follow a similar pattern overall, countercurrents result in some places due to the influence of Earth’s rotation.
- Page 4: Sample answer: Temperatures are highest near the equator (where sunlight is most intense and relatively constant throughout the year), and lowest at the poles (where sunlight is less intense and varies greatly depending on the season). The global pattern of sea surface temperature corresponds with the differing amounts of energy received at the various latitudes.
Page 5: Sample answers:
- Currents near the equator are generally warm (reds and oranges).
- Currents near the poles are generally cool (greens and blues).
- Within the Atlantic, warmer currents flow from the equatorial regions northward.
- Heat is carried by the surface currents, and moves along in the direction that the currents are flowing.
- Eddies transfer heat as they swirl along.
Page 7: Remind students to focus on the North Atlantic basin. Sample answer: The sinking occurs where the ocean is very cold and dense (darkest blue on this color scale).
- It may be helpful to explain to students that seawater can become denser than surrounding waters (and sink to form deep water masses) through several mechanisms—density increases when high salinity water becomes colder, or when cold water increases in salinity. (A “water mass” is a mass of water with a distinct set of physical properties such as temperature and salinity.)
Note: Although the Arctic receives much less solar energy than the North Atlantic, deep water masses do not form in the Arctic because the water there is too “fresh”, due in part to freshwater runoff from many major rivers that drain into the Arctic basin.
In the Southern Ocean around Antarctica, waters do convect and sink; however, the deep waters formed there become part of a different, deeper circulation cell. The volume of deep water formation in the Antarctic is not very large compared to the volume formed in the North Atlantic (but is still important to the ocean system.)
Page 8: Note: Students may wonder how deep dense waters in the Southern Ocean rise and return to the surface. The answer is complicated, but involves mechanisms related to the wind-driven Antarctic Circumpolar Current which encircles Antarctica.
Page 9: Sample answer: The eddies are always changing in size and shape; some form and disappear, while others persist. The pattern of eddies at the beginning of the video is not the same as the pattern at the end of the video. The temperature data shows that ever-changing eddies transport heat throughout the year.
- Page 10: Ensure students expand the videos to full screen in order to observe the flows in detail. Discuss with students that the videos are data visualizations that show a simulation that has been produced by a computer model, not direct observation of actual ocean flows.
Page 11: Students should be able to describe and explain several mechanisms that together redistribute heat from equatorial regions towards the poles. Remind students that, while the videos on page 10 focus on surface and deep flows in the North Atlantic, at a global scale, the meridional circulation spans both hemispheres.
- Sample answer: The ocean’s circulation helps redistribute the Sun’s energy on Earth through its system of surface and deep currents, that bring heat from regions that receive more intense solar energy over the course of the year to regions that receive less. For example, in the North Atlantic, the circulation involves the flow of warm surface water from the tropics northwards to the pole. The data map of sea surface temperature on page 4 shows the unequal heating on the planet, and the visualizations on page 10 show the general flow of surface and deep currents in the North Atlantic.
- After going through the interactive lesson, students should have a better understanding of how the fluid ocean circulates and contributes to heat redistribution across the globe. Emphasize that without fluids—both the waters of the ocean and the air of the atmosphere—and the global transport of heat by the ocean from equatorial regions to the poles, the difference in temperature between the equator and the poles would be many times greater than the relatively moderate 30°C that it is today. (Surface temperatures on the Moon—which has only an ultra-thin layer of atoms and not a dense atmosphere like Earth—vary by almost 300°C between the hottest and coldest places!).
- Note: It’s important to keep in mind that overturning circulation in the ocean is not the only way that solar energy is redistributed on Earth. While the MOC is the overturning circulation in latitude and depth that spans pole-to-pole, also linked and very important are the horizontal, basin-wide subtropical ocean gyres. These large-scale swirling flows fill the subtropical Pacific, Atlantic, and Indian ocean basins and are also significant transporters of heat in the ocean. The atmosphere also transports a large amount of heat. The relative transport of heat by the ocean or atmosphere varies with latitude and region on Earth. For example, at about 35°N/S, atmospheric heat transport dominates, accounting for over three quarters of the heat transport. From the equator to about 17°N in the tropics, ocean heat transport dominates. In the Pacific and Indian basins, wind-driven gyres transport much of heat in the ocean.
- Present students with a historical account of the first instance of measurements being made of the deep ocean. Distribute the An Early Observation of the Deep Ocean handout. Have students read the account and use their understanding of the meridional overturning circulation model to explain Captain Ellis’ observations.
- Sample answers:
- (a) 84°F; (b) about 24 degrees colder; (c) 53°F; (d) no change; the water remained cold.
- They were warmer by about 10 to 15 degrees.
- The water was probably even colder but warmed in the time it took to come up to the surface.
- He was sampling cold, dense waters that had originated in the North Atlantic.
- Note: It was not until almost 50 years later that a scientist studying heat in fluids first recognized the important implications of Ellis’ interesting observations. The American-born British physicist Count Benjamin Thompson Rumford was the first to deduce that the cold, deep waters must have originated near the poles. In his essay on the matter, “Of the propagation of heat in fluids,” first published in 1797, Rumford reasoned:
- But if the water of the ocean, which, on being deprived of a great part of its Heat by cold winds, descends to the bottom of the sea, cannot be warmed where it descends, as its specific gravity is greater than that of water at the same depth in warmer latitudes, it will immediately begin to spread on the bottom of the sea, and to flow towards the equator, and this must necessarily produce a current at the surface in an opposite direction. [Rumford, 1797]
The study of physical oceanography has come a long way since measurements were made with a line and bucket. With advances in research tools, powerful satellite observation capabilities, and computer ocean circulation models that can resolve far finer-scaled features such as eddies, scientists continue to build upon the global ocean circulation model first postulated by Count Rumford, and to piece together the dynamic story of ocean flows in increasing detail.
- Hold a class discussion to evaluate students’ understanding of the role of global ocean circulation in redistributing heat across the globe. Ask:
- What is the significance of planetary fluids such as the ocean to Earth’s climate? What are some key mechanisms that move heat from the equatorial region toward the poles?
- What roles do data visualizations that show Earth processes at a global scale play in helping scientists build a more complete and detailed picture of global ocean circulation?
- Based on what you have learned and the visualizations you have seen, why is it an oversimplification to refer to ocean circulation as a “conveyor belt”?