A few kilometers from the shores of Palm Beach County, Florida, the Gulf Stream current—a remarkable “river” within an ocean—makes its closest approach to land. The current’s journey across the Atlantic Ocean connects southeast Florida and southwest Great Britain as it streams steadily north at speeds of 97 km a day; moving 100 times as much water as all the rivers on Earth (Perlman 2005). To help my ninth-grade Integrated Science students understand why and how the Gulf Stream flows, I use the 5E constructivist instructional model—Engage, Explore, Explain, Elaborate and Evaluate (Bybee 1997)—to analyze a single problem: Why wasn’t the iceberg that sank the Titanic either dissolved or deflected eastward by the Gulf Stream before the collision?
Setting the stage
To engage students, I offer an idea that the Gulf Stream current should have steered the iceberg away from the shipping lanes of the ocean liner Titanic. Shortly after the disaster, U.S. Senator William Alden Smith, chairman of the subcommittee overseeing hearings about Titanic’s sinking (and no relation to the Titanic’s skipper), put into the record a memorandum from Captain John Knapp, a hydrographer in the U.S. Navy’s Bureau of Navigation. Knapp wrote the following regarding the drift of ice on and near Grand Banks, Canada, where Titanic sank on April 15, 1912:
- The Labrador Current, which brings both berg and field ice down past Newfoundland, sweeps across the banks in a generally south to southwest direction, flowing more westerly on its surface as it approaches the warm Gulf Stream water in about latitude 43°, with a set of about 12 miles a day. The speed of the Gulf Stream drift at its northern edge is only about 6 miles a day at the 15th meridian and its depth is probably less than 300 feet.
- An icefield arriving at the edge of the Gulf Stream drift finds itself impelled less and less to southward and more and more to eastward and north-eastward; but a deeply floating iceberg may continue to plow southward into the warm east-flowing current and end its career south of latitude 40° by melting and breaking up (Titanic Inquiry Project 2006).
Captain Knapp concludes point 2 with an explanation that the cold, south-moving current actually underruns the warm surface water, continuing to push the berg. Instead of revealing Knapp’s conclusion to my students, I encourage them to explore and experiment themselves as to why the deeply floating iceberg kept going south and hit the Titanic, instead of moving eastward with the Gulf Stream as would an object floating more on the surface. In doing so, students learn about the Gulf Stream current as part of the ocean’s “conveyor belt,” experiment with fluid density differences and thermohaline circulation through hands-on labs and teacher demonstrations, and make extensions to the chemistry of climate change.
Starting the ocean unit
Figure 1. SeaWiFS
Polar Projections September
1997 – July 1998. Provided by the
SeaWiFS Project, NASA/Goddard
Space Flight Center and ORBIMAGE
The ocean currents and circulation unit begins with math problems to calculate the number of soda bottles and swimming pools our oceans could fill. Examples: How many liters does an Olympic swimming pool 50 × 20 × 2 m hold? 2000 m3, or 2 × 106 L. The Earth’s oceans contain approximately 1.34 × 1018 m3 of water (Perlman 2005). How many pools would the oceans fill? About 6.7 × 1014 (670,000,000,000,000) pools, about 100,000 Olympic-sized swimming pools for every person on Earth. Prior knowledge is also accessed with students defining terms such as density, salinity, thermocline, halocline, current, and estuary (Bernstein et al. 2005).
The ocean’s over one billion km3 of water are set sloshing circularly by interaction of Newton’s laws of motion and the Coriolis effect. Using an overhead of the Earth’s polar region (Figure 1) and a pendulum bob, I demonstrate how fluid “fluid motion” can be. Objects in motion such as cold currents coming down from the North Pole should maintain motion in a straight line. And yet as a Foucault pendulum proves, they are acted upon by the outside force of Earth’s rotation to deflect their path. The curvature of the Gulf Stream into a great North Atlantic gyre is caused in part by the spin swirling it (Dobson, Holman, and Roberts 2001).
In the North Atlantic, currents collide. Figure 2 shows waters warmed by the equatorial Sun flow northward along the entire eastern seaboard of the United States.
Figure 2. The Gulf Stream by Benjamin Franklin.
The colder Labrador Current along the eastern Canadian coast and the East Greenland Current both run southward from the Arctic Ocean, and meet the Gulf Steam, bringing icebergs into the balmier and brinier river—the Titanic was sunk here in April 1912, at 41° North latitude, 49° West longitude (Figure 3).
|Figure 3. The Titanic and the Gulf Stream. Graphic created by Erich Landstrom from an image provided by the National Oceanographic and Atmospheric Administration (NOAA). This interface displays Expendable Bathythermographs (XBT) data stored in the databases of the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) and Global Telecommunication System data from NOAA. The AOML Environmental Data Server provides interactive, online access to various oceanographic and atmospheric datasets residing at AOML. The inhouse datasets include Atlantic Expendable Bathythermograph (XBT), Global Lagrangian Drifting Buoy, Hurricane Flight Level, and Atlantic Hurricane Tracks (North Atlantic Best Track and Synoptic). Other available datasets include Pacific Conductivity/Temperature/Depth Recorder and World Ocean Atlas 1998. www.aoml.noaa.gov/phod/trinanes/xbt.html.|
The mingled waters continue clockwise across the Atlantic to reach the North Sea by the British Isles. Or they may deflect down along the European and North African side of the Atlantic as the Canary Current. This current can roll with the Earth’s rotation right back into the Caribbean Sea and the Gulf of Mexico. Or the Canary Current can cross the equator and join other ocean circulations to circumnavigate the globe in a millennium-long marathon.
To “explore” density differences between warm and cold fresh water, student teams completely fill one baby food jar with warm tap water and add yellow food coloring, then completely fill a second baby food jar with ice cold fresh water and add blue food coloring (each team will have two jars each). These jars are placed in a plastic bin, and carefully swirled so the coloring is evenly mixed. A water-resistant playing card is placed over the warm water jar’s top. While tightly holding the card over the jar mouth, the team carefully turns the jar upside down and positions it directly over the cold water so the tops line up exactly. Students write short-answer predictions to explain how the fluids will mix when the playing card partition is removed between the warm fresh water and the cold fresh water, and make drawings with colored pencils to illustrate how the fluids will appear 10 seconds and 1 minute after the partition is removed (Figure 4).
|Figure 4. Sample student prediction of warm and cold water mixing. (The warm water contains yellow food coloring and the cold water contains blue food coloring. The drawing on the far left is a student drawing; the student prediction is incorrect since the waters do not mix.)|
I solicit student responses through the possible permutations at the mixing minute’s end: (a) the waters combine, completely turning totally green, (b) the waters do not mix at all, remaining distinctly blue and yellow, or (c) the waters mix at their thermocline creating a blue, green, and yellow banding. Students are polled as to the most likely outcome. Following the discussion, teams slowly and carefully remove the card between so the waters contact. Students observe what happens, and compare their predictions to the actual outcome. Because cold fresh water has greater density, it does not rise, so the waters mix only slightly at their thermocline. [Note: It is very important that the jars are completely filled, otherwise the force of falling water does cause the waters to mix.]
Next, the experiment repeats with the reverse setup of warm on bottom and cold on top (Figure 5). Again, we run through possible outcomes, poll the probabilities, pull the partitions, and compare predictions to products. Because cold fresh water has the greater density, it sinks through the warm fresh water, which at the same time is rising because it has a lower density. As the waters completely turn greenish-blue, students are asked to explain why the fluids changed position during the second trial. Supporting questions lead to discussions about whether warm or cold water is denser, and whether a sensitive scale weighing equal volumes of warm and cold water could detect the difference in mass and density.
|Figure 5. Sample student prediction of warm and cold water mixing (reverse setup).|
As students clean up, I prepare an aquarium in the front of the classroom to demonstrate density difference between fresh and salt water. The Density Flow Model is a demonstration model for simulating the realistic flow of fluids in nature (e.g., weather systems, ocean circulation, and plate tectonics) invented by veteran science teacher Paul Herder and available through What If Scientific (www.whatifsci.com). The original model is an acrylic tank that measures 110 × 15 × 9.5 cm wide, holds 15 L of water, and has two removable clear partitions (a junior model half the size and half the cost is also available). The temperature of water is the same on both sides, so the only difference is a saline solution added to one side. This solution is created in front of the students by scooping 1 L of water out of the tank, adding blue food coloring and 100 g of table salt to it, stirring to mix in the dye and dissolve the salt completely, and then returning the mixture to one side of the tank.
Figure 6. Sample student prediction of
fresh and salt water mixing.
Students are asked how fresh and salt water will mix when the clear acrylic divider is removed (Figure 6). Because of the change in orientation from the horizontal separation used in the previous activity to vertical separation, there is an additional possible outcome: (a) salt on the bottom, horizontal halocline, fresh on the top, (b) fresh on the bottom, horizontal halocline, salt on the top, (c) the entire tank will uniformly mix with no halocline, or (d) salt and fresh sides will not comingle, leaving a vertical halocline. After polling the possible outcomes, the acrylic divider is removed. As the internal waves calm and the waters stratify, students judge their predictions. Because salt water has the greater density, it sinks dowanward under the warm fresh water, which at the same time is rising because it has less density.
The final demonstration models the Gulf Stream against the Labrador Currents. Given that cold fresh water is denser than warm fresh water, and that warm salt water is denser then warm fresh water, the “Titanic time” question is “Which is denser, warm salt water or cold fresh water?” The tank, now containing room temperature water separated at a halocline with saline submerged below the fresh water, is partitioned again. Red food coloring and ice cold fresh water to lower the salinity and temperature are added to one side along with a toy boat, which represents the doomed Titanic.
Figure 7. Sample student
prediction of cold fresh
and warm salt water mixing.
Students are asked if and how mixed will the waters be when the acrylic divider is removed (Figure 7), and they are evaluated on correctly ordering the red cold fresh water, clear warm fresh water, and blue warm salt water in the tank. I assemble the class around the tank and pull the divider. I invite students to look down into the tank. In the side view the thermocline is easily visible between the clear warm fresh water on top and the red cold fresh water below it, and the halocline and thermocline between the red cold fresh water in the midzone and blue warm salt water on bottom is easily visible. However, the top view completely masks the thermoclines and the halocline in a purple haze. It is worth waiting several minutes for strong stratification to surprise students as they step back. I then “explain” Captain Knapp’s reason for an iceberg in Titanic’s path: “The reason for this is that the cold, south-moving current actually underruns the warm surface water.” With only the tip of an iceberg above water, enough of it extends down to reach the south-moving currents of cold, fresh water to be propelled despite the north-moving current of warm salt water on the surface.
Presenting another example
To evaluate educational outcome, student lab reports are assessed with a rubric (which is accessible by clicking here) and a related but unique example (Bloom 1956): the emptying of the Mississippi River into the Gulf of Mexico. Students are asked to model the Mississippi Delta in profile, and explain how the season of the year is a factor, namely that in spring, the Mississippi is awash in melt water from snow and ice. Students should recognize its greater density due to lower temperature will make it underrun the warmer waters in the Gulf.
To “extend” the lesson with multiple intelligences, activities include reading and reporting on the poem “The Convergence of the Twain (lines on the loss of the Titanic)” by Thomas Hardy (1915):
And as the smart ship grew
In stature, grace, and hue
In shadowy silent distance grew the Iceberg too.
After students have read the entire poem through twice, they note any background knowledge that helps to connect to the people, animals, or object in the poem; for example, “Convergence” consists of eleven stanzas each of three lines, following an AAA rhyme pattern. Students next illustrate each stanza with the picture in their head of what is happening in the poem. Finally, they link textual evidence to background knowledge to judge what the poem is about ultimately.
To “elaborate” on how temperature and salinity cause the ocean’s “conveyor belt” effect, my class discussion focuses on thermohaline circulation and climate change. Starting with a clip from the 2004 eco-catastrophe movie “The Day After Tomorrow,” we watch the prediction that deep convection disruption caused by global warming ironically leads to a new Ice Age. The Gulf Stream becomes both colder and saltier as it reaches the Norwegian Sea. Polar air from Canada lowers the Gulf Stream’s average temperature and removes water by evaporation, leaving behind boreal briny water. As discussed, some water mixes to circle clockwise from Labrador to Canary Currents. But the increase in the current’s water density also causes open-ocean convection. By the time it approaches the coast of Newfoundland, or farther northeast in the Norwegian Sea, the increased salinity and lowered temperature makes it dense enough to sink directly down in a process is called “overturning.” Overturning leads to formation of a cold, salty deep water layer in the North Atlantic at depths of 1500–2000 m. This deep water current then slowly travels along avove the ocean floor to the Southern Hemisphere, with the return flow to the north occurring at the surface.
Changes in overturning can affect major climate change, especially with global warming melting more polar ice and adding more fresh water to the oceans (Bettwy 2004). While “The Day After Tomorrow” scenario was exaggerated, some scientists believe adding fresh water might have a dramatic effect on climate change. Models created by researchers funded by the National Science Foundation paleoclimate program suggest that at the end of the last Ice Age the north Atlantic deep water circulation system may have shut down because of melting glaciers and increased icebergs (NSF 2001). When fresh water mixes with the salty water in the north Atlantic, it makes the water less dense, which slows the overturning process and the ocean circulation. As a result, the reduced Gulf Stream would transport only about half as much heat northward, thereby cooling the northeastern United States and Western Europe titanically.
Erich Landstrom (firstname.lastname@example.org) is a teacher at Boynton Beach Community High School, 4975 Park Ridge Boulevard, Boynton Beach, FL 33426.
Bernstein, L., M. Schachter, A. Winkler, and S. Wolfe. 2004. Concepts and challenges: Earth Science. 4th ed. Parsippany, NJ: Pearson Education.
Bettwy, M. 2004. NASA Goddard Web Feature: Modeling ocean behavior: The key to understanding our future climate.
Bloom, B. 1956. Taxonomy of educational objectives. New York: David McKay Company.
Bybee, R.W. 1997. Achieving scientific literacy. Portsmouth, NH: Heinemann.
Dobson, K., J. Holman, and M. Roberts. 2001. Holt science spectrum: A physical approach. Austin, TX: Holt, Rinehart and Winston.
Hardy, T. 1914. Satires of circumstance, lyrics and reveries, with miscellaneous pieces. London: Macmillan and Co.
National Science Foundation (NSF). 2001. Melting glaciers diminished Gulf Stream, cooled western Europe during last Ice Age. www.nsf.gov/od/lpa/news/press/01/pr0192.htm.
Perlman, H. 2005. The water cycle: Water storage in oceans. U.S. Geological Survey water science basics. http://ga.water.usgs.gov/edu/watercycleoceans.html.
Titanic Inquiry Project. 2006. United States Senate inquiry memorandum for Senator Smith: Drift of ice on and near the grand banks. www.titanicinquiry.org/USInq/AmInq17KnappMemo04.html.