Oceanic Circulation: What Keeps the Ocean in Motion?
OverviewWhy is the ocean vital to our planet? There are many reasons, but one important one is that the ocean is a major player in regulating our weather and climate through currents. In this lesson plan, your students will model ocean currents with cups, water, and food coloring, and explore how temperature and density differences set deep ocean waters in motion to create a global oceanic circulation system.
- Understand the ocean's impact on global climate.
- Investigate and describe how temperature differences drive oceanic water movement.
- Explain how climate change can affect oceanic circulation systems.
NGSS AlignmentThis lesson helps students prepare for these Next Generation Science Standards Performance Expectations:
- MS-ESS2-6. Develop and use a model to describe how unequal heating and rotation of the Earth cause patterns of atmospheric and oceanic circulation that determine regional climates.
Science & Engineering Practices
Developing and Using Models. Develop and use a model to describe phenomena.
Planning and Carrying Out Investigations. Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
Analyzing and Interpreting Data. Analyze and interpret data to provide evidence for phenomena.
Disciplinary Core Ideas
ESS2.C: The Roles of Water in Earth's Surface Processes. Variations in density due to variations in temperature and salinity drive a global pattern of interconnected ocean currents.
ESS2.D: Weather and Climate. The ocean exerts a major influence on weather and climate by absorbing energy from the sun, releasing it over time, and globally redistributing it through ocean currents.
Energy and Matter. Within a natural or designed system, the transfer of energy drives the motion and/or cycling of matter.
Systems and System Models. Models can be used to represent systems and their interactions—such as inputs, processes and outputs—and energy, matter, and information flows within systems.
Scale Proportion and Quantity. Time, space, and energy phenomena can be observed at various scales using models to study systems that are too large or too small.
Materials per group of 3–4 students:
- Plastic cups, 9 oz. (2 of the same size)
- Laminated paper or stiff plastic sheet; available from Amazon. Plastic strips cut from the lid of clamshell boxes for fresh produce also work well.
- Food coloring (dark colors such as blue and green work better than light colors)
- Hot water
- Ice water
- Spoon for mixing
- Small flashlight
- Immersion thermometer; available from Amazon
- Aluminum tray
- Smartphone with a sensor app such as phyphox, available for free on
Google Play for Android devices (version 4.0 or newer) or from the App Store for iOS devices (iOS 9.0 or newer). Note: Phyphox does not support the light sensor on iOS devices. If you need the light sensor, you have to use Android devices for your experiment. Note that on some devices the light sensor is only updated when there is a coarse change of illuminance. This means that if the light intensity does not change or only changes slightly, the sensor appears to not record any data. The recording will continue once the light intensity changes again. If your experiments allows, it helps to wiggle the phone or the light source (e.g. flashlight) slightly to induce minimal reading fluctuations and keep the sensor active.
Materials for teacher preparation and demonstration:
- World map or globe
- Plastic cup
- Food coloring
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Background Information for TeachersThis section contains a quick review for teachers of the science and concepts covered in this lesson.
Water covers about 70% of Earth's surface. Seen from space, the blue of the oceans and the white of clouds are the dominant visual features of our planet. The ocean is of chief importance to us—it provides us with a major source of food, water (through the water cycle), and microscopic phytoplankton that live in the ocean produce more than 50% of the oxygen we breathe. In fact, more than 80% of life on Earth is in the ocean! In addition to that, the ocean is a major player in regulating our weather and climate. In short, the ocean is the life support system of our planet. Similar to a heart pumping blood, ocean water is in constant motion and transports life-sustaining heat, nutrients, and oxygen around the world.
This constant motion or water movement is driven by ocean currents. In general, there is a lot of water movement in the ocean. The most obvious examples—the ones we can see—are the waves and ripples on the water's surface that are generated by wind, or ocean currents due to tides. However, water can also be moved without wind or tides, which is what happens in the deep ocean. There, currents are set in motion by variations in water density caused by differences in temperature and salinity (amount of dissolved salt), a process called convection.
The water of the oceans is not uniform. Unequal heating by the sun and climatic processes create large-scale differences in ocean water temperature and salinity, as illustrated in Figures 1 and 2. As you might expect, ocean waters near the equator tend to be warmer than those at higher latitudes as these areas receive more solar radiation. Figure 1 shows sea surface temperature, coded in color (see legend).
Temperature on a heatmap is color coded, red areas are the hottest and purple areas are the coolest. The sea surface temperatures are hottest around the equator and gradually cool down the closer the water is to the poles.
Figure 1. Color-coded map of sea surface temperature in degrees Celsius.
Figure 2 shows global differences in ocean surface salinity. At the surface, in general, salinity is higher in equatorial regions and lower at high latitudes.
Map showing areas of high and low salinity in the worlds oceans. Areas of high salinity are represented in red and low salinity in dark blue. The area of highest surface salinity is in the Mediterranean Sea and extends outward into the Atlantic ocean from the coast of Western Europe to the Eastern coast of North America. The areas of lowest surface salinity are near the arctic circle and the waters around Alaska.
Figure 2. Color-coded map of sea surface salinity in PSU (Practical Salinity Units), which is similar to parts per thousand (ppt or 0/00). (Image credit: NASA's Goddard Space Flight Center Scientific Visualization Studio).
The density of water varies with temperature (warm water is less dense than cold water) and salinity (more salt makes water heavier). These two factors are the main driving forces behind the global ocean conveyor belt, also called thermohaline circulation, which is a huge water circulation system in the deep ocean, as shown in Figure 3. Currents begin near the pole in the North Atlantic, where the surface of the ocean gets cooled by the arctic temperatures. As sea ice forms, the water becomes saltier (the majority of the salt does not freeze into the ice, but remains in the liquid water below the ice). This water is now denser and will sink to the bottom of the ocean floor. This creates a current, as warm surface water moves in to replace the sinking cold water. The cold, deep water moves all the way south to Antarctica, then to the Indian and Pacific Oceans. Once the water reaches warmer regions, it warms up, becomes less dense, and rises to the surface. Eventually, it finds its way back to the North Atlantic where the whole cycle begins again. The completion of one full cycle is estimated to take about 1,000 years!
Figure 3. Global ocean conveyor belt. (Image credit: Courtesy NASA/JPL-Caltech).
What would happen if, due to climate change, temperature and salinity differences in the ocean became less pronounced? Scientists are concerned that heating the globe, which increases ocean temperatures and melts large quantities of polar ice, will lead to a decrease in ocean temperature and salinity differences. This could potentially have devastating effects on the ocean currents of the global conveyor belt.
In this lesson plan, you will do experiments with your students to see what happens when layers of water at different temperatures are brought together. You will monitor how temperature differences result in the movement of water using a mobile phone equipped with a sensor app and simulate how a decrease in temperature variances could affect ocean currents.