Lesson Six: Surface Ocean Currents Which factors in the earth system create gyres? Why does pollution collect in the gyres? Objective: Develop a model that shows how patterns in atmospheric and ocean currents create gyres, and explain why plastic pollution collects in them. Introduction: Gyres are circular, wind-driven ocean currents. Look at this map of the five main subtropical gyres, where the colors illustrate enormous areas of floating waste. These are places in the ocean that accumulate floating debris—in particular, plastic pollution. How do you think ocean gyres form? What patterns do you notice in terms of where the gyres are located in the ocean? The Earth is a system: a group of parts (or components) that all work together. The components of a system have different structures and functions, but if you take a component away, the system is affected. The system of the Earth is made up of four main subsystems: hydrosphere (water), atmosphere (air), geosphere (land), and biosphere (organisms). The ocean is part of our planet’s hydrosphere, but it is also its own system. Which components of the Earth system do you think create gyres, and why would pollution collect in them? Activity 1 Gyre Model: How do you think patterns in ocean currents create gyres and cause pollution to collect in the gyres? Use labels and arrows to answer this question. Keep your diagram very basic; we will explore this question more in depth as we go through each part of the lesson and you will be able to revise it. www.5gyres.org 2 Activity 2 How Do Gyres Form? A gyre is a circulating system of ocean boundary currents powered by the uneven heating of air masses and the shape of the Earth’s coastlines. These gyres occur north and south of the equator. There are five main gyres where trash accumulates in the ocean: The North Pacific Gyre, The South Pacific Gyre, The North Atlantic Gyre, The South Atlantic Gyre, and The Indian Oceanic Gyre. An ocean current is a river of water moving in the ocean; they can be at the surface, as well as deep in the ocean. Surface ocean currents are called convection currents; they form due to differences in temperature. Surface ocean currents move as wind drags on the ocean’s surface. The water moves and builds up in the direction that the wind is blowing. Differences in air temperature cause wind. Air masses move from areas of high pressure toward areas of low pressure. In general, air masses are colder at the poles and have higher density (high pressure) than air at the equator, which is warmer and has a lower density (low pressure). These differences in temperature and pressure cause colder air masses to move toward the equator, while warmer air masses move from the equator toward the poles. If the Earth remained still, the atmosphere would only circulate between the poles and the equator. However, the Earth spins on its axis, resulting in the Coriolis Effect, which describes the ground moving at a different speed than an object in the air. The Earth makes a complete rotation about its axis approximately once every 24 hours, and has a greater circumference at the equator than at the poles—so it spins faster at the equator than at the poles. As the Earth spins, it causes the earth’s air masses to curve toward the right in the Northern Hemisphere (in a clockwise motion) and to curve toward the left in the Southern Hemisphere (in a counterclockwise motion). This sets up global wind patterns that circulate air masses around the Earth and influence surface ocean currents. The air masses pull on the surface ocean water and drag it in the direction that the air is moving. This combination of factors create the gyres. Use what you learned from this reading to revise your Activity 1 model (page 2). www.5gyres.org 3 Activity 3 Modeling The Coriolis Effect: What effect might the Earth’s spinning have on the way air and water move? Model a fluid (either water or air) traveling across the Earth. Materials • Round fruit with a peel and without a stem, such as an orange or melon* • A globe or world map • Dry erase markers, in different colors Directions 1. Form into partners or groups. 2. Refer to the map, and use the dry erase to mark: a. Arctic Circle and North Pole b. Antarctica and the South Pole c. Equator 3. One student holds the fruit still, while another marks a line from the North Pole to the Equator. Observe what happens as the line is drawn from the colder area of high pressure to the warmer area of lower pressure. How does the line appear to move? 4. The student holding the fruit slowly turns it in a circle from left to right, while a second line is drawn from the North Pole to the Equator, using a different colored marker. Watch the line from the perspective of the North Pole. What happened to the line? www.5gyres.org 4 5. One student holds the fruit still, while another marks a line from the South Pole to the Equator, using another colored marker. Observe what happens as the line is drawn from the warmer area of high pressure to the colder area of lower pressure. How does the line appear to move? 6. The student holding the fruit slowly turns it in a circle from left to right, while a second line is drawn from the South Pole to the Equator, using a different colored marker. Watch the line from the perspective of the South Pole. What happened to the line? What pattern do you see among all of your data? Optional: Watch “Surface Ocean Currents” video to learn more. www.5gyres.org 5 Activity 4 Modeling Ocean Currents: Develop and use a model to describe how unequal heating, the rotation of the earth, and land boundaries causes patterns of oceanic circulation that create gyres in the ocean system. Carefully review this thermal map of the Earth. What pattern do you see in the earth’s temperature? Which zones of the earth are the warmest? The coldest? We will explore how the different temperatures of the earth affect the ocean by conducting an investigation using an ice cube and water. What happens when different temperatures of water come in contact with each other? What do you think will happen when you place the ice cube in the cup of room temperature water? www.5gyres.org 6 Materials • Ice cube • Clear glass filled with warm water • Liquid food coloring Directions 1. Place the ice cube in the water. 2. After one minute, add a drop of food coloring to the top of the ice cube. Observe the glass. Which direction is the colored water moving? Moving water within another body of water is called a current. Is this what you have created in the glass? Draw and describe your model below. One characteristic that we use to describe matter is its density. Density is the amount of matter in a given volume of an object. When we compare the density of one piece of matter to another, we generally use the idea of heaviness. Objects that are denser than their surroundings tend to sink, while objects that are less dense than their surroundings tend to float. In denser objects, molecules tend to be closer together, while a less dense object’s molecules tend to be further apart. Did the ice cube float or sink when it was placed in the water? Based on your response, how would you expect the molecules to look inside the ice cube, as compared to the room temperature water—would they be closer together or farther apart? www.5gyres.org 7 Temperature affects the density of a substance. Temperature measures the kinetic energy or movement of molecules within a substance. Would you expect molecules to move faster or slower at higher temperatures? Would a substance at a higher temperature be denser or less dense than at a lower temperature? We would expect ice to be denser than liquid water because it has a lower temperature. The density of ice is unique because it does not follow this general rule. Water molecules contain a type of bonding, called hydrogen bonding. The hydrogen bonds hold water molecules slightly farther apart from one another as solid ice forms, in what is known as a crystal lattice formation. This is why ice is less dense than water. It is a good thing too because life as we know it would not exist if ice did not float. The thin layer of ice that floats atop many lakes and bodies of water during winter provides warmth for the organisms that live in the water and is an important part of maintaining our ecosystem. Ice melt is fresh water that melts from a block of ice in the ocean. We demonstrated this in your experiment by placing a block of ice into room temperature water. What happens to the temperature of ice melt as it interacts with room temperature water? Use evidence from your experiment to explain your reasoning. Which was denser, the ice melt or room temperature water? Use evidence from your experiment to explain your reasoning. www.5gyres.org 8 A current should have formed in the cup as the ice melted. Describe both the temperature and density of water molecules that you would expect to sink versus those that expected to rise. Think about the thermal map of the Earth and the experiment of the ice being placed in the glass of water.