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Thermohaline Circulation ACTIVITY TIME Sea Ice Oceanography Unit I: Sea Ice Dynamics Thermohaline Circulation ACTIVITY TIME 90 minutes LEARNING OUTCOMES • Describe the concept of the global ocean conveyor and how it relates to the Arctic Ocean. • Describe the role of sea ice formation in thermohaline circulation and its relation to salinity and density differenc- es. OVERVIEW When sea ice forms in the fall, salt is expelled into the water below. This, along with frigid temperatures, changes the density of the water and causes it to sink. The movement of this water, which sometimes sinks all the way to the ocean floor, only happens where ocean water is really cold, like in the Arctic Ocean. This mechanism is an important part of what drives global ocean circulation. In this lesson, which can be done as a demonstration or as a hands-on activity, students simulate thermohaline circulation – “thermo” for heat and “saline” for salt – on a smaller scale. FLOW 1. Explore background section 3. Conduct experiment; record 2. Student brainstorming activity results and pre-activity questions 4. Analyze a graphic 5. Discussion questions ArcticSeaIce.com THE ARCTIC SEA ICE EDUCATIONAL PACKAGE TEACHER VERSION 1 Thermohaline Circulation B CONTENTS Student Overview 2 Experiment Instructions 10 Background 3 Worksheet 12 Activity Description 6 Extensions 15 Preparation 6 Links 15 Procedure 8 Sources 15 Discussion Questions 9 Attribution 15 STUDENT OVERVIEW WHY? HOW? To understand seawater density and how it Read background section influences ocean circulation (creates open ocean and coastal polynyas) and recognizing sea ice as an important control factor in the global climate. Pre-activity questions WHAT? • Salt water density Perform experiment • Relationship between frozen seawater and freshwater Post-discussion questions • Ocean circulation and its effects • Sea ice and the global climate Image 1 Salt is pushed out into the water as sea ice forms in the late fall (J.Heath). 2 TEACHER VERSION THE ARCTIC SEA ICE EDUCATIONAL PACKAGE ArcticSeaIce.com Sea Ice Oceanography Unit I: Sea Ice Dynamics BACKGROUND VOCABULARY arctic Deep Ocean Layer: Large ocean currents are driven, for the most part, by wind and water A water mass in the Arctic density differences. Wind is the most important driver of large surface Ocean characterized by currents such as the Gulf Stream. On the other hand, water density salty, low temperature sea- differences are important drivers for ocean currents that transport water water that generally begins from the ocean surface to near the ocean bottom. 900m below the surface. arctic Ocean Surface The density of seawater depends on its temperature and its saltiness. Layer: The surface layer of Cold salty water is denser and less buoyant than warmer fresher seawater. water in the Arctic Ocean, Less dense layers of water “float” on layers with greater density, similar to characterized by a rela- oil (less dense) floating on water (more dense). This concept is easy if you tively low salinity due to freshwater runoff, generally extending down to remember that seawater has a higher density when it’s colder and saltier. 200m below the surface. There are distinct layers that occur in most ocean basins, separated atlantic Layer: The middle water because of temperature and salinity differences. The Arctic Ocean is mass in the Arctic Ocean, which is actually made up of three quite distinct layers of water. warmer than the surface layer but is much saltier, and thus, has a higher 1 Formation of Sea Ice density. It originates from ocean 0 Sea Ice water carried northwards by the 2 Brine Rejection Arctic Ocean North Atlantic Current, an extension 200 Surface Layer of the Gulf Stream. Generally found between 200m to 900m below the 400 surface. 3 Increased water density Atlantic Layer 600 Brine rejection: A process that 800 occurs during sea ice formation where salt ions are forced out of 1000 developing sea ice crystals and into 4 Water sinks as part of the water below. 2000 thermohaline circulation Density: A measurement of the 3000 Deep Ocean Layer mass per unit volume of a material 4000 or object. 5000 global Ocean Conveyor: The inter- connected path of surface and deep FIGURE 1 There are three distinct water masses in the Arctic Ocean characterized by their different temperature and salinity differences: the Polar water currents that connect all of Surface Water, the Atlantic Layer, and the Deep Ocean Layer. the world’s oceans, transporting matter and energy across ocean THE ARCTIC OCEAN SURFACE LAYER basins. Also referred to as thermo- haline circulation. This is the top layer of the Arctic Ocean and extends down about 200m. Sequester: To store, in the context The surface water here has some of the highest freshwater content of the carbon cycle, it is when car- of any ocean in the world. This is partly because the Arctic Ocean is bon is unavailable to circulate with- nearly landlocked and partly because there are many large river basins in the carbon cycle, and remains in Europe, Asia, and North America that drain tremendous volumes of unavailable for long periods of time, freshwater into the Arctic Ocean each year. The freshwater layer sits atop such as at the bottom of the ocean. the denser layers below it. ArcticSeaIce.com THE ARCTIC SEA ICE EDUCATIONAL PACKAGE TEACHER VERSION 3 Thermohaline Circulation B ATLANTIC LAYER Below the low salinity surface layer, about 200m deep in the water column, there is a warmer layer of water called Atlantic Layer. Water in this layer originates from the Atlantic Ocean and is very salty. The high salinity of this water makes it is denser than the surface layer and thus sits below it. The transition between the Arctic Ocean Surface Layer and the Atlantic Water is defined by sharp changes in temperature and salinity, called the thermocline and pycnocline. DEEP WATER FORMATION DEEP CURRENT SURFACE CURRENT . FIgure 2 The Global Ocean Conveyor is an interconnected path of currents that transports water between all of Earth’s ocean basins. 4 TEACHER VERSION THE ARCTIC SEA ICE EDUCATIONAL PACKAGE ArcticSeaIce.com Sea Ice Oceanography Unit I: Sea Ice Dynamics THe DeeP OCEAN LaYer Below the Atlantic Layer, starting at about 900m below the surface and extending to the seafloor, the water gets colder and saltier the deeper it gets. This water is called Arctic Deep Water. It moves very slowly around the deep ocean basins of the Arctic Ocean before eventually leaving the Arctic Ocean through the Fram Strait, a relatively deep trough located between Greenland and the Svalbard Archipelago. THERMOHALINE CIRCULATION What causes surface ocean water to sink to the deep ocean? With the formation of sea ice in the North Atlantic and the Arctic Ocean in early fall, the ocean surface water becomes more saline as salt is pushed out of the developing sea ice into the water below. This process is called brine rejection. The increased salinity of surface water in the North Atlantic and Arctic Ocean combined with the low temperature of the seawater causes the surface layer of water to become denser than the underlying layers of seawater and therefore, to sink. This water movement is called thermohaline circulation, “thermo” for heat and “saline” for salt. Remember, it is these temperature and salinity differences, both of which determine the density of seawater, that drive the exchange of ocean water between the surface and the deep ocean. This thermohaline movement of water helps drive what is called the Global Ocean Conveyor, which is the name for the connected system of surface and deep sea currents that connect all of the world’s oceans. In fact, the Arctic Ocean is an extremely important area of exchange between the surface and the deep bottoms of the world’s oceans. The surface water brings oxygen to the deep and carries a large amount of dissolved carbon dioxide, which is an important greenhouse gas, sequestering it near the ocean floor. The opposite movement of water, from the deep ocean to the surface, occurs in certain areas of the world’s oceans as well. This kind of water movement, from the deep ocean to the surface, is called upwelling, and it brings organic nutrients to the surface that plankton and other microorganisms rely on for nutrition. Much more freshwater is flowing into the Arctic Ocean than a century ago due to climate change. In particular, the freshwater ice sheets of Greenland are melting much faster than scientists had originally predicted. As more and more freshwater flows into the Arctic Ocean and North Atlantic, the ocean surface layer becomes diluted. Some scientists have been concerned that this may affect the thermohaline currents in these waters. Increased freshwater could mean that the surface layer will not sink as often or as deep. This could drastically affect life in the Arctic. Animal habitats and safe ice conditions for hunting are at risk if salinity levels of the ocean surface continue to change. ArcticSeaIce.com THE ARCTIC SEA ICE EDUCATIONAL PACKAGE TEACHER VERSION 5 Thermohaline Circulation B ACTIVITY DESCRIPTION This activity illustrates the effects of salinity and temperature on the density of water, and how density affects the movement of ocean water from the surface to the ocean depths. Students will use blue food dye mixed with cold salty water to imitate the Deep Ocean Layer and the cold salty water created by sea ice formation. Red food dye mixed with cold freshwater will imitate the more dilute surface waters. Armed with this understanding students will investigate how likely it is that the thermohaline circulation will slow down. PREPARATION MATERIALS • Two different colours of food dye - blue food dye represents cold, salty water, and red for the less saline water. • Clear receptacle (graduated cylinders or Mason jars work well) • Access to a freezer before the activity • Ice trays • Access to warm water during the lesson Per group Image 2 Simple materials for the experiment.
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