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

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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).

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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.

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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.

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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 calledbrine 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 theGlobal 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.

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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. • 3 beakers • 1 eye dropper or pipette • 35 g table salt per litre of solution prepared • A sheet of white paper to better observe the water layering in the container • 1 thermometer

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SETUP

Figure 3 Cold salty water is blue, cold freshwater is red and warm tap water is uncoloured.

1. Cold Saltwater Solution (Blue)

A This water is meant to represent both the salty cold Ocean Deep Water, as well as surface water that is salty and cold as a result of brine rejection from developing sea ice and frigid air temperatures. B Prepare before class by mixing 35g (2 and 1/3 tablespoons) of salt for each litre of water in a container and place in the freezer. Keep in freezer until ice starts to form, or, if freezing overnight, make sure to remove the container an hour or two before the lesson so that most of the ice thaws, but the temperature is kept near the freezing point. Add a few drops of blue food dye.

2. Cold Freshwater (Red)

A This water represents a fresh, more dilute, layer of ocean surface water. B Prepare ice from tap water in ice trays beforehand and add to tap water along with red food colouring.

3. Warm Tap Water (Not Coloured)

A Should be available to students during the lesson, this water is used to help illustrate the separation of the two other water types.

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PROCEDURE 1. Review the Background (pg. 3) with students, along with the necessary Vocabulary (pg. 3) prior to beginning the experiment. 2. In groups, brainstorm what changes in the ocean are experienced at deepening depths, right down to the ocean floor. Students can decide to experience this as a diver, an eider duck or other animal, as explorers in a submarine, etc. Consider temperature changes, salinity levels, visual changes, and currents. Have each group present to the class. 3. Handout the Experiment Instructions (pg. 10) and Worksheet (pg. 12). Refresh students on the concepts of salinity and density in relation to seawater and what drives ocean currents. Review ocean layer diagram (Figure 1). 4. Complete pre-activity questions on the worksheet. 5. Students collect required materials. 6. Follow experiment instructions. 7. At this point, explain that the blue salty layer represents the Arctic Ocean deep water (as seen in the diagram on the worksheet). Mention that the Atlantic Layer has been omitted for simplicity 8. Students will simulate conditions in winter in the Arctic Ocean and North Atlantic when the surface water loses heat, and the formation of sea ice results in brine rejection. 9. Next, students will simulate conditions where the surface layer is cold and much less saline (in this case, hardly saline at all). This scenario is meant to represent potential conditions in surface water with the release of freshwater due to climate change related glacier and ice-sheet melting. 10. When adding the red water, slowly, there should be a separation in colours. If everything has gone right, the red water should sit just atop the blue water. 11. Share the graphic of the Global Ocean Conveyor (Figure 2) with students. Discuss the importance of the circulation of surface water to the deep ocean. Why does it only occur near Antarctica and in the Arctic Ocean and North Atlantic? How might a freshening of the surface layer from melting glaciers and the ice sheets of Greenland affect the Arctic thermohaline circulation? This activity should illustrate that when less saline water is present in the surface layer of the Arctic Ocean, the strength of the convective current is much less strong. As the saline water that occurs when sea ice forms will be more dilute, there will be less vertical movement of surface water. It is the highly saline water formed along with sea ice that allows ocean surface water to sink to the deep ocean, and it is this thermohaline circulation driven by density and temperature differences that influences the entire ocean network of deepwater currents, called the Global Ocean Conveyor. Thermohaline circulation makes life possible in the deep ocean basins by bringing oxygen to the depths. The Global Ocean Conveyor is an incredibly important regulator of regional and global climate.

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DISCUSSION QUESTIONS 1. In words or a diagram, explain the thermohaline circulation of the Arctic Ocean. Ocean currents driven by water density, which is controlled by temperature and salinity. 2. What happens to CO2 during thermohaline circulation? The current through the Arctic Ocean sinks the CO2 dense waters to the bottom of the ocean floor. 3. How does thermohaline circulation influence the global climate? Surface waters are rich in CO2. Thermohaline circulation draws down water, along with the dissolved greenhouse gas, deep into the ocean and ocean floor, sequestering it. 4. Has your community noticed any changes in the environment that could affect thermohaline circulation? Elders and hunters in Hudson Bay have reported that currents are slowing down, there is more freshwater in the system and in some areas less sea ice extent could mean less brine rejection.

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EXPERIMENT INSTRUCTIONS

Figure 4 Cold salty water is blue, cold freshwater is red and warm tap water is uncoloured.

1. In small groups, spend 5 minutes brainstorming what changes in the ocean would be experienced at deepening depths, right down to the ocean floor. Decide to experience this as a diver, an eider duck or other animal, as explorers in a submarine, etc. Consider temperature changes, salinity levels, visual changes, and currents. Each group presents to the class quickly from their tables taking on their chosen character as they describe their experiences. 2. Review the diagram of the ocean layers (Figure 5). 3. Collect your materials:

• Clear receptacles, such as a graduated cylinder or glass bowl • Blue saltwater solution in a beaker, with enough saltwater ice to keep the solution near freezing • Red, cold tap water in a beaker • Eyedropper or pipette • Thermometer • Piece of white paper to be used to observe water layering and movement.

4. Fill the container with warm tap water. Do not fill it so high that you will be unable to reach the bottom of the water with your eyedropper or pipette. The water in the container needs to sit for at least 3 minutes at rest on the workspace before you can add the blue saltwater solution. 5. Record the temperature of the tap water, the blue saltwater, and the red icewater on the worksheet. 6. Before continuing with the experiment, predict what will happen when you add the blue saltwater solution. Record on the worksheet.

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7. Add the blue water 1 Formation of Sea Ice 0 slowly, down the Sea Ice 2 Brine Rejection Arctic Ocean side of the container, 200 Surface Layer instead of dropping it from a height. Repeat 400 3 Increased water density Atlantic Layer this a couple of times, 600 adding until there is a blue layer visible 800 at the bottom of the 1000 container. It may be 4 Water sinks as part of 2000 necessary to place a thermohaline circulation white piece of paper 3000 Deep Ocean Layer on one side of the container so that it is 4000 easier to observe the 5000 colours. Figure 5 There are three distinct water masses in the Arctic Ocean characterized by their different temperature and salinity differences.

The blue salty layer represents the deep ocean layer. The Atlantic layer is not included in this setup due to the difficulty of establishing the layers without substantial mixing occurring. 8. Did the observations meet the predictions? Record your results on the worksheet. 9. Before adding the red water, hypothesize the results on the worksheet. 10. Add the red water as you did the blue saltwater, slowly and down the side of the container. Is there a difference? Did the observations meet the predictions? Record your results. 11. Review the graphic of the Global Ocean Conveyor (Figure 2). Discuss the importance of the circulation of surface water to the deep ocean. Why does it only occur near Antarctica and in the Arctic Ocean and North Atlantic? How might a freshening of the surface layer from melting glaciers and the ice sheets of Greenland affect the Arctic thermohaline circulation?

Image 3 The cold blue salty water is denser than the Image 4 While denser than the warm tap water, the warm tap water so it sinks to the bottom. red freshwater is not as dense as the blue saltwater so it rests it the middle.

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WORKSHEET

PRE-ACTIVITY QUESTIONS: 1. Define density:

2. Define thermohaline circulation:

3. Describe the characteristics and origin of each water layer in the Arctic Ocean:

A Surface Layer:

B Atlantic Layer:

C Deep Ocean Layer:

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ACTIVITY QUESTIONS You are modelling the layering of water in the Arctic Ocean. You will be using water in this activity with the characteristics listed below. Add your temperature reading next to each description:

Cold Saltwater Solution (Blue): 4. This water is meant to represent both the salty cold Ocean Deep Water, as well as surface water that is salty and cold as a result of brine rejection from developing sea ice and frigid air temperatures. Preparation: mixed 35 grams of salt per litre of water. Added blue food dye.

• Temperature: ______ºC

Cold Freshwater (Red): 5. This water represents a fresh, more dilute, layer of ocean surface water. Preparation: Added ice cubes and red food dye to tap water.

• Temperature: ______ºC

Warm Tap Water (Not Coloured): 6. Used to illustrate the movement of the water.

• Temperature: ______ºC

7. You will be adding the cold saltwater (blue) solution to the surface water to imitate the formation of cold salty water when sea ice forms in the fall and winter. What do you think will happen when you add this water and why?

8. What happened when you added the cold saltwater solution? Did it agree with your prediction?

9. Next, you will be adding the cold freshwater (red) solution to the surface water to imitate a fresher surface water layer due to melting glaciers and ice sheets. What do you think will happen when you add this water and why?

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10. What happened when you added the cold freshwater solution? Did it agree with your prediction?

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EXTENSIONS

Move it into the community Interview elders or hunters. Find out how thermohaline circulation affects Arctic animals, the climate or the Inuit population. Create a presentation with your findings, this could be in a media format, research paper, video, music, etc.

Hands-on Learning Create a diorama or model demonstrating the processes involved in thermohaline circulation in the Arctic Ocean.

LINKS  Why We Care: Sea-Ice Basics Overview of sea ice concepts such as formation, density, brine rejection, extent, and circulation which can be used by students or teacher to fulfill prerequisites for the lesson. https://arcticeider.com/links/thc01

SOURCES Cover Image by Joel Heath

ATTRIBUTION

PRIMARY • Karl Hardin CONTRIBUTORS • Rian Dickson • Joel Heath • Jackie Kidd • Misha Warbanski • Evan Warner

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