Ocean Currents: Keeping the heat down

Transcription

1 Biology Ocean Currents: Keeping the heat down Ocean currents have always been important to humans from the voyages of discovery in past centuries through to shipping and commercial fishing today. But their ability to transport vast quantities of heat that have been absorbed into ocean waters means currents play a role on our planet far beyond just these direct interactions with people. In this lesson you will investigate the following: What are ocean currents and how do they form? Why are salinity and temperature so important? What are the North Atlantic chimneys? What is the link between ocean currents and climate? Let s set sail on a voyage of discovery! This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to

2 Introduction: Ocean Currents Despite expectations about global warming, average air temperatures today are not much higher than they were in 2000 the atmosphere, at least, is much cooler than computer models predicted. At the start of the millennium climate scientists thought temperatures would rise steadily at about 0.2 C per decade. That might not seem a lot, but to heat the entire atmosphere even a little takes a lot of energy. But the predicted warming hasn't happened. On the other hand, other indicators of global warming haven't taken a break. Ocean temperatures, for example, have continued to rise, making scientists think that the ocean has been absorbing the heat that they thought would go into the air. But no-one knew how that could happen until now. The first clue came from the mid-1990's when scientists realized that there was a dramatic increase in the winds blowing across the Pacific from South America towards Indonesia. In fact, for the past 20 years these winds have been 30% stronger than the longterm average. Factoring this into their climate models they saw a major increase in the amount of heat being drawn down into the ocean. The blustering winds were churning up the water and driving the heat into the cooler waters below. After nearly two decades of this it s no surprise that we are registering lower air temperatures than predicted. Scientists still don t know why the Pacific winds have picked up so much but they do know that these winds come and go in a decades-long cycle. Soon the winds are due to calm again and we can expect atmospheric temperatures to start rising once more. Whatever the future holds, the discovery clearly shows how important the oceans are to climate across the whole planet. Read the full Cosmos Magazine article here.

3 Question 1 Identify: Label each of the Earth's five oceans on the map below. The oceans are: Arctic Ocean Atlantic Ocean Indian Ocean Pacific Ocean Southern Ocean

4 Gather: Ocean Currents When you go to the beach do you sometimes think how exciting it would be to find a message in a bottle? Every now and then it happens. In 2013 a person walking on a beach in Croatia found a bottle with a message: "Mary, you really are a great person. I hope we can keep in correspondence. I said I would write. Your friend always, Jonathon, Nova Scotia, 1985". The bottle never reached Mary, but it did go a very long way. Jonathon had kept his promise to write to Mary, though you might wonder how much he really wanted to stay in touch.

5 Question 1 Trace: Nova Scotia and Croatia are marked in the map below. Given the major ocean current shown, draw a likely route for Jonathon's bottle. Ocean Conveyor Belt Being in a bottle, Jonathon's message floated to Croatia it was blown by the wind and carried by currents running across the ocean's surface. But not all ocean currents are on the surface there are also deep currents that flow along the ocean floors. 1:10

6 Question 2 Question 3 Remember: Which of the following is true? Wind and ocean currents move heat from the poles to the equator. Ocean currents alone move heat from the equator to the poles. Wind and ocean currents move heat from the Recall: The video refers to which of the factors below as determining the density of ocean water? Wind Climate Salt Temperature equator to the poles. Wind alone moves heat from the poles to the equator. The Ocean Conveyor Belt stretches in a continuous cycle around the Earth's oceans. In some sections, where it is a surface current, it is warm. In other sections it is a cold current on the ocean floor. It is estimated that water takes 1,000 years to complete a full cycle. The map below shows the Ocean Conveyor Belt, representing warm surface sections in red and cold deep sections in blue.

7 Question 4 Track: Use the map to complete the table below, tracking the route of the Ocean Conveyor Belt. Start in the North Atlantic. For row 2, current depth, select between: rising falling bottom surface For row 3, water temperature, select between: cooling warming cold warm Ocean North Atlantic Atlantic Indian (Branch 1) Pacific (Branch 2) Current depth bottom surface Atlantic Water temperature cooling warming Wind and water density drive currents Wind is the main factor driving the warm, surface sections of the belt, but deep sections are powered by differences in water density. Dense water sinks and less dense water rises. At certain locations on Earth conditions are such that they create differences in water density that act like pumps, keeping the conveyor belt moving. The main factors that make water more or less dense are temperature and salinity: cold water is denser than warm water saline water is denser than fresh water Let's look a little more closely at these factors. Temperature Everyone knows that it's hotter in the low latitudes, near the Equator, and colder at the high latitudes, near the poles but why? The main reason is illustrated in the diagram below. Sunlight striking the Earth near the North Pole (a) and on the Equator (b) has the same amount of warming energy, but at b the sunlight is concentrated into a small area while at a it is distributed over a much larger area, because the Earth's surface curves away from the sunlight here. The higher the latitude the greater the surface area that the sunlight has to warm, so it's colder.

8 Question 5 Question 6 Remember: Sunlight entering the Earth's atmosphere near the poles has less energy than sunlight entering the Earth's atmosphere near the equator. True False Think: Regions near the South Pole receive much more sunlight than regions at the same latitudes near the North Pole. True False Salinity Salinity is the measure of how much salt there is dissolved in water. The average salinity of ocean water is 35 grams per kilogram (g/kg). The main factors influencing ocean salinity are: Lowering salinity freshwater input from rivers, rain and snow, and seasonal snow and ice melt. Increasing salinity high evaporation, particularly in equatorial waters where the sunlight is most intense. Temperature and salinity data from the ocean surface to 1500 m depth for two locations. Question 7 Question 8 Interpret: The surface waters at location A have: High temperature and high salinity Match: In light of your answer to Question 7, is location A more likely to be in equatorial or polar waters? Explain your answer. High temperature and low salinity Low temperature and high salinity Low temperature and low salinity

9 Question 9 Question 10 Interpret: The surface waters at location B have: High temperature and high salinity Match: In light of your answer to Question 9, is location B more likely to be in equatorial or polar waters? Explain your answer. High temperature and low salinity Low temperature and high salinity Low temperature and low salinity The North Atlantic "chimneys" Imagine waterfalls over three kilometres high and 15 kilometres wide! They exist! but not on land. In a small area in the North Atlantic up to a dozen so-called "chimneys" form, plunging cold, dense, saline water straight down to the ocean floor. While such chimneys also form in the Southern Ocean, those in the North Atlantic create one of the main engines driving the Ocean Conveyor Belt. The main factor creating the chimneys is ice formation. Ice is made from pure water no salt so as ice forms in these cold waters it leaves the surrounding seawater highly saline. The increased density of this salty water makes it sink, drawing in more water from the ocean surface and sending it to the bottom to start its journey around the world. Question 11 Explain: In your own words, describe how the North Atlantic chimneys are formed.

10 Process: Ocean Currents Map showing the average surface temperature of the world's oceans. Question 1 Question 2 Describe: Examine the map of ocean surface temperature above and describe how the temperature varies with latitude from the Equator to the poles. Use the scale to give a rough estimate of the amount of temperature variation. Analyze: Now focus on the temperature variation along the eastern coast of North America and in the North Atlantic. What evidence can you see for the presence of the Ocean Conveyor Belt in this region?

11 Map showing the average surface salinity of the world's oceans. Question 3 Question 4 Describe: Examine the map above and describe how the surface salinity of the Pacific Ocean varies with latitude. Is it highest at the Equator, in middle latitudes or near the poles? Where is it lowest? Explain: How could you explain the variation you described in the previous question in terms of the combined effects of evaporation and rainfall? Hint: In the Pacific Ocean, rainfall is consistently high at the Equator but is outweighed by evaporation in middle latitudes. Ocean currents and climate As discussed in the Cosmos Magazine article, the oceans absorb large quantities of heat from the atmosphere. Then currents can transport the heat over vast distances. A great example of this is the Gulf Stream the part of the Ocean Conveyor Belt that carries warm water across the North Atlantic towards Europe. 1:08

12 Question 5 Calculate: Use the speed and distance figures for the Gulf Stream provided in the video to calculate the time it takes for water to travel from one end of the current to the other. Round your answer to the nearest whole number of days. Hint: To convert from metres per second (m/s) to kilometres per hour (km/h), multiply by 3.6. Question 6 Compare: The video also states that water in the Gulf Stream flows at the rate of 100,000,000 m /s. By contrast, the average flow rate at the mouth of the Amazon River is 175,000 m /s. 3 How many times larger is the Gulf Stream compared to the Amazon River in terms of flow rate? Round your answer to the nearest whole number. 3 The map shows the locations of Nuuk in Greenland (1) and Sandviksberget in Norway (2). The graphs show average monthly minimum (blue) and maximum (red) temperatures for the two towns. Question 7 Infer: The towns of Nuuk in Greenland and Sandviksberget in Norway are located at about the same latitude. 1. Compare their average monthly temperature data as shown in the graphs above. Which town has warmer weather and by how much? 2. Suggest a reason for this difference based on your understanding of the Gulf Stream.

13 Question 8 Speculate: Take another look at the global ocean temperature and salinity maps above and find one or more unusual variations or anomalies in the data. For example, in the temperature map the arm of warm water along the east coast of North America the Gulf Stream is an anomaly because most of the water at this latitude is cooler. Suggest possible explanations for the anomalies you find based on everything you've learnt in this lesson. If you have time you can research the possibilities. If you find an anomaly that you can't explain, note why the usual factors don't seem to apply. Hint: Remember the main factors: sunlight, rainfall, snowfall, rivers, evaporation, freezing and the melting of ice, currents.

14 Apply: Ocean Currents Experiment: Modelling ocean currents Background The main driving force behind thermohaline circulation is the difference in water densities, mostly brought about by differences in temperature and salinity. Aim To demonstrate how salinity differences create thermohaline circulation. Hypothesis Question 1 Predict: When salty water and fresh water come together in the same container... [complete the sentence and explain why you think this will occur].

15 Materials 1 oblong see-through plastic container 20 cm long x 14 cm wide x 10 cm deep is a good size, but other sizes will work too. You need a minimum 8 cm depth. 1 piece of stiff card or plastic large enough to form a dividing wall across the container Scissors and/or Stanley knife Blu-Tack, putty, or similar for water-proof seals Food dye Two beakers or jugs, at least one with a measuring scale Teaspoon Salt Procedure This experiment is best done in small groups we suggest 3 students per group. 1. Measure how much water it takes to fill the container to a centimetre or so below the rim. 2. Halve the water between the two beakers or jugs. 3. Add 1 heaped teaspoon of salt per 200 ml to one of the beakers and stir till dissolved. 4. Add enough food dye to the beaker with the saline solution to make it strongly coloured. 5. Cut the card or plastic so it fits snugly into the container, dividing it into two equal sections as illustrated above. 6. Cut 2 holes approximately 1 cm diameter in the card. Put one in the middle about 1 cm from the base of the card, and the other in the middle so it will be about 2 cm under the water level. 7. Plug or cover the holes with card, plastic and/or putty. You want a good seal but also you must be able to remove the plugs easily while under water. 8. Place the card in position in the container and seal the edges with putty. 9. Pour the fresh water into one end of the container and the dyed saline solution into the other. This must be done AT THE SAME TIME so the dividing card stays in position. 10. Remove the plugs at the same time. 11. Observe for up to 4 minutes. Take notes and sketch, video and/or photograph what you see. Results Question 2 Present the results from your experiment in the project space below. Discussion Question 3 Describe: Summarize what happened when the holes between the section were opened. Was this consistent with your hypothesis?

16 Question 4 Reflect: How well did the experimental setup work? How would you improve it if you were to do the experiment again? Question 5 Calculate: A heaped teaspoon of salt is about 9 g. You added 1 heaped teaspoon for every 200 ml of water so what was the salinity of your solution in g/kg? Note: one litre of water has a mass of 1 kg. How does this compare to the average salinity of seawater 35 g/kg? Question 6 Discuss: How well does the experiment model thermohaline circulation in the ocean?

17 Career: Ocean Currents If you've ever stood on the beach and enjoyed the cool sea breeze you already have an idea of how the oceans affect weather and climate. Matthew England, an oceanographer and climate scientist at the University of New South Wales, knows this better than most. Growing up in Sydney, Australia, Matthew always adored the sea. At first he wanted to become a marine biologist, thinking it would allow him to spend more time surfing. But he didn t study biology in school. The field of oceanography, however, combined his love for the ocean with his talent in maths and physics. Matthew knew he d found his calling. Oceanographers study everything about the oceans, from currents through to nutrient cycles and ecosystems. Matthew focuses on large-scale ocean currents and uses sophisticated computer models to understand how they affect climate. The oceans, which cover more than 70% of the Earth s surface, have a great capacity to store heat, says Matthew. And although their average depth is over 4000 m, the upper metre can store as much heat as the whole atmosphere! The absorption of heat by the oceans creates an apparent slowing in global warming but it doesn't solve the problem, Matthew warns. For example, water expands when heated so warmer oceans will add to rising sea levels. Matthew spends most of his time at work brainstorming with his research group as they plan out their next experiments his favourite part of the job. He also spends a lot of time analyzing data from both direct observations and satellite images. The ocean is always on Matthew s mind, even when he isn t working. He is an avid body-surfer and relishes any chance to enjoy the waves. Question 1 Consider: What feature of the ocean do you find most intriguing? If you could study anything about the ocean what would it be, and why?

18 Cosmos Lessons team Lesson author: Jim Driscoll Introduction and profile author: Yi-Di Ng Editor: Jim Rountree Art director: Wendy Johns Education director: Daniel Pikler Image credits: NASA, World Ocean Atlas, Google Earth, istock, Shutterstock Video credits: Kurz Gesagt In a Nutshell, YouTube