Earth and space sciences Ocean Currents
Ocean currents have always been important to humans – from the voyages of discovery in past centuries to shipping and commercial fishing today. But with their ability to transport vast quantities of heat, currents play a role far beyond these direct interactions with people.
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 www.cosmoslessons.com 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 밄rst clue came from the mid-1990's when scientists realized that there was a dramatic increase in the winds blowing across the Paci밄c from South America towards Indonesia. In fact, for the past 20 years these winds have been 30% stronger than the long-term 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 Paci밄c 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.
1 Question 1
Identify: The location of the Paci밄c Ocean is shown on the world map below. Drag the names of the remaining four oceans to their correct locations.
2 Gather: Ocean Currents
When you go to the beach do you sometimes think how exciting it would be to 欭nd 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.
3 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 彐oated 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 彐ow along the ocean 彐oors.
0:00 / 1:10
Credit: The Gulf Stream Explained by In a Nutshell – Kurzgesagt (YouTube).
4 Question 2
Notes: Use this space to take notes for the video.
Note: This is not a question and is optional, but we recommend taking notes – they will help you remember the main points of the video and also help if you need to come back to answer a question or review the lesson.
Question 3 Question 4
Remember: Which of the following is true? Recall: The video refers to which of the factors below as determining the density of ocean water? Wind and ocean currents move heat from the equator to the poles Temperature
Wind and ocean currents move heat from the Wind poles to the equator
Ocean currents alone move heat from the Salt equator to the poles Climate Wind alone moves heat from the poles to the equator I'm not sure
I'm not sure
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 彐oor. 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.
5 Question 5
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
Indian Paci欭c (Branch Ocean North Atlantic Atlantic Atlantic (Branch 1) 2)
Current depth bottom surface
Water cooling warming temperature
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 dierences in water density. Dense water sinks and less dense water rises. At certain locations on Earth conditions are such that they create dierences 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.
6 Question 6 Question 7
Remember: Sunlight entering the Earth's atmosphere Think: Regions near the South Pole receive much more near the poles has less energy than sunlight entering the sunlight than regions at the same latitudes near the North Earth's atmosphere near the equator. Pole.
True True
False False
I'm not sure I'm not sure
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 in彐uencing 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 8 Question 9
Interpret: The surface waters at location A have: Match: In light of your answer to Question 7, is location A more likely to be in equatorial or polar waters? Explain High temperature and high salinity your answer.
High temperature and low salinity
Low temperature and high salinity
Low temperature and low salinity
I'm not sure
7 Question 10 Question 11
Interpret: The surface waters at location B have: Match: In light of your answer to Question 9, is location B more likely to be in equatorial or polar waters? Explain High temperature and high salinity your answer.
High temperature and low salinity
Low temperature and high salinity
Low temperature and low salinity
I'm not sure
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 彐oor. 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 12
Explain: In your own words, describe how the North Atlantic chimneys are formed.
8 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 Analyze: Now focus on the temperature variation along above and describe how the temperature varies with the eastern coast of North America and in the North latitude from the Equator to the poles. Use the scale to Atlantic. What evidence can you see for the presence of give a rough estimate of the amount of temperature the Ocean Conveyor Belt in this region? variation.
9 Map showing the average surface salinity of the world's oceans.
Question 3 Question 4
Describe: Examine the map above and describe how the Explain: How could you explain the variation you surface salinity of the Paci哷c Ocean varies with latitude. Is described in the previous question in terms of the it highest at the Equator, in middle latitudes or near the combined e⤃ects of evaporation and rainfall? poles? Where is it lowest? Hint: In the Paci褅c 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.
0:00 / 1:08
Credit: The Gulf Stream Explained by In a Nutshell – Kurzgesagt (YouTube).
10 Question 5
Notes: Use this space to take notes for the video.
Note: This is not a question and is optional.
Question 6
Calculate: Use the speed and distance 哷gures 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 7
Compare: The video also states that water in the Gulf Stream 䒋ows at the rate of 100,000,000 m3/s. By contrast, the average 䒋ow rate at the mouth of the Amazon River is 175,000 m3/s.
How many times larger is the Gulf Stream compared to the Amazon River in terms of 䒋ow rate? Round your answer to the nearest whole number.
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.
11 Question 8
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 di⤃erence based on your understanding of the Gulf Stream.
Question 9
Speculate: Take another look at the global ocean temperature and salinity maps above and 哷nd 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 哷nd based on everything you've learnt in this lesson. If you have time you can research the possibilities. If you 哷nd 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.
12 Experiment: Ocean Currents
Modelling ocean currents
Background
The main driving force behind thermohaline circulation is the di erence in water densities, mostly brought about by di erences in temperature and salinity.
Aim
To demonstrate how salinity di erences 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].
13 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 sti card or plastic large enough to form a dividing wall across the container Scissors and/or sharp 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 ll 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 ts 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.
14 Discussion
Question 3
Describe: Summarize what happened when the holes between the section were opened. Was this consistent with your hypothesis?
Question 4
Reৌect: 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?
15 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 a၈ect 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 ‵rst he wanted to become a marine biologist, thinking it would allow him to spend more time sur‵ng. But he didn’t study biology in school. The ‵eld 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 a�ect 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.
16 Question 1
Consider: What feature of the ocean do you ‵nd most intriguing? If you could study anything about the ocean what would it be, and why?
Image credits
NASA, World Ocean Atlas, Google Earth, iStock, Shutterstock.
17 Test: Ocean Currents
Note: There may be more than one correct answer to the multiple-choice questions below.
Question 1 (1 mark) Question 2 (1 mark)
Except at the edges where they meet, the waters of Earth's The main factor driving ocean currents is: ᓀve oceans do not mix. wind True river outᓀows False underwater volcanoes I'm not sure diᓘerences in the density of sea water
rain distribution
I'm not sure
Question 3 (1 mark) Question 4 (1 mark)
One eᓘect of ocean currents is to: The currents that make up the Ocean Conveyor Belt:
transport heat away from the equator to the all run along the surface of the ocean poles all run along the bottom of the ocean transport heat away from the poles to the equator (leaving polar waters cold) run on the surface and the bottom at diᓘerent locations transport heat from the northern hemisphere to the southern hemisphere are diᓘerent temperatures at diᓘerent None of the above – ocean currents do not locations transport heat are a consistent temperature everywhere I'm not sure I'm not sure
18 Question 5 (1 mark) Question 6 (1 mark)
In the term thermohaline circulation, thermo refers to Denser water will ᓀow so that it is below less dense water. ______and haline refers to ______. True temperature; quality False temperature; salinity I'm not sure heat; movement
density; salinity
insulation; cloudiness
I'm not sure
Question 7 (1 mark) Question 8 (1 mark)
Which of the following make sea water more dense? Temperatures near the Equator are overall much warmer than at the poles because: Evaporation the Earth's crust is thinner at the Equator, so Rain seas and land are warmed by magma under the continental plates Becoming colder there is a lot more cloud at the poles than at Becoming warmer the Equator, blocking sunlight
A higher concentration of salt ocean currents and winds concentrate warm water and air around the Equator I'm not sure the Equator faces the Sun directly, so captures more sunlight per square metre
I'm not sure
Question 9 (1 mark) Question 10 (1 mark)
Samples from a particular ocean location indicate that the Surface samples from another location show a water water on the surface averages around 20°C and has a high temperature just above freezing and high salinity. Which of salinity of 35.8 g/kg. The location is probably ______the following scenarios might explain this? because ______. Close to a glacier outfall near a pole; a lot of sunlight is reᓀected oᓘ ice into the water, warming it Iceberg formation
near a pole; 20°C is a low temperature for Very deep water ocean water An area with strong winds near the Equator; high water temperature and high salinity indicate high levels of sunlight I'm not sure warming and evaporation
near the Equator; ocean currents have concentrated warm salty water there
I'm not sure
19 Question 11 (1 mark) Question 12 (1 mark)
In the North Atlantic Chimneys, water that is ______, Which statement more accurately captures the role of the ______and ______falls to the ocean ᓀoor. North Atlantic Chimneys within the Ocean Conveyor Belt?
cold; dense; salty The North Atlantic Chimneys are major drivers in the Ocean Conveyor Belt, helping to keep it cold; dense; not very salty moving
cold; not very dense; not very salty The North Atlantic Chimneys are created by the Ocean Conveyor Belt but do not drive it warm; dense; fresh I'm not sure warm; dense; salty
I'm not sure
Question 13 (1 mark) Question 14 (1 mark)
Lima is a city on the west coast of South America. Lobito, Western Europe has a much milder climate than other Angola, is at the same latitude –12 degrees south – on the parts of the world at the same latitude, for example in west coast of Africa. Because they are both on the west North America. This is because: coast of large continents and at the same latitude we can assume that, on average, temperatures in both cities are it is protected from cold northerlies by high about the same. mountains the Gulf Stream carries heat from the Gulf of True Mexico to the waters surrounding Europe
False the Gulf Stream carries heat from waters around North Africa to the waters surrounding I'm not sure Europe
warm southerly winds blow from northern Africa
I'm not sure
Use the two maps below to answer Questions 15 and 16.
Left: The Ocean Conveyor Belt. Warm water is shown in red, cold in blue. Right: Average surface water temperature.
20 Question 15 (1 mark) Question 16 (1 mark)
If the Ocean Conveyor Belt stopped the climate of India If the Ocean Conveyor Belt stopped the climate of Western would likely: Europe would likely:
become cooler become cooler
become hotter become hotter
not change not change
I'm not sure I'm not sure
Question 17 (1 mark)
Learning goal 1: What is the main factor that drives ocean currents? Name three aspects of Earth's natural systems that aᓘect this factor, causing it to change. Explain.
Question 18 (1 mark)
Learning goal 2: In general, how would you expect surface ocean water temperature and salinity to diᓘer between the Equator and the poles? Why?
Question 19 (1 mark)
Learning goal 3: Do ocean currents have any inᓀuence on the climate over land? Explain, giving an example.
Total available marks: 19
21