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 www.cosmosforschools.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 notseem 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 torise, making scientists think that the ocean has been absorbing the heat that they thought would go into the air. But no-one knewhow 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 acrossthe Pacific 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. Theblustering winds were churning up the water and driving the heat into the cooler waters below. After nearly two decades of this it’sno 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 ina 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.

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

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 ithappens. 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 mightwonder how much he really wanted to stay in touch.

Question 1

Trace: Nova Scotia and Croatia are marked in the map below. Given the major ocean current shown, draw a likely route forJonathon's bottle.

Being in a bottle, Jonathon's message floated to Croatia – it was blown by the wind and carried by currents running across theocean's surface. But not all ocean currents are on the surface – there are also deep currents that flow along the ocean floors. 

Ocean Conveyor Belt

1:10

Question 2

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

equator to the poles.

Wind alone moves heat from the poles to the

equator.

Question 3

Recall: The video refers to which of the factors below asdetermining the density of ocean water?

Wind

Climate

Salt

Temperature

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 fullcycle.

The map below shows the Ocean Conveyor Belt, representing warm surface sections in red and cold deep sections in blue.

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 (Branch1)

Pacific (Branch2)

Atlantic

Current depth bottom surface

Watertemperature

cooling warming

Wind is the main factor driving the warm, surface sections of the belt, but deep sections are powered by differences in waterdensity. Dense water sinks and less dense water rises. At certain locations on Earth conditions are such that they create differencesin 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.

Wind and water density drive currents

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 amuch larger area, because the Earth's surface curves away from the sunlight here. The higher the latitude the greater the surfacearea that the sunlight has to warm, so it's colder.

Temperature

Question 5

Remember: Sunlight entering the Earth's atmosphere near thepoles has less energy than sunlight entering the Earth'satmosphere near the equator.

True

False

Question 6

Think: Regions near the South Pole receive much more sunlightthan regions at the same latitudes near the North Pole.

True

False

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. 

Salinity

Temperature and salinity data from the ocean surface to 1500 m depth for two locations.

Question 7

Interpret: The surface waters at location A have:

High temperature and high salinity

High temperature and low salinity

Low temperature and high salinity

Low temperature and low salinity

Question 8

Match: In light of your answer to Question 7, is location A morelikely to be in equatorial or polar waters? Explain your answer.

Question 9

Interpret: The surface waters at location B have:

High temperature and high salinity

High temperature and low salinity

Low temperature and high salinity

Low temperature and low salinity

Question 10

Match: In light of your answer to Question 9, is location B morelikely to be in equatorial or polar waters? Explain your answer.

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 Atlanticcreate 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 watersit leaves the surrounding seawater highly saline. The increased density of this salty water makes it sink, drawing in more water fromthe 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.

Process: Ocean Currents

Map showing the average surface temperature of the world's oceans.

Question 1

Describe: Examine the map of ocean surface temperatureabove and describe how the temperature varies with latitudefrom the Equator to the poles. Use the scale to give a roughestimate of the amount of temperature variation.

Question 2

Analyze: Now focus on the temperature variation along theeastern coast of North America and in the North Atlantic. Whatevidence can you see for the presence of the Ocean ConveyorBelt in this region?

Map showing the average surface salinity of the world's oceans.

Question 3

Describe: Examine the map above and describe how thesurface salinity of the Pacific Ocean varies with latitude. Is ithighest at the Equator, in middle latitudes or near the poles?Where is it lowest?

Question 4

Explain: How could you explain the variation you described inthe previous question in terms of the combined effects ofevaporation and rainfall?

Hint: In the Pacific Ocean, rainfall is consistently high at the Equator

but is outweighed by evaporation in middle latitudes.

As discussed in the Cosmos Magazine article, the oceans absorb large quantities of heat from the atmosphere. Then currents cantransport 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 Atlantictowards Europe.

Ocean currents and climate

1:08

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 totravel 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 flowrate at the mouth of the Amazon River is 175,000 m /s.

How many times larger is the Gulf Stream compared to the Amazon River in terms of flow rate? Round your answer to the nearestwhole number.

3

3

The map shows the locations of Nuuk in Greenland (1) and Sandviksberget in Norway (2). The graphs show averagemonthly 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 byhow much?

2. Suggest a reason for this difference based on your understanding of the Gulf Stream.

Question 8

Speculate: Take another look at the global ocean temperature and salinity maps above and find one or more unusual variations oranomalies in the data. For example, in the temperature map the arm of warm water along the east coast of North America – theGulf 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 canresearch 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.

Apply: Ocean Currents

Experiment: Modelling ocean currents 

The main driving force behind thermohaline circulation is the difference in water densities, mostly brought about by differences intemperature and salinity.

To demonstrate how salinity differences create thermohaline circulation.

Background

Aim

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].

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 

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 theother 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 toremove 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 THESAME 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.

Procedure

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?

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

Materials

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 salinityof 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?

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 weatherand climate. Matthew England, an oceanographer and climate scientist at the University of New South Wales, knows this betterthan most.

Growing up in Sydney, Australia, Matthew always adored thesea. At first he wanted to become a marine biologist, thinking itwould allow him to spend more time surfing. But he didn’t studybiology in school. The field of oceanography, however,combined his love for the ocean with his talent in maths andphysics. Matthew knew he’d found his calling.

Oceanographers study everything about the oceans, fromcurrents through to nutrient cycles and ecosystems. Matthewfocuses on large-scale ocean currents and uses sophisticatedcomputer 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 althoughtheir average depth is over 4000 m, the upper metre can storeas much heat as the whole atmosphere!

The absorption of heat by the oceans creates anapparent slowing in global warming but it doesn't solve theproblem, Matthew warns. For example, water expands whenheated so warmer oceans will add to rising sea levels.

Matthew spends most of his time at work brainstorming withhis research group as they plan out their next experiments – hisfavourite part of the job. He also spends a lot of time analyzingdata from both direct observations and satellite images.

The ocean is always on Matthew’s mind, even when he isn’tworking. He is an avid body-surfer and relishes any chance toenjoy 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?

Cosmos Lessons team

Lesson author: Jim DriscollIntroduction and profile author: Yi-Di NgEditor: Jim RountreeArt director: Wendy JohnsEducation director: Daniel Pikler

Image credits: NASA, World Ocean Atlas, Google Earth, iStock,ShutterstockVideo credits: Kurz Gesagt – In a Nutshell, YouTube