Ocean circulation relies on different densities of water falling and rising. The density of water is affected by salinity (the amount of dissolved salt) and temperature. With this activity, you can predict and observe how different types of water behave.

Activity preparation

To prepare for this activity, you will need to prepare four different types of water for students to work with.

Ocean water

• Create one litre of ocean water per group by dissolving 32 grams of salt per litre of tap water.

Gulf Stream seawater (red)

• Create 25 ml of warm Gulf Stream seawater per group with red dye and keep this in a thermos.
• The Gulf Stream seawater will need 32 grams of dissolved salt per litre.

Arctic seawater (blue)

• Create 25 ml of Arctic seawater per group with blue dye and keep this in the fridge.
• The Arctic sea water will need 64 grams of dissolved salt per litre.

Iceberg (green)

• Create on ice cube with green dye and kept in the freezer.
• Add no salt.

Activity steps

1. Fill three large glasses with a third of the ocean water each.
2. These represent the Arctic Ocean. The system of ocean circulation relies on cold, salty water sinking in the Arctic Ocean.

Gulf Stream seawater

1. Take the red-dyed Gulf Stream seawater.
2. The Gulf Stream is an ocean current that carries warm water from the Caribbean towards the coast of north western Europe.
3. Predict how you think the Gulf Stream seawater will behave. Why?
4. Gently, add the Gulf Stream seawater using a small spoon or pipette.
5. Did the Gulf Stream seawater behave in the way you predicted?

Arctic Ocean seawater

1. Take the blue-dyed Arctic Ocean seawater.
2. The surface water in the Arctic is both cold and salty. The additional saltiness is caused by the formation of sea ice. This is because when salt water freezes it exudes salt into the surface waters of the Arctic Ocean.
3. How do you think the Arctic Ocean seawater will behave? Why?
4. Gently, add the Arctic Ocean seawater using a small spoon or pipette.
5. Did the Arctic Ocean seawater behave in the way you predicted?

Iceberg

1. Take the green-dyed iceberg.
2. Icebergs are formed when land-based ice, such as glaciers and ice sheets, breaks off into the ocean.
3. How you think meltwater from the iceberg will behave? Why?
4. Place the iceberg gently on top of the mini Arctic Ocean.
5. Did the iceberg behave in the way you predicted?

Activity materials (per group)

• 3 large clear containers, at least 500 ml capacity, such as glasses, beakers or plastic cups
• 3 small containers for the different coloured water and ice cube
• Salt
• Water
• Food dye (red, blue, and green)
• Ice cube tray
• Teaspoon or pipette

Ocean circulation diagram

In general, we think about temperatures becoming colder as you move further north, but England, for example, enjoys a relatively mild climate compared to North America. The reason for this is the system of global ocean currents, and specifically for the British Isles and north west Europe, the current known as the Gulf Stream which brings warm waters from the Caribbean. Ocean circulation has a huge impact on our climate.

The movement of the ocean current in the North Atlantic is a bit like a swimmer doing a tumble turn. The warm salty water from the tropics moves northward releasing its heat close to the English coast. This current continues to move further north until it enters the Arctic Ocean. Here, the surface water cools and becomes denser, sinking down towards the seabed.

This water then flows out of the Arctic back towards the equator, creating a dragging effect that sustains global ocean currents. This mechanism of warm water floating and cold saltier water sinking is known as thermohaline circulation (thermo = temperature and haline = salt).

Both salt and temperature play a big role in this process as these two factors affect the water’s density. Cold water is heavier than warm water. Salt water is heavier than fresh water, as it is not just water, but also contains dissolved salts.

Scientists have modelled scenarios where thermohaline circulation slows abruptly. This slowing of the ocean conveyor (the global system of ocean currents) could have dramatic effects on the climate around the world. The Met Office predict that if the global ocean currents stopped completely, the whole northern hemisphere would cool by an average of 1-2°C in the first 50 years, which may not sound too bad, except that in the UK this cooling could be as much as 8°C.