Ocean circulation relies on different densities of water falling and rising. The density of water is affected by salinity and temperature. With this demonstration, you can understand how the ocean behaves more like a river with deep currents.

Activity preparation

Standard seawater (no color)

• 2 litres of water at room temperature.
• Create the brine by dissolving 32 grams of salt per litre of tap water.

Gulf Stream seawater (Red)

• 0.2 litres of warm water with red food dye and kept in a thermos.
• Create the brine by dissolving 32 grams of salt per litre of tap water.

Arctic seawater (Blue)

• 0.2 litres of cold water with blue food dye and kept in the fridge (4°C).
• Create the brine by dissolving 64 grams of salt per litre of tap water.

Ice sheet meltwater (Green)

• 0.2 litres of ‘ice sheet meltwater’ with green food dye and kept in the fridge (4°C).
• Add no salt.

Activity steps

1. Fill the container halfway with up to 2 litres of the undyed ‘standard seawater’.
2. This is 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 carries warm water from the Caribbean towards the coast of England.
3. Predict how you think this warm ‘Gulf Stream seawater’ will behave. Why?
4. Pour the ‘Gulf Stream seawater’ gently down the side of the container.
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 waters of the Arctic Ocean are not only cold, but also very salty due to the salt being exuded during sea ice formation.
3. How do you think the ‘Arctic Ocean seawater’ will behave? Why?
4. Pour the ‘Arctic Ocean seawater’ gently down the side of the container.
5. Did the ‘Arctic Ocean seawater’ behave in the way you predicted?

Ice sheet meltwater

1. Take the green-dyed ‘ice sheet meltwater’.
2. The meltwater from the Greenland ice sheet and other glaciated Arctic regions, although cold is made from freshwater rather than salty seawater.
3. How do you think the ‘Ice sheet meltwater’ will behave? Why?
4. Pour the ‘ice sheet meltwater’ gently down the side of the container.
5. Did the ‘ice sheet meltwater’ behave in the way you predicted?

• 1 large clear container to act as a mini-ocean (at least 3 litres capacity)
• 3 small containers for the different coloured waters
• Salt
• Water
• Food dye (red, blue and green)
• Ladle

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.