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. This activity looks at just the impact of temperature, but you can experiment with salt as well, with some guidance in the 'Further ideas' section below.

Activity preparation

Arctic water (blue)

• Ice cubes made with blue food dye

Gulf Stream water (red)

• Put some hot water in a glass and add red food dye

Activity steps

1. Fill your jug or container with cold water from the tap.
2. Take your Arctic water ice cubes from the freezer and place them in the jug.
3. Observe what happens.
4. Place the straw all the way to the bottom of glass of hot red water.
5. Pinch the top or place a finger over the top to use the straw like a pipette.
6. Use this to add the hot water as close to the bottom of the jug or container by releasing the end of the straw.
7. Observe what happens.

Activity materials

• Water
• Food dye (red and blue)
• Ice tray and freezer
• Large clear container to act as a mini-ocean (anything from a large jug and upwards)
• Kettle
• Straw

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.