Ocean currents

Impact of ocean currents

Ocean currents refer to the movement of water from one location to another. They transport water long distances and re-distribute heat, salt, oxygen, and nutrients.

These currents have a big impact on:

• weather and climate, by moving heat toward the poles
• the biological characteristics of the waters through which they flow
• human activities such as fishing and boating.

How ocean currents form

The following forces generate and drive ocean currents.

Direction

The direction of the current is influenced by the:

• rotation of the Earth
• depth and shape of the sea floor
• shape of coasts
• wind direction
• position of other currents.

Short-term events such as earthquakes can also create changes in currents.

Speed

Currents are generally measured in metres per second (m/s) or in knots. One knot is 1.85 km/h or 0.5144 m/s.

The Gulf Stream is the world's fastest current, reaching up to 2.5 m/s.

Typical speeds for the Leeuwin Current are 0.5 m/s. The core of the East Australian Current can reach 2 m/s.

Australian region ocean currents

There are several significant currents in our region. The major ones are the East Australian Current and the Leeuwin Current. Others include the:

• Indonesian Throughflow
• South Equatorial Current
• Antarctic Circumpolar Current.

Currents on ocean maps

Compared to the diagram above, currents on our ocean maps appear much more chaotic. They look less like rivers and more like turbulent wind patterns with swirling, temporary features called ocean eddies. How they appear on these maps is also shaped by the seasons and other forces.

How salinity and temperature drive currents

Sea surface salinity measures the concentration of dissolved salt in sea water, near or at the ocean surface.

Measuring salinity is key to understanding global water cycles and how water moves around the oceans.

About 85% of evaporation and 80% of precipitation – rain and snow – happen over the Earth's oceans.

• Evaporation removes fresh water, increasing the salinity of the local ocean.
• Rainfall, snow and rivers add fresh water, decreasing salinity.

These changes in salt concentration affect the density of surface waters, which affects how water moves around.

Salinity values

Sea surface salinity in the open ocean generally ranges between 32 and 37 practical salinity units (psu – grams of salt per litre or kilogram of water).

It may be much lower near fresh water sources and can be as high as 42 psu in the Red Sea – one of the saltiest bodies of water in the world.

Impact on water movement

Salinity and temperature determine the density and buoyancy of sea water.

• The volume of water contracts as it cools.
• When it contracts, there is the same amount of salt in a smaller volume of water.
• The saltier water is more dense and heavier.
• Heavier water tends to sink, which lowers the sea-surface height.

In warmer waters:

• Water volume expands.
• There is the same amount of salt to fill the greater volume.
• This makes the water less dense and lighter.
• The sea surface is higher.

These temperature and salinity variations drive ocean circulation, creating a 'global conveyor belt'.

Upwelling, downwelling and the Ekman transport

When winds that blow across the ocean push surface water away, more water rises from below to replace it. The water that rises is generally colder and has more nutrients. This is called upwelling. It can happen in the open ocean or near the coast.

Downwelling is the reverse process. It happens when wind pushes surface water against the coast. The surface water piles up and eventually sinks.

Ekman transport is the name given to seawater movement that occurs under certain wind conditions. This movement can cause coastal upwelling zones, critical for some of Australia's marine life.