Ocean currents

Learn how water moves around the oceans through currents, upwelling and downwelling

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Ocean currents explained

Ocean currents refer to the movement of water from one location to another.

Currents transport water long distances and re-distribute heat, salt, oxygen, and nutrients.

They 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

These forces generate and drive ocean currents.

Tides

Tidal currents are strongest near the shore, tend to be very predictable and can move fast – sometimes 8 knots or more. Learn more about tides.

Winds

Strong winds drive currents at or near the ocean's surface. This may be localised or at global scale, circulating water for thousands of kilometres. Learn more about wind, gusts and squalls.

Water temperature and density

Differences in sea temperature and salinity (density) drive slow water movement at depth, on a global scale. This is called thermohaline circulation, or the 'global conveyor belt'. Learn more about sea temperature.

Gravity

Gravity causes water that is heavy in salts to fall, pushing less dense, lighter water to the surface.

Direction of ocean currents

The direction of the current is influenced by the:

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

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

Speed of ocean currents

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 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.

Ocean currents in the Australian region

There are several significant currents in our region. The 2 major ones are the East Australian Current and the Leeuwin Current. Others include the:

  • Indonesian Throughflow
  • South Equatorial Current
  • Antarctic Circumpolar Current.
Map of Australia with arrows in the surrounding oceans, showing the direction of major currents. From the top, the Indonesian Throughflow travels along Australia's north-west coast to join the Leeuwin Current, which flows around into Bass Strait, tailing off near Tasmania. At the bottom, the Antarctic Circumpolar Current flows west to east. The East Australian Current flows from the Pacific. It divides into the north-flowing South Equatorial Current and the southerly East Australian Current.
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Map showing directions of major currents in the Australian region

East Australian Current

From its source in the Coral Sea, the East Australian Current tracks down the east coast of Australia. Sometimes it comes within 5 or 6 km of the coast.

This current is well known for forming eddies. These may be up to 200 km across and persist for several weeks.

It has a big impact for Australians living on the east coast. By bringing warmer ocean water south, it:

  • adds several degrees of temperature to the coastal waters
  • supports fish and other marine animals by sweeping other species down from the north
  • helps flush waste from waterways and coastal areas along the way.

Leeuwin Current

The Leeuwin Current sweeps down Australia's west coast, from about the North West Cape. It can extend as far as the Great Australian Bight and Tasmania's south-west.

Turning east at Cape Leeuwin, it becomes the South Australian Current. The current flows eastwards below most of South Australia to Tasmania, where it is called the Zeehan Current.

During winter this entire system can act as a single current. It brings warm tropical water to Western Australia, raising the ocean temperatures several degrees. The current carries tropical species south along the Western Australian coast and into the waters of South Australia.

Though carrying about a quarter of the East Australian Current's water volume, it has a big impact on our western and southern coasts. For example:

  • The life cycles and abundance of salmon and tuna in Australian waters depend on the current. Swept along, salmon avoid the warm waters. Seeking cooler conditions, they circle around into South Australia.
  • The current carries juvenile tuna after they spawn between Australia and Indonesia, to an area west of Broome. The tuna ride the current south and eastwards off South Australia as they grow.

The Leeuwin Current can sometimes reach very close to the coast. It can sometimes be as close as several kilometres from the coast – for example, at Ningaloo.

How salinity and temperature drive currents

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

Why measure salinity

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).

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.

How salinity and temperature affect 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'.

Map of the global conveyor belt, or thermohaline circulation. High salinity water cools and sinks in the north Atlantic. It flows south and turns east at Antarctica. The flow branches into the Indian and Pacific oceans, where the deep water surfaces. Turning south, the branches reunite in the Indian Ocean. As the water flows north, it cools and gets saltier. It sinks in the north Atlantic and starts another cycle.
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It takes about 1000 years to take a trip on the global conveyor belt, which moves heat and salt around the world

Global conveyor belt

The global conveyor belt starts in the Norwegian Sea.

It's powered by thermohaline circulation – the movement of heat (thermo) and salt (haline) around the world.

The conveyor belt starts when warm water from the Gulf Stream heats the atmosphere in the cold northern latitudes.

Losing heat to the atmosphere, the water cools and becomes more dense. It sinks to the bottom of the ocean.

As more warm water is carried north, the cooler water is pushed south. It flows south of the equator, travelling to Antarctica.

The water returns to the surface through upwelling and mixing. Mixing is an irreversible change in the temperature, salt, gas and nutrients of a body of water, due to forces such as wind and tides.

It takes about 1000 years to make a full trip on the 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.

Graphic showing a square section of ocean with a sea bed rising to the left. A red arrow indicates the wind blowing along the coastal sea water. A dark blue arrow shows the direction of upwelling, pulling water from the deep and circulating to the shore and back out to sea at the surface. Three lighter arrows label this movement the Ekman flow.
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Winds blowing along the coast can lead to upwelling of colder water to the surface

Ekman transport

Ekman transport is the movement of seawater that occurs under certain wind conditions. It is named after Swedish oceanographer Vagn Walfrid Ekman, who first described the phenomenon in 1902.

Sustained winds in a consistent direction over the ocean move the top layer (about 30 metres depth) of seawater. In the southern hemisphere, the seawater layer moves to the left of the wind direction, due to the Earth's rotation (the Coriolis effect).

As the top layer of water is moved by the wind, it needs to be replaced. If the coast is to the right of the wind direction, and the winds persist for more than a day, an upwelling process draws up colder and more nutrient-rich water from the depths of the ocean to the surface.

The longer the winds persist, and the longer the stretch of coastline that experiences a similar wind direction, the colder the water brought to the surface. This upwelled water can last for days or longer, until wind conditions change and the seawater mixes.

The reverse process – downwelling – can also occur. It brings warmer water towards Australia's coast from boundary currents such as the East Australian Current or Leeuwin Current.

Ekman transport in Australia

Upwelling is more likely along certain parts of the Australian coastline, particularly along New South Wales, south-east Queensland and the Bonney Coast in South Australia.

An important factor is the width of the continental shelf – the landmass that extends with a shallow gradient from the continent underneath the ocean.

Upwelling occurs when the continental shelf is narrow and the sea becomes very deep relatively close to the shore. The deeper water requires less time and energy to reach the coastline.

Headlands and bays along the coastline may also vary the effects from beach to beach.

If you look out to sea from the shore, the wind needs to be consistently blowing from left to right for upwelling to happen. This means that northerly or north-easterly winds are required along most of the east coast, but south-easterly winds in the eastern Great Australian Bight.

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