What drives global ocean circulation and how does the East Australian Current affect Australian seas?
Explain how surface and deep ocean circulation are driven by wind, density and the Coriolis effect, and describe the East Australian Current and the global thermohaline conveyor
A focused answer to the QCE Marine Science Unit 4 sub-topic on ocean circulation. Explains wind-driven surface gyres, the Coriolis effect, the thermohaline conveyor and upwelling, and details the East Australian Current and its poleward extension under warming.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
QCAA wants you to explain the engines of ocean circulation, wind, density and the Earth's rotation, and to describe both the wind-driven surface gyres and the deep thermohaline conveyor. You should be able to describe the East Australian Current as a named Australian example and link circulation to heat, nutrient and larval transport.
Surface circulation and gyres
Steady global winds drag on the sea surface and push surface water along. The Earth's rotation deflects this moving water, the Coriolis effect, to the left in the Southern Hemisphere and to the right in the Northern. The combination organises surface currents into large, roughly circular systems called gyres, one in each major ocean basin. Currents on the western side of each basin (such as the East Australian Current) are warm, fast and narrow; those on the eastern side are cool, slow and broad.
Deep circulation: the thermohaline conveyor
Below the wind-driven surface lies a much slower circulation driven by density. Water density rises when water is cold and when it is salty. In polar regions, cold, salty water becomes dense enough to sink, then spreads through the deep ocean and eventually rises again elsewhere. This global overturning, the thermohaline circulation or great ocean conveyor, takes around a thousand years to complete a loop and moves heat, oxygen and nutrients around the planet.
Upwelling and downwelling
Where winds and currents push surface water away from a coast, deep water rises to replace it, an upwelling. Upwelled water is cold and rich in nutrients, so upwelling zones are highly productive and support major fisheries (linking to the Unit 3 productivity material). Where surface water piles up and sinks, a downwelling, it carries oxygen down but suppresses surface productivity.
The East Australian Current
The East Australian Current (EAC) is the warm western-boundary current of the South Pacific gyre. It flows south down the Queensland and New South Wales coast, carrying warm tropical water, and breaks into eddies as it turns away from the coast near southern New South Wales. Its roles are large:
- It transports heat poleward, moderating the climate of the east coast.
- It carries larvae of corals, fish and other animals southward, driving connectivity between reefs and seeding temperate reefs with tropical species.
- It influences weather and the distribution of marine life along the whole east coast.
Under climate change the EAC is strengthening and extending further south, pushing warm water into Tasmanian waters. This has shifted species ranges poleward, for example allowing the long-spined sea urchin to spread south and overgraze Tasmanian kelp forests, a clear case of circulation change driving ecological change.
Why this matters
Circulation ties the whole of Unit 4 together. It distributes the heat that drives climate, the nutrients that drive productivity and fisheries, and the larvae that connect ecosystems, and it spreads pollutants and warm-water stress. Changes to circulation, such as a strengthening EAC or a weakening conveyor, are among the most important climate-related risks the ocean faces.
Exam-style practice questions
Practice questions written in the style of QCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2022 QCAA4 marksExplain how wind, the Coriolis effect and density differences drive ocean circulation, distinguishing surface circulation from deep circulation.Show worked answer →
Four marks: the surface engine, the deep engine, and the Coriolis effect.
Surface circulation (2 marks). Steady global winds drag on the sea surface and push water along; the Coriolis effect from Earth's rotation deflects this water (to the left in the Southern Hemisphere), organising fast surface currents into large rotating gyres.
Deep circulation (2 marks). The thermohaline conveyor is driven by density: cold, salty water in polar regions becomes dense and sinks, then spreads slowly through the deep ocean over centuries before upwelling elsewhere.
Markers reward the wind-versus-density distinction, the role of the Coriolis effect, and the contrasting timescales.
2023 QCAA6 marksThe East Australian Current is strengthening and extending further south with climate change. Analyse how this change affects marine species distribution, using a named Australian example.Show worked answer →
Six marks: the current's roles, then the analysed ecological consequence.
- Roles of the EAC (2 marks)
- As the warm western-boundary current of the South Pacific gyre, the EAC transports heat poleward and carries tropical larvae southward, driving connectivity.
- The change (2 marks)
- Warming strengthens the EAC and pushes its warm water further south into Tasmanian waters, raising temperatures and delivering tropical and warm-temperate larvae beyond their former range.
- Named example (2 marks)
- The long-spined sea urchin (Centrostephanus rodgersii) has spread south into Tasmania, where it overgrazes kelp forests, creating barrens and displacing temperate species, a clear case of circulation change driving ecological change.
Markers reward the EAC roles, the poleward range shift, and a specific worked example with an ecosystem consequence.