False-colour MODIS brightness-temperature image showing the Gulf Stream and warm eddies in the North Atlantic.

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Model scale

Geographic globe. current routes schematic. widths and tracers enlarged.

Gulf Stream: warmer poleward western boundary (amber). Canary Current: cooler equatorward eastern boundary (blue). Grey connects the branches.

How to read these results

Viewed on a map, northern subtropical gyres turn clockwise and southern ones anticlockwise. Play shows direction only. UK winters reflect westerlies, ocean heat and geography together; the Gulf Stream is one contribution.

Ocean currents

Follow the major surface currents and explain how winds and ocean heat transport contribute to regional climate.

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Why it happens

Surface winds and Earth’s rotation help organise subtropical ocean gyres. Warm western boundary currents generally carry water poleward, while cooler eastern boundary currents carry it towards the equator.

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MODIS, 2 May 2001: thermal contrast traces the Gulf Stream and eddies. Brightness temperature includes the sea surface and overlying moist atmosphere.

Try a comparison

Compare South Atlantic and explain the reversed sense of rotation. Locate all five subtropical gyres.

Reading the result

The globe shows the broad routes and directions of currents; their width and tracer speed are enlarged for reading.

Film

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How Do Oceans Circulate? Crash Course Geography #9 — Crash Course.

A common misunderstanding

Ocean currents are driven only by differences in water temperature.

Winds and pressure gradients drive much upper-ocean circulation, with Earth’s rotation and coastlines shaping the flow. Temperature and salinity affect density and contribute to overturning, while mixing connects different depths. Surface gyres and deep circulation are related but cannot be explained by temperature alone.

Cases

Exam language

Gyre

A large-scale rotating system of ocean currents. Winds, Earth’s rotation and the shapes of ocean basins help organise subtropical gyres. A gyre includes currents with different widths, speeds and temperatures rather than one uniform loop.

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MODIS observations trace warm Gulf Stream water in the western North Atlantic. The Gulf Stream is one part of the basin-scale subtropical gyre; a single current image does not show the whole circulation.

Western boundary current

A narrow, fast current along the western side of an ocean basin’s subtropical gyre, such as the Gulf Stream or Kuroshio. These currents generally carry warm water poleward. Western refers to the ocean basin, not the coast of the neighbouring continent.

Eastern boundary current

A current along the eastern side of an ocean basin, such as the Canary Current. In subtropical gyres these currents are usually broader, slower and cooler than western boundary currents, and coastal winds can also support upwelling.

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Aqua MODIS, 5 May 2020. Upwelling in the Canary Current system supplies nutrients off Mauritania. The visible green water contains phytoplankton and sediment; it is not a temperature map.

Gulf Stream

A fast, warm western-boundary current in the North Atlantic, flowing from the Florida Straits along the eastern United States before turning offshore.

Canary Current

The relatively cool eastern-boundary current flowing southward in the North Atlantic subtropical gyre. Coastal upwelling along northwest Africa can bring colder, nutrient-rich water to the surface. The current is a moving water mass, not simply a strip of cold weather.

Subtropical high

A region of relatively high atmospheric pressure commonly found near the poleward side of the Hadley circulation. Descending air tends to suppress cloud formation, helping explain many subtropical dry regions. Its position and strength vary with season and geography.

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GOES-East cloud observations over the Americas, 22 April 2014. Tropical cloud bands and midlatitude systems provide evidence of atmospheric circulation. The three-cell pattern is a long-term zonal average, not three fixed cloud rings in this single image.

Surface current

Flow in the upper ocean, driven by winds and pressure gradients and modified by rotation, coastlines and mixing.

Process0.40Ocean gyre