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Surface Circulation: Key Ideas

Ocean water circulates in currents.

Surface currents are caused mainly by wind and affectthe uppermost 10% of the ocean.

Circulation of the other 90% of the ocean is driven bygravity, as dense water sinks and less dense waterrises.

Upwelling and downwelling describe the verticalmovement of water masses.

The Coriolis effect modifies the course of oceancurrents.

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Ocean Currents

Transfer heat from tropical to polar regions

Influence weather and climate

Distribute nutrients and scatter organisms

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Surface Currents

Surface currents are driven by wind

Most of Earth’s surface wind energy isconcentrated in the easterlies and westerlies.

Due to the forces of gravity, the Coriolis effect,and winds, water often moves in a circularpattern called a gyre.

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Surface Currents

Water movesclockwise in theNorthern Hemispheregyres andcounterclockwise inthe SouthernHemisphere gyres.

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Gyres can be further subdivided into distinct currents. The fourinterconnected currents in the North Atlantic Gyre have different flowcharacteristics and temperatures.

Flow Within a Gyre

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Ekman Spiral

The motion of the water at the surface is driven by the wind. Eachsubsequent layer below is driven by the movement of the water above,and slowed down by friction. This is termed the Ekman spiral, althoughthe water itself does not spiral.

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Figure 8.2

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Ekman Spiral and Coriolis Effect

The effect of Ekman spiraling and the Coriolis effect causethe water within a gyre to move in a circular pattern.

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Figure 8.3The overall effect

of Ekmantransport is topush surface

water toward themiddle of the

gyre.

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Flow Within a Gyre

The surface of the North Atlantic is raised through wind motion andEkman transport to form a low hill. Why is the center of the hill offsetto the west?

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Geostrophic Gyres

Geostrophic gyres - a balance between the pressure gradient andthe Coriolis effect. Of the six great current systems in the ocean, fiveare geostrophic gyres.

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Narrow, deep, fast currents on the western sides of oceanbasins; their edges are sharp.

Gulf Stream

Japan Current

Brazil Current

Agulhas Current

Eastern Australian Current

Western Boundary Currents

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Cold, shallow, broad and slow currents on the easternsides of ocean basins; their boundaries are not welldefined.

Canary Current

Benguela Current

California Current

West Australian Current

Peru Current

Eastern Boundary Currents

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Water flow in the Gulf Stream and the CanaryCurrent, parts of the North Atlantic gyre.

Currents Within Gyres

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Currents Within Gyres

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Countercurrents return water to theopposite sides of ocean basins

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Countercurrents and Undercurrents

Countercurrents flow on the surface in the oppositedirection from the main currents.

Undercurrents are countercurrents that flow beneaththe surface current.

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Figure 8.7Cold and

warmwater

eddies

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Wind induced vertical circulation - vertical movementinduced by wind-driven horizontal movement of water.

Upwelling is the upward motion of water. This motion bringscold, nutrient-rich water to the surface.

Downwelling is downward motion of water. It supplies thedeeper ocean with dissolved gases.

Upwelling And Downwelling

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Figure 8.14

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Surface currents distribute tropical heat. Thewarm ocean currents are shown in red and coldcurrents in blue.

Effects of Surface Currents onClimate

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Float methods depend on the movement of afree-floating object.

Flow methods measure the current as it flowspast a fixed object

Studying Currents

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Ocean water, circulating in currents, distributes heat from the equatorto the poles. Currents have a profound effect on Earth’s weather andclimates.

Summary