2. Windgedreven oppervlaktestromen


In this chapter we look at the wind-driven surface-ocean currents that dominate the central regions of the open ocean. In the subtropics, these currents form huge gyres with warm, fast boundary currents on the western side of the oceans and slow, cold boundary currents on the eastern side.

Zoom Sign
Mean global surface currents
Zoom Sign
Map of global surface currents. Bron: NOC

The long-term average pattern of ocean surface currents is plotted in the map above. Taking a closer look at the map you should be able to identify the main features:

  • Five subtropical gyres: two in the Atlantic, two in the Pacific and one in the Indian Ocean
  • A belt of eastward-flowing equatorial currents to the north and south of the equator in all three oceans
  • Weak equatorial counter currents that separate the northern and southern hemisphere gyres in each ocean
  • Mid-latitude belts of westward-flowing currents: the main one circling the Antarctic in the southern hemisphere, and two smaller belts diverted northwards by land in the North Atlantic and North Pacific.
  • Two subpolar gyres: one in the North Pacific and one in the North Atlantic

This global pattern is created by interaction between the prevailing (average) winds, the rotation of the Earth and the shape of the ocean basins. The rest of this chapter will look at what controls these currents and examine their characteristics in more detail.

Why the big difference?

In the 18th century mail ships from London to New York took two weeks longer than merchant ships from Falmouth to Rhode Island. Benjamin Franklin worked out why. Can you?

Zoom Sign
Franklin and 18th century ship
Left: New York Harbour in the 18th Century. Courtesy of Vallejo Gallery.
Right: Benjamin Franklin. Courtesy of National Science Foundation.

Using satellites to study current flow

The global current patterns have been known for some time. Satellites are now making it easier to study how the flow of these currents vary with with seasons and from one year to the next. As a result we are now able to link changes in ocean currents to recruitment of fish stocks, bleaching of coral reefs, or changes in weather patterns that cause floods or droughts. El Nino - shown below - is an example of this.

Zoom Sign
January 1997: Normal pattern with cool water from the Humboldt current reaching the eastern tropical Pacific and seasonal upwelling off Peru (click on image for a larger version). Bron: NOAA.

This chapter will cover

  • The global belts of prevailing winds
  • The ocean surface circulation driven by these winds
  • The ocean gyres formed by the main surface currents
  • The mid-latitude westward-flowing currents