Chapter 3 · Try it yourself

The Current That Never Hits Land

Every other big current eventually bumps into a continent. Around Antarctica, between about 45° and 65° south, there is nothing in the way, so the westerly winds push the ocean around and around the whole planet. Speed it up, slow it down, or build the land bridge that used to block it.

Westerly winds

The Drake Passage

Map centred on the South Pole. 60°S40°SSouth AmericaAfricaAustraliaAntarcticaNew Zealand Polar Front Drake Passage
  • Water flow
  • Polar Front (simplified position)

Map centred on the South Pole, simplified outlines. The current flows clockwise in this view, which is west to east.

Flow all the way around Antarctica
≈ 173 Sv

  • This current
  • All the world’s rivers together (≈ 1.2 Sv)
  • Amazon River (≈ 0.2 Sv)

The Polar Front is a boundary that circles Antarctica inside the current. Crossing it southward, the surface water turns suddenly colder and fresher, and the plankton, fish and seabirds change too. It works like a fence for marine life.

Simplified model. 1 sverdrup (Sv) = 1 million m³ of water per second. “Normal” uses the 173 Sv measured through the Drake Passage by Donohue and others (2016); older estimates were nearer 135 Sv. The weak and strong values are illustrative: in the real ocean, swirling eddies soak up much of any extra wind push, so the current speeds up less than the wind. Rivers: about 1.2 Sv in total, Amazon about 0.2 Sv. The Drake Passage probably opened to deep water sometime between about 41 and 30 million years ago (estimates differ); the gyres shown with it closed are a sketch, not a reconstruction.