Ocean currents

Imagine a river flowing inside the sea, in one fixed direction, mile after mile. That is an ocean current: a steady stream of water moving across the ocean in a definite direction (Fig. 1.9).

What keeps currents moving?

• Winds push the surface water along.
• Earth's spin: as the Earth turns, anything moving freely gets bent off course: in the northern half of the world it bends to the right, and in the southern half to the left. This turning push is the Coriolis force.
• Differences in temperature and saltiness: cold water and saltier water are heavier and sink, and lighter water moves in to take their place.
• The shape of the continents and ocean basins, which steer the water.

Warm or cold? It depends on where the water comes from. Water flowing from warmer places, near the equator, makes a warm current. Water flowing from colder places, near the poles, makes a cold current. On maps, warm currents are usually drawn red and cold ones blue.

Currents change the weather on the coast. Two examples from the book:

• The Gulf Stream is a warm current that starts in the Gulf of Mexico, on the Atlantic side, and flows all the way towards Norway. It keeps those waters warm, so sea ice does not form easily there, even though Norway is far north, at 60°-70° latitude.
• The Labrador Current is a cold current along the east coast of Canada. It carries freezing water, and sea ice and icebergs form even at lower latitudes.

Where warm meets cold, fish gather. Where a warm current and a cold current meet, the water is rich in nutrients. Tiny plants grow, small fish eat them, bigger fish eat those. Such places are rich fishing grounds.