Ocean currents and gyres
Wind and Earth's spin drive great circling currents in each ocean basin, with fast currents such as the Gulf Stream on their western sides.
The surface of the ocean is always moving. Steady winds drag on the water, and Earth's rotation deflects moving water, to the right in the Northern Hemisphere and to the left in the Southern, through what is called the Coriolis effect. The result is a set of enormous, slowly circling current systems called gyres. There are five major subtropical gyres: in the North and South Atlantic, the North and South Pacific, and the Indian Ocean.
Gyres are lopsided. The currents on their western sides are narrow, deep and fast, an effect called western intensification, while the return flow on the eastern side is broad and slow. The Gulf Stream, which carries warm water from the Caribbean and the Gulf of Mexico up the American coast and across the Atlantic, is the best known of these western boundary currents. A ship sailing with it gets a free boost and one sailing against it loses ground, because its speed over the sea floor is its speed through the water plus or minus the current.
Anything that floats goes along for the ride. Drift bottles, buoys, lost cargo and plastic have all traced the currents, and floating debris tends to collect in the quiet centres of the gyres. Sea ice drifts too: in the Arctic, the Transpolar Drift carries ice from the Siberian side across the pole towards the Atlantic.
Coming up in future stories
- The Old Phone Cable That Weighs the Gulf Stream April 1982 → today · Straits of Florida
- The Oldest Message in a Bottle Was a Science Experiment 1886 / 2018 · Indian Ocean & Wedge Island, Western Australia
- Why Britain's Mail Was Two Weeks Late 1768 · London & the North Atlantic