Some deep ocean water takes about a thousand years to resurface
Warm Atlantic water flows north, cools, grows dense and sinks near the Arctic. From there it creeps through the deep basins of the world, and the oldest of it rises again in the North Pacific roughly a millennium later. This slow global loop, nicknamed the conveyor belt, helps set the climate of entire continents.
Two engines move the sea. Near the surface, prevailing winds push water along, and Earth's rotation deflects it so that currents run at an angle to the wind and curl into clockwise gyres in the Northern Hemisphere and anticlockwise ones in the south. Deeper down, the driver is density, which depends on temperature and saltiness; hence the name thermohaline, from the Greek-derived words for heat and salt. Oceanographers measure flow in sverdrups, each equal to a million cubic metres of water per second.
Currents come in grades. A drift, such as the North Atlantic Drift, is a slow wind-pushed surge of surface water; a current like the Labrador moves faster in a firmer direction; and a stream such as the Gulf Stream shifts the largest volumes at the highest speeds. The biggest of all is the Antarctic Circumpolar Current, which circles Antarctica without interruption, linking every ocean basin.
Their effect on climate can be startling. The Gulf Stream and its northern extension keep northwest Europe far milder than other places at the same latitude and help keep its harbours free of ice. Lima, in Peru, stays surprisingly cool for the tropics because the cold Humboldt Current runs past it. Cold upwelling water carries nutrients to the surface, feeding the plankton at the base of marine food webs. Currents also carry life across oceans: European eels depend on them during their early drifting stages, and tortoises and lizards have reached distant islands riding on floating debris.
Sailors learned these lessons the hard way. In the age of sail, the Agulhas Current off southeast Africa long frustrated attempts to reach India, and ignorance of currents likely doomed some expeditions. Today shipping companies save fuel by riding them, and round-the-world yachtsmen use them for speed. Since the 2000s, a fleet of drifting Argo floats has been mapping ocean temperature and salinity. Researchers expect the Antarctic Circumpolar Current to lose about a fifth of its strength by 2050.
Source: Ocean current
