Ocean Currents

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Ocean water moves along fairly consistent, well mapped paths for three main reasons operating together, at different depths and speeds. At the surface, wind drags directly on the water, and because the Earth is rotating, that wind-driven water does not travel in a straight line; the Coriolis effect deflects it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, bending steady wind-driven flows into the large circular current systems oceanographers call gyres. The Gulf Stream, carrying warm tropical water up the eastern coast of North America and across the North Atlantic toward Europe, and the Kuroshio, its Pacific counterpart off Japan, are both wind-driven surface currents shaped by this rotational deflection, and both move enormous volumes of heat poleward as they go, which is a major reason northwestern Europe stays markedly milder in winter than other places at the same latitude.

Beneath the wind-driven surface layer, a slower, deeper circulation operates on temperature and salinity rather than wind. Cold, salty water is denser than warm, less salty water, and in specific regions, most importantly the North Atlantic near Greenland and parts of the Southern Ocean around Antarctica, surface water becomes cold and salty enough to sink all the way to the deep ocean floor. That sinking water spreads slowly through the world's deep ocean basins, eventually rising back toward the surface elsewhere, in a circuit so vast that oceanographers estimate it takes on the order of a thousand years for a single parcel of water to complete one full loop, a slow global circulation often called the thermohaline circulation or, more informally, the ocean conveyor belt.

A third, ecologically crucial effect currents produce is upwelling, in which wind blowing along certain coastlines pushes surface water away from the shore, and cold, nutrient-rich water from the deep ocean rises to replace it. That nutrient-rich water fuels some of the most productive fisheries and richest coastal ecosystems on the planet, including the kelp forests that depend on a steady supply of upwelled nutrients to sustain their unusually fast growth. Currents are not perfectly stable from year to year, either; the clearest large-scale disruption is El Nino, during which the normal wind and current pattern across the equatorial Pacific weakens or reverses outright, temporarily shutting down the upwelling that ordinarily sustains some of the region's richest fishing grounds.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

Benjamin Franklin, serving as a colonial postal official in the 1760s, became curious about a specific practical mystery: British packet ships carrying mail from England to the American colonies routinely took two weeks longer to make the crossing than ordinary merchant ships sailing the same general route, despite the mail ships supposedly being faster vessels. Franklin's cousin, Timothy Folger, a Nantucket whaling ship captain, had a ready explanation. Whalers, who spent long stretches tracking their prey across the Atlantic, had long since learned to recognize a warm, fast-moving current running against a westbound ship and to steer around it, while the British packet captains, unaware of the current's exact path, sailed straight through it, fighting a current running directly against them the entire way.

Franklin and Folger worked together to chart the current's path in 1769, producing what is generally regarded as the first systematic map of the Gulf Stream, based on temperature readings, water color observations and the accumulated sailing experience of Nantucket's whaling captains rather than on any theory of ocean circulation, since the physical mechanisms driving the current, wind, the Coriolis effect and basin geography, would not be understood in any real detail for more than a century afterward. Franklin published the chart to help British ships avoid the current on the westbound crossing and use it to advantage heading east, a practical navigational fix built entirely from empirical observation.

Modern tracking of the Gulf Stream and currents like it bears almost no resemblance to Franklin's method beyond the basic goal of knowing where the water is moving and how fast. Satellite altimetry, measuring tiny variations in sea surface height that reveal a current's location and strength from orbit, combined with drifting buoys and autonomous underwater gliders, now lets oceanographers track the Gulf Stream's position and strength continuously rather than reconstructing it after the fact from a captain's logbook. That continuous tracking has taken on a significance Franklin could not have anticipated: some climate models project that a warming climate could weaken the Atlantic's deep overturning circulation, the same broader current system the Gulf Stream is part of, and monitoring its strength in something close to real time has become a genuine, actively studied question in climate science rather than only a matter of shipping schedules.