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.