Ocean Current
An ocean current is a continuous, directed flow of seawater driven by wind, differences in temperature and salinity, Earth's rotation, and tides.
Definition
An ocean current is a large-scale, directed flow of seawater, some driven by wind at the surface and others by density differences that make cold, salty water sink and creep along the ocean floor. Surface currents, shaped by wind stress, the Coriolis effect, and the position of continents, form circular gyres that rotate clockwise in the Northern Hemisphere, with Ekman transport moving water $90^\circ$ to the right of the wind and Sverdrup theory explaining why flow intensifies into strong western boundary currents like the Gulf Stream. Deep currents form the global thermohaline circulation, or conveyor belt, driven by dense water sinking near Greenland and around Antarctica during sea-ice formation, and the Atlantic Meridional Overturning Circulation alone carries about $1.2 \times 10^{15}$ watts of heat northward, redistributing warmth between the tropics and the poles.
Example
The Gulf Stream is a warm current flowing up the East Coast past the Outer Banks toward Europe, carrying warm water at speeds up to $2\ \text{m/s}$ and keeping western European winters far milder than Labrador's at the same latitude, while the cold Labrador Current meets it off the Outer Banks, creating rich fishing grounds and frequent fog. By the time it passes Cape Hatteras the Gulf Stream transports over $100$ million cubic meters per second, more than a hundred times the combined flow of all the world's rivers, and during Heinrich events of the last ice age, freshwater floods weakened this overturning circulation and cooled the North Atlantic within decades.
Key Insight
Currents move heat as much as water, which is why nearby coasts warmed by a current stay milder and coasts cooled by one stay drier, and deep ocean water takes about a thousand years to complete one lap of the global conveyor belt, meaning water sinking near Greenland today may not resurface until the year 3000. The overturning circulation is also considered a possible tipping element in the climate system, since freshening of the North Atlantic from Greenland's melting ice could weaken the sinking that drives it, and paleoclimate records show it has switched states abruptly before.