When global winds blow across the ocean, they don't just pass over the water; they grab it. This grip is called frictional drag, and it is the primary mechanism that transfers kinetic energy from the moving air directly into the seawater. This energy transfer stirs the thin upper layer of the sea, setting massive volumes of water into horizontal motion across entire ocean basins.

Not all wind-driven water moves at the same pace or in the same way. Oceanographers classify these horizontal movements into three categories based on their speed and structure. A drift, such as the North Atlantic Drift, is a broad, slow movement of water directly controlled by the prevailing winds. In contrast, a current like the Labrador Current is swifter and flows in a more definite direction. At the extreme end, a stream like the Gulf Stream is a highly defined, high-velocity highway of water.