Why wind-driven water moves sideways
Wind transfers momentum to the ocean by dragging across its surface. But the water does not simply travel in the wind’s direction: as Earth rotates, moving water is deflected by the Coriolis effect. The deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
A turning stack of water
The surface layer pulls on the water below it through friction. That deeper layer moves more slowly and is deflected too, but its direction turns farther from the wind. Each successive layer moves more slowly and at a different angle, producing a spiral pattern known as the Ekman spiral.
Those layers do not all move sideways by the same amount. When their movements are added together, the net transport of the wind-driven upper layer—the Ekman transport—is roughly 90 degrees from the wind: rightward in the north and leftward in the south. The angle describes this combined transport, not the path of every parcel of water at the surface.
Why the distinction matters
This sideways transport can move surface water away from a coastline. Water from below then rises to replace it, a process called upwelling. The rising water often carries nutrients that support productive marine food webs. The same wind can therefore help shape both the direction of ocean movement and the conditions that make some coastal waters especially rich in sea life.
