How does wind drive surface ocean currents?

Wind drives surface ocean currents primarily through the transfer of momentum from the atmosphere to the sea surface, a process known as wind stress. As wind blows across the water, frictional forces drag the surface layer along, setting it in motion.12

This wind-driven circulation is significantly influenced by the Coriolis effect, which deflects the path of moving water due to Earth's rotation. In the Ekman layer—the upper portion of the ocean affected by wind—this interaction creates the Ekman spiral. In this theoretical model, the surface water is deflected at an angle of 45° to the wind direction (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere). As depth increases, the current speed decreases and the direction of flow rotates further away from the wind direction.342

The net transport of water within this layer, known as Ekman transport, occurs at an average angle of 90° to the wind. Variations in this transport lead to upwelling and downwelling, which help extend wind-driven effects into deeper waters.2