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Wind stress on the ocean surface creates an Ekman spiral: the surface layer moves at ~45° to the wind direction due to Coriolis forcing, with successive deeper layers rotating further until flow reverses at the Ekman depth (~100 m). Net Ekman transport is perpendicular to the wind, enabling coastal upwelling when alongshore winds blow equatorward.
You already know that the Coriolis effect deflects moving objects on a rotating planet — to the right in the Northern Hemisphere, to the left in the Southern. The Ekman boundary layer is what happens when you combine that deflection with friction between layers of water. When wind blows steadily across the ocean surface, it drags the topmost layer of water along with it. But the Coriolis effect immediately begins deflecting that surface water — roughly 45° to the right of the wind direction in the Northern Hemisphere. This deflected surface layer then drags the layer beneath it, which gets deflected further, and so on down through the water column.
The result is the Ekman spiral: each successive layer moves more slowly and at a greater angle from the wind direction than the layer above it. By the time you reach the Ekman depth — typically around 100 meters, though it varies with wind strength and latitude — the current has rotated so far that it actually opposes the surface flow, and its speed has decayed to near zero. Picture a deck of cards fanned out: the top card points one way, each card below rotates a bit further, and the bottom card points almost the opposite direction. That fanning pattern, viewed from above, traces the spiral.
The critical insight is what happens when you add up all these layers. The net Ekman transport — the total movement of water integrated over the entire Ekman layer — points 90° to the right of the wind in the Northern Hemisphere (90° to the left in the Southern). This perpendicular transport is not intuitive, but it follows directly from the mathematics of balancing wind stress against Coriolis deflection through a frictional boundary layer. The individual layers each move at different angles, but their vector sum lands squarely at 90° from the wind.
This perpendicular transport has enormous consequences. Along a coastline where the wind blows parallel to shore — say, equatorward along a west coast in the Northern Hemisphere — Ekman transport pushes surface water offshore, away from the coast. That displaced surface water must be replaced, and the replacement comes from below: cold, nutrient-rich deep water rises to the surface in a process called coastal upwelling. This is why the world's most productive fisheries cluster along eastern boundary currents — the California Current, Peru Current, and Benguela Current all owe their biological richness to Ekman-driven upwelling. In the open ocean, converging or diverging Ekman transport also drives Ekman pumping, which pushes water downward or upward and helps shape the great subtropical gyres you will study next.