Surface currents, the Ekman spiral, and Ekman transport
As wind moves across the surface of the ocean, friction between moving air and the water surface causes water to begin to move as well. This transfer of energy by friction is how wind causes surface currents. Once water at the surface begins to move, some energy gets transferred to deeper layers allowing water movement to penetrate to depths of 50 to 100 meters.
Video Overview
Walford Ekman first investigated the movement of water in response to surface winds in 1905. The arctic explorer Fridtjof Nansen gave Ekman the idea to study surface currents. While on an expedition in the Arctic, Nansen noticed that windblown sea ice did not move in the same direction as the wind, Instead, it moved 20 to 40 degrees to the right of the wind. He correctly speculated that this was due to the influence of Earth's rotation.
Eckman took Nansen's idea and built a mathematical model to explain it. The model treats water as a series of layers that move independently of each other. As each layer moves the energy from the motion of each layer can transfer to the layer just below it.
Movement of energy in the model can be tracked by considering a vertical column of water that starts at the surface and extends towards the bottom. As energy moves from the surface to deeper layers, two things happen:
- Because there is energy lost during each transfer, water speed diminishes quickly with depth.
- As energy transfers from layer to layer, the Coriolis effect deflects the path of each layer to the right of the one above it. This creates a spiraling pattern called an Ekman spiral.
One result of this deflection pattern is that water near the bottom of the spiral moves in the opposite direction of the wind driving the motion.
Because water movement diminishes quickly with depth, Ekman showed that the net transfer of water is at a 90-degree angle to wind direction. Since the Coriolis effect drives this, the deflection is 90 degrees to the right in the Northern Hemisphere and 90-degrees to the left in the southern hemisphere. The net movement of water at right angles to the direction of the prevailing winds is called Ekman transport.
Consistent with Nansen's observations of the icebergs, water right at the surface moves 20 to 40 degrees to the right or left of the wind direction. The exact angle depends on how long and how consistently the wind blows.
Ekman transport contributes to fundamental features of the surface ocean. In coastal areas where prevailing winds blow along the coast, Ekman transport causes surface water to flow offshore. This movement of water away from the coast at the surface causes deeper water to upwell. This upwelled water tends to be rich in nutrients, making coastal upwelling zones highly productive areas.
Wind blowing in the opposite direction pushes surface water towards the coast. As water piles up at the shore, it is forced down, creating downwelling. The concentration* of relatively nutrient-poor surface water makes these regions less productive.
Eckman transport driven up and downwelling is not limited to coastal environments. In the open ocean, trade winds also caused Ekman transport.
Along the equator prevailing winds blow from east to west. This causes water to move away from the equator in both the northern and southern hemispheres. Diverging currents lower the water level right along the equator resulting in the upwelling of deeper water.
Ekman transport causes water to pile up at mid-latitudes in the zone where prevailing winds transition from easterly to westerly, water piles up as the change in wind direction pushes surface currents towards each other. This convergence causes downwelling.
Just like along the coast, upwelling along the equator brings nutrient-rich water to the surface stimulating primary productivity. The piling up of water at mid-latitudes contributes to the formation of gyres that circulate in all of Earth's major ocean basins.