The Ekman spiral refers to a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

 oceanographer

Oceanography

Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...

 Vagn Walfrid Ekman

Vagn Walfrid Ekman

Vagn Walfrid Ekman was a Swedish oceanographer.Born in Stockholm to Fredrik Laurentz Ekman, himself an oceanographer, he became committed to oceanography while studying physics at the University of Uppsala and, in particular, on hearing Vilhelm Bjerknes lecture on fluid dynamics.During the...

. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen

Fridtjof Nansen

Fridtjof Wedel-Jarlsberg Nansen was a Norwegian explorer, scientist, diplomat, humanitarian and Nobel Peace Prize laureate. In his youth a champion skier and ice skater, he led the team that made the first crossing of the Greenland interior in 1888, and won international fame after reaching a...

 during the Fram expedition (1893–1896).

The effect is a consequence of the Coriolis effect

Coriolis effect

In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

 which subjects moving objects to a force to the right of their direction of motion in the northern hemisphere (and to the left in the Southern Hemisphere). Thus, when a persistent wind blows over an extended area of the ocean surface in the northern hemisphere, it causes a surface current which accelerates in that direction, which then experiences a Coriolis force and acceleration to the right of the wind: the current will turn gradually to the right as it gains speed. As the flow is now somewhat right of the wind, the Coriolis force perpendicular to the flow's motion is now partly directed against the wind. Eventually, the current will reach a top speed when the force of the wind, of the Coriolis effect, and the resistant drag of the subsurface water balance, and the current will flow at a constant speed and direction as long as the wind persists. This surface current drags on the water layer below it, applying a force in its own direction of motion to that layer, repeating the process whereby that layer eventually becomes a steady current even further to the right of the wind, and so on for deeper layers of water, resulting in a continuous rotation (or spiraling) of current direction with changing depth. As depth increases, the force transmitted from the driving wind declines and thus the speed of the resultant steady current decreases, hence the tapered spiral representation in the accompanying diagram. The depth to which the Ekman spiral penetrates is determined by how far turbulent mixing can penetrate over the course of a pendulum day.

The diagram above attempts to show the forces associated with the Ekman spiral as applied to the Northern hemisphere. The force from above is in red (beginning with the wind blowing over the water surface), the Coriolis force (which is shown at right angles to the force from above when it should in fact be at right angles to the actual water flow) is in dark yellow, and the net resultant water movement is in pink, which then becomes the force from above for the layer below it, accounting for the gradual clockwise spiral motion as you move down.

The classic Ekman spiral has been observed under sea ice, but is not found in most open-ocean conditions. This is due both to the fact that the turbulent mixing in the surface layer

Surface layer

The surface layer is the layer of a turbulent fluid most affected by interaction with a solid surface or the surface separating a gas and a liquid where the characteristics of the turbulence depend on distance from the interface...

 of the ocean has a strong diurnal cycle

Diurnal cycle

A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth.In climatology, the diurnal cycle is one of the most basic forms of climate patterns. The most familiar such pattern is the diurnal temperature variation...

and to the fact that surface waves can destabilize the Ekman spiral. Ekman spirals are, however, found in the atmosphere. Surface winds in the Northern Hemisphere tend to blow to the left of winds aloft.