The Ekman spiral, named after Swedish scientist Vagn Walfrid Ekman (1874-1954) who first theorized it in 1902, is a consequence of the Coriolis Effect.  

• When surface water molecules move by the force of the wind, they, in turn, drag deeper layers of water molecules below them.  
• Each layer of water molecules is moved by friction from the shallower layer, and each deeper layer moves more slowly than the layer above it, until the movement ceases at a depth of about 100 meters (330 feet).  
• Like the surface water, however, the deeper water is deflected by the Coriolis Effect-to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.  
• As a result, each successively deeper layer of water moves more slowly to the right or left, creating a spiral effect.  
• Because the deeper layers of water move more slowly than the shallower layers, they tend to “twist around” and flow opposite to the surface current. 

Source: G C Leong