Surface Currents are 100-200 thick. Surface currents are driven by wind and the rotation of the Earth.

3.1. Ekman Current

Ekman current is the surface current that develops from steady wind at the ocean surface. As wind blows over the ocean a surface current develops due to the drag at the wind-water interface. The surface current, under the Coriolis effect is deflected to the right (northern hemisphere) as shown below.

Ekman-transport

As an Ekman current is transmitted to grater depths by viscous forces, each subsequent layer is forced by viscous forces (internal friction) to flow in the direction of the layer above. The Coriolis force will, again, cause thedeflection of the current to the right; thus, each subsequent layer as we godeeper is deflected even further to the right compared to the initial winddirection.

Ekman's spiral indicates that each moving layer is deflected to the right of the overlying layer's movement; hence, the direction of watermovement changes with increasing depth. As depth increases, the force transmitted from the driving wind declines andthus the speed of the resultant steady current decreases, hence the tapered spiral representation.

The depth to which the Ekman spiral penetrates depends on wind speed. A windblowing steadily over deep water for 12 hrs at an average speed of about 100 cmper sec (2.2 mi per hr) would produce a 2 cm per sec current (about 2% of thewind speed). The Ekman spiral indicates tha each successive layer moves moretoward the right and at a slower speed. At a depth of about 100 to 150 m, theEkman spiral has gone through less than half a turn and the current is so weakthus we it generally assumed that this is the typical thickens of wind-drivensurface-currents.

Ekman transport is the total water transport in the Ekmanlayer. It is calculated by integrating the mass transport though a window ofunit with and height dz from the surface to the bottom of the Ekman current.The unit width is taken to be normal to the current velocity at every depth inorder to get the net transport.

 

Equations

 

fv= −Az∂2u∂z2

 

Example 1

 

 

Example 2

 

 

 

3.2. Upwelling

Coastal upwelling occurs where Ekman transport moves surface waters awayfrom the coast; surface waters are replaced by water that wells up frombelow.

upwelling-ekman-current

3.3. Downwelling

Coastal downwelling occurs where Ekman transport moves surface waterstowards the coast; water piles up and sinks below the surface current.

downwelling-ekman-current 

 

 

Futher Reading

http://oceanmotion.org/html/background/ocean-in-motion.htm.

 

3.4. Gyres

Gyres are large circular currents that flow over vast ocean areas