Rules of Thumb & Things to Remember
- Surface currents are 100-200 m thick.
- Surface currents are driven by wind and the rotation of the Earth.
3.1. Ekman Current
Ekman current is the surface current caused by steady wind at the surface (wind-driven, surface-current). As wind blows over the ocean surface, an Ekman current develops due to the drag at the wind-water interface and it deflected to the right due to the Coriolis effect as shown below. Deflection to the right is observed in northern hemisphere, whereas deflection to the left is observed in southern hemisphere.
The Ekman current is acts to grater depths due to viscous forces in the water mass. Each subsequent layer is forced by viscous forces (internal friction) to flow in the direction of the layer above. At the same time, the Coriolis force cause the deflection of the current to the right; thus, each subsequent layer as we go deeper is deflected even further to the right compared to the initial wind direction.
Ekman’s spiral indicates that each moving layer is deflected to the right of the overlying layer’s movement; hence, the direction of water movement changes with increasing depth. As depth increases, the force transmitted from the driving wind declines and thus the velocity of the resultant steady current decreases, hence the tapered spiral representation.
The depth to which the Ekman spiral penetrates depends on wind speed. A wind blowing steadily over deep water for 12 hours at an average speed of about 1 m/s (3.6 km/h or 2.2 mph) would produce a 0.02 m/s current (about 2% of the wind speed). The Ekman spiral indicates that each successive layer moves more toward the right and at a slower speed. At a depth of about 100 to 150 m, the Ekman spiral has gone through less than half a turn and the current speed reaches zero.
Ekman transport is the total water transport in the Ekman layer. It is calculated by integrating the mass transport though a window of unit width 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 in order to get the net transport.
Coastal upwelling occurs where Ekman transport moves surface waters away from the coast; surface waters are replaced by water that wells up from below.
Coastal downwelling occurs where Ekman transport moves surface waters towards the coast; water piles up and sinks below the surface current.
Gyres are large circular currents that flow over vast ocean areas