There are two type of Ocean Currents:
1. Surface Currents--Surface Circulation
These waters make up about 10% of all the water in the ocean.
These waters are the upper 400 meters of the ocean.
2. Deep Water Currents--Thermohaline Circulation
These waters make up the other 90% of the ocean
These waters move around the ocean basins by density driven forces and gravity.
The density difference is a function of different temperatures and salinity
These deep waters sink into the deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
Ocean Currents are influenced by two types of forces
1. Primary Forces--start the water moving
The primary forces are:
2. Secondary Forces--influence where the currents flow
Solar heating cause water to expand. Near the equator the water is about 8 centimeters high than in middle latitudes. This cause a very slight slope and water wants to flow down the slope.
Winds blowing on the surface of the ocean push the water. Friction is the coupling between the wind and the water's surface.
A wind blowing for 10 hours across the ocean will cause the surface waters to flow at about 2% of the wind speed.
Water will pile up in the direction the wind is blowing.
Gravity will tend to pull the water down the "hill" or pile of water against the pressure gradient.
But the Coriolis Force intervenes and cause the water to move to the right (in the northern hemisphere) around the mound of water.
These large mounds of water and the flow around them are called Gyres. The produce large circular currents in all the ocean basins.
Note how the North Atlantic Gyre is separated into four distinct
Currents, The North Equatorial Current, the Gulf Stream, the North Atlantic Current, and the Canary Current.
But why doesn't the water spin towards the center of the ocean? Why does it flow around the hill in this circular motion.
Remember the hill of water-- This hill is formed by the inward push of water through a process call Ekman Transport
Remember the Coriolis Force move objects to the right in the northern hemisphere
Wind blowing on the surface of the ocean has the greatest effect on the surface. However, for the lower layers of the ocean to move they must be pushed by the friction between the layers of water above. Consequently, the lower layer moves slower than the layer above. With each successive layer down in the water column the speed is reduce. This leads to the spiral affect seen in the above diagram.
The net movement of water (averaged over the entire upper 330 meters of the ocean) is 90o to the right of the wind direction (in the northern hemisphere).
When the water is pushed to the right it forms the hill we described above. So, when water is pushed along by the wind it wants to be turned to the right by the Coriolis force (in the northern hemisphere) but it must fight against gravity (trying to move up the hill of water formed by Ekman transport). A balance is met between the Coriolis and the gravity (pressure gradient force). This balance produces a balanced flow called a Geostrophic current.
Boundary Currents are the major geostrophic currents around the gyre
Note the difference is strength (Sv) between the western and eastern boundary currents. This is caused by the effect of the rotating Earth which tends to move the "hill" of water to the western sides of the ocean basins