Surface Currents involve large masses of water moving horizontally on the surface.
• The transfer of wind energy to water is not very efficient
(only about 2% energy transfer of “friction” between water and air).
• Wind produces both waves and currents (more on waves in Chapter 10).
• Surface currents occur in the mixing zone within and above the pycnocline (layer of rapidly changing density).
• Effects of surface currents is to redistribute heat from equatorial to polar regions.
Mechanisms moving surface currents include:
• Wind: major mechanism (result of atmospheric circulation patterns).
• Solar heating: (direct heating by the sun) - a minor mechanism (influences surface waters but not water at depth).
• Tides: (affect currents in coastal regions - tidal currents are discussed in Chapter 10).
• Geography: locations of continents (and islands) influence direction and flow currents (acting as barriers to flow).
Subtropical gyres are large system of rotating ocean surface currents driven by global wind currents with the influence of Ekman Transport (see below) and continental geography (land masses restrict and deflect the flow of water currents).
Movement of Surface Currents
Moving water (like wind) are influenced by the Coriolis effect (Figure 9.7):
• Moving water is deflected to the RIGHT in the Northern Hemisphere.
• Moving water is deflected to the LEFT in the Southern Hemisphere.
The Coriolis effect has a large influence on the movement of both surface water and deeper water. However, wind-driven currents move fastest near the ocean surface and diminish with depth. The difference in rate of movement results in a rotational process called Ekman transport.