8.1: Energy Sources

Winds are the primary energy source for currents that flow horizontally in the ocean surface layers (less than 100 to 200 m deep). Hence, surface currents are often called “wind-driven currents” or “wind drift currents.” Currents that flow deep in the oceans below the level affected by winds are generated primarily by convection caused by variations in water density (Chap. 5, CC3). Higher-density waters sink and displace (push aside or up) less-dense deeper water. Because density is determined by temperature and salinity (CC6), deep-water circulation is called thermohaline circulation.

The primary energy source for both wind-driven circulation and thermohaline circulation is the sun. Winds are generated by density variations in the Earth’s atmosphere caused by solar heating and radiative and evaporative cooling (Chap. 7). Thermohaline circulation is driven by density differences between ocean water masses caused by temperature and salinity variations. Variations in temperature and salinity are controlled, in turn, primarily by balances between solar heating and radiative cooling, and between solar evaporation and precipitation, respectively (Chap. 7).

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