Forces driving ocean currents
Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"
Meaning
Ocean currents are set moving and steered by two kinds of force.
- Primary forces start the flow. Solar heating makes equatorial water expand, so it stands about 8 cm higher than water in the mid-latitudes and flows down that slight slope. Wind drags the surface water along. Gravity pulls piled-up water back down. The Coriolis force (the turning effect of Earth's rotation) bends moving water to the right in the Northern Hemisphere and to the left in the Southern.
- Secondary forces are differences in water density caused by temperature and salinity. Colder or saltier water is denser and sinks, which drives vertical movement.
Example
In the North Atlantic, the north-east trade winds push the North Equatorial Current west, and the Coriolis force bends the flow northward into the Gulf Stream. Far to the north, near Greenland, the Gulf Stream's salty water cools, becomes denser and sinks.