FTV 2/17/2012 Deep Ocean Currents

In addition to the surface currents we looked at yesterday, currents may also be generated by density differences in water caused by temperature and salinity variations. These currents move water masses through the deep ocean taking nutrients, oxygen and temperature with them.

Occasional events also trigger serious currents. Huge storms move water masses. Underwater earthquakes may trigger devastating tsunamis. Both move masses of water inland when they reach shallow water and coastlines. Earthquakes may also trigger rapid down-slope movement of water-saturated sediments, creating turbidity currents strong enough to snap submarine
Temperature, salinity and pressure affect the density of seawater. Large water masses of different densities are important in the layering of the ocean water (more dense water sinks).

As temperature increases water becomes less dense. As salinity increases water becomes more dense. As pressure increases water becomes denser. A cold, highly saline, deep mass of water is very dense whereas a warm, less saline, surface water mass is less dense. When large water masses with different densities meet the denser water mass slips under the less dense mass. These responses to density are the reason for some of the deep ocean currents

Major subsurface currents in the oceans are most often due to differences in the density of water masses. A slow subsurface circulation of water develops with the sinking of cold water at the poles and its creeping across the ocean bottom with the meeting of north polar water and south polar water. There is a layering (due to density) near the equator. This is called thermohaline circulation (due to density differences in seawater caused by temperature and salinity) and some scientists predict it takes around 400 years for the water to complete this cycle. In the Atlantic ocean the Antarctic bottom water is denser than the North Atlantic bottom water and may creep up to 35 degrees north on the bottom. In the Pacific Ocean the North Pacific bottom water is denser and creeps down to nearly 15 degrees south latitude on the bottom. Each water mass has its own signature salinity, temperature and density.

Bottom water results from the sinking of polar water (blue = Antarctic, orange = Arctic) and their densities. A general stratification of the Pacific Ocean (left), Atlantic Ocean (middle), and Indian Ocean (right) shows their differences.

Try this activity on density currents you may need to refer to this picture when setting up your experiment.

Learn more about the density of saltwater in this activity. 

Use this activity to learn about deep ocean currents 

Try this Demo for Density to see how density affects currents.