What two forces primarily drive deep ocean currents?
a. winds and salinity
b. winds and temperature
c. temperature and salinity
d. wind and earth's rotation

The best answer is C. Deep ocean currents are driven by differences in the water's density, which is controlled by temperature (thermo) and salinity(haline). This process is called thermohaline circulation. In the earth's polar region, ocean water gets very cold forming sea ice. As a consequence, the surrounding sea water gets saltier because when sea ice is forms, the salt is left behind. As the sea water gets saltier, the density increases and it starts to sink. Surface water is pulled in to replace the sinking water which in turn eventually becomes cold and salty enough to sink. This initiates the deep ocean currents driving the global conveyer belt.