The two factors that contribute to differences in water density and drive the formation of deep ocean currents are temperature and salinity.
Cold water is denser than warm water, so as water cools, it becomes more dense and sinks. This is why deep ocean currents are often associated with regions of the ocean where water is very cold, such as the polar regions.
Salinity also affects water density. Water with a higher salt content is denser than water with a lower salt content. This is why deep ocean currents are often associated with regions of the ocean where water is very salty, such as the Atlantic Ocean near the equator.
When water with different temperatures and salinities come into contact with each other, the denser water sinks and the less dense water rises. This creates a circulation pattern in the ocean, with cold, salty water sinking in some regions and warm, less salty water rising in others. These circulation patterns drive the formation of deep ocean currents.