Why are density differences in water one of the primary forces that cause ocean currents?

Denser liquids sink, causing dense water to move under water that is less
dense.

Denser liquids are warmer, and warmer water tends to move toward the
equator.

Fast-moving winds move denser water faster than water that is less
dense.

Less dense liquids sink, resulting in upwelling that moves denser water to
the surface.