The two most effective generating forces of surface ocean currents are ____________.

The correct choice highlights that wind and density differences in the water are the primary forces behind the generation of surface ocean currents.

Wind is a significant driving force for surface currents, creating friction as it blows across the ocean’s surface. This friction results in the movement of the water, forming currents that can flow for long distances. The Coriolis effect, which is influenced by the rotation of the Earth, also plays a role, causing these currents to veer to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Density differences, which result from variations in temperature and salinity of ocean water, are also crucial. When water becomes denser due to cooling or increased salinity, it tends to sink, while less dense, warmer water rises. This process contributes to the establishment of currents and plays a vital role in the vertical movement of water in oceans, further influencing surface currents.

Understanding these mechanisms is essential, as they illustrate how wind can initiate movement, and density differences can sustain and modify the currents over time. Together, these forces create a complex, dynamic system driven by both atmospheric and oceanographic processes.