Which one of the following is NOT a primary force initiating ocean currents?

The correct option is C.

Explanation

Ocean currents are categorized into two types based on the forces that drive them: primary forces that initiate the movement of water and secondary forces that influence the direction and flow of the currents. Primary forces include solar heating, gravity, wind, and the Coriolis force.

• Option A: Heating by solar energy (Correct): Solar heating causes water to expand. Near the equator, the ocean water level is approximately 8 cm higher than in middle latitudes. This slight gradient causes water to flow down the slope, initiating movement.
• Option B: Gravity (Correct): Gravity acts as a primary force by pulling water down the gradient created by solar expansion or surface accumulation. It also plays a role in the vertical movement of water masses.
• Option D: Prevailing winds (Correct): Wind blowing on the surface of the ocean exerts friction, pushing the water forward. This is a dominant primary force in the initiation of surface currents.
• Option C: Differences in water density (Incorrect): Density differences, caused by variations in temperature and salinity, are classified as secondary forces. While they drive the deep-ocean vertical circulation (thermohaline circulation), they are responsible for the movement of water rather than the initial horizontal initiation of surface currents.

Key Takeaway: Primary forces (Solar heating, Wind, Gravity, Coriolis force) initiate the movement of ocean water, while secondary forces (Density differences, Temperature gradients) influence the vertical movement and speed of the currents.