Previous Year Question

Ocean currents are regular and directed movements of seawater. Surface and deep-ocean currents are driven by different forces, but Earth’s rotation, winds and differences in water density are among their major controlling factors.

1. Rotation of the Earth—correct: Earth’s rotation produces the Coriolis effect. It deflects moving ocean water towards the right of its direction of motion in the Northern Hemisphere and towards the left in the Southern Hemisphere. The Coriolis effect is not the primary force that initiates a current, but it influences its direction and the structure of large oceanic circulations called gyres.

2. Air pressure and wind—correct: Differences in atmospheric pressure generate winds. Persistent winds transfer momentum to the ocean surface through friction and drive surface currents. Trade winds, westerlies and polar winds help operate large-scale surface-current systems. Near coastlines, winds can also cause upwelling and downwelling.

3. Density of ocean water—correct: Seawater density is governed mainly by temperature and salinity. Cold water is denser than warm water, while water with higher salinity is denser than less saline water. Cold, saline water sinks, whereas relatively warm or fresher water rises. These density differences drive the deep-ocean thermohaline circulation.

4. Revolution of the Earth—incorrect: Earth’s revolution around the Sun influences seasons and seasonal atmospheric conditions. It may consequently exert indirect seasonal effects on currents, but it is not treated as a direct principal factor comparable to rotation, wind and density differences.

Other factors affecting currents include the configuration of coastlines and ocean basins, gravity, tides and submarine topography.