An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current’s direction and strength. Ocean currents are primarily horizontal water movements.

Factors leading to origin and modification of ocean currents:

• There are a variety of factors that affect how ocean currents (water in motion) are created, including a combination of two or more factors.
• The different types of currents (referred to as surface or thermohaline, depending on their depth) are created by, among other things, wind, water density, the topography of the ocean floor and the coriolis effect. 

Primary Forces Responsible for Ocean Currents: 

• Insolation
  • Heating by solar energy causes the water to expand. That is why, near the equator the ocean water is about 8 cm higher in level than in the middle latitudes.
  • This causes a very slight gradient and water tends to flow down the slope. The flow is normally from east to west.
• Wind (atmospheric circulation)
  • Wind blowing on the surface of the ocean pushes the water to move. Friction between the wind and the water surface affects the movement of the water body in its course.
  • Winds are responsible for both magnitude and direction [Coriolis force also affects direction] of the ocean currents. Example: Monsoon winds are responsible for the seasonal reversal of ocean currents in the Indian ocean.
  • The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern.
  • The air circulation over the oceans in the middle latitudes is mainly anticyclonic [Sub-tropical High Pressure Belt] (more pronounced in the southern hemisphere than in the northern hemisphere due to differences in the extent of landmass). The oceanic circulation pattern also corresponds with the same.
  • At higher latitudes, where the wind flow is mostly cyclonic [Sub-polar Low Pressure Belt], the oceanic circulation follows this pattern.
  • In regions of pronounced monsoonal flow [Northern Indian Ocean], the monsoon winds influence the current movements which change directions according to seasons.
• Gravity:
  • Gravity tends to pull the water down to pile and create gradient variation.
• Coriolis force:
  • The Coriolis force intervenes and causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere.
  • These large accumulations of water and the flow around them are called Gyres. These produce large circular currents in all the ocean basins. One such circular current is the Sargasso Sea.

Secondary Forces Responsible for Ocean Currents: 

• Temperature difference and salinity difference are the secondary forces.
• Differences in water density affect vertical mobility of ocean currents (vertical currents).
• Water with high salinity is denser than water with low salinity and in the same way cold water is denser than warm water.
• Denser water tends to sink, while relatively lighter water tends to rise.
• Cold-water ocean currents occur when the cold water at the poles sinks and slowly moves towards the equator.
• Warm-water currents travel out from the equator along the surface, flowing towards the poles to replace the sinking cold water.

Impact on climate of surrounding regions:

• Desert formation
  • Cold ocean currents have a direct effect on desert formationin west coast regions of the tropical and subtropical continents.
  • There is fogand most of the areas are arid due to desiccating effect (loss of moisture).
• Rains
  • Warm ocean currents bring rain to coastal areas and even interiors. Example: Summer Rainfall in British Type climate.
  • Warm currents flow parallel to the east coasts of the continents in tropical and subtropical latitudes. This results in warm and rainy climates. These areas lie in the western margins of the subtropical anti-cyclones.
• Moderating effect
  • They are responsible for moderate temperatures at coasts. [North Atlantic Drift brings warmness to England. Canary cold current brings cooling effect to Spain, Portugal etc.]
• Drizzle
  • Mixing of cold and warm ocean currents create foggy weather where precipitation occurs in the form of drizzle [Newfoundland].
• Climate
  • Warm and rainy climates in tropical and subtropical latitudes [Florida, Natal etc.],
  • Cold and dry climates on the western margins in the sub-tropics due to desiccating effect,
  • Foggy weather and drizzle in the mixing zones,
  • Moderate clime along the western costs in the sub-tropics.
• Tropical cyclones
  • They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.

An ocean current flows for great distances and together they create the global conveyor belt, which plays a dominant role in determining the climate of many of Earth’s regions. More specifically, ocean currents influence the temperature of the regions through which they travel. Knowledge of surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. Ocean currents can also be used for marine power generation, with areas off of Japan, Florida and Hawaii being considered for test projects.