Ocean Currents (SL IB Geography)

• Ocean currents move the water in our oceans around the world 
• There are two major types of ocean currents:
  • Surface currents – the currents that flow over the surface of the ocean
  • Deep currents – the currents deeper in the ocean, around 300 meters deep

Surface currents

• Wind is one of the main drivers of surface current distribution
• The wind is caused by the Atmospheric Circulation:
  • Surface currents follow common wind belt paths, such as Trade Winds, the Westerlies and the Polar easterlies
• The Coriolis Effect causes a deflection of the atmosphere as it rotates. This means that wind patterns vary:
  • The wind deflects to the west in the Southern Hemisphere and east in the Northern Hemisphere
  • Ocean currents flow in the deflection direction – currents will spiral clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere 
  • These swirling currents are called gyres and occur on either side of the equator
  • In a low-pressure system (where storms form), ocean currents spiral in the opposite direction (clockwise in the southern hemisphere and anticlockwise in the northern hemisphere)

Image showing the deflection of wind from the Coriolis Effect

Image showing the deflection of wind from the Coriolis Effect

• Ocean topography, e.g. ocean basins or other landmasses, can also affect surface currents
• The main drivers of deeper ocean currents are temperature and salinity

• The Oceanic Conveyor Belt, powered by Thermohaline Circulation, drives the movement of deeper currents:
  • The Oceanic Conveyor Belt moves waters from the north to the south of the world and all the way back again, just like a conveyor belt 
• Ocean water moves in a cyclical pattern. Warmer waters flow downwards, and colder waters flow upwards
• Heat and salinity control Thermohaline Circulation:
  • Heat and Salinity affect water density
  • Cold water is more dense than warm water
  • The higher the salinity, the denser the water
• Starting in polar areas, the cold and salty (dense) water sinks
• Surface water then replaces this sinking water
• This process repeats, forming the deep currents 
• These currents then make their way around the world, into areas where the water will heat up again
• This warmer (less dense) water returns to the surface, moving further around the world and eventually reaching the point where the process started
• The cycle repeats
• One full loop of the Oceanic Conveyor Belt could take anywhere between 100 and 1000 years

 Map showing thermohaline circulation process

Image of the Thermohaline Circulation process

• Without the Oceanic Conveyor Belts, average temperatures across the world would drop 
• Weather events like Hurricanes would become more common
• Marine life, like fisheries, would be severely affected 

• The Coriolis Effect can cause upwelling:
• The wind and atmospheric deflection cause the water on the surface of the ocean to blow away
• Upwelling occurs when deep ocean currents rise upwards to replace surface waters
• Upwelling brings nutrient-dense waters to the surface, which is perfect for species like plankton. This provides a food source for other marine life
• Upwelling at the coast can provide a perfect climate for fishing industries

Diagram showing the process of upwelling 

Image showing the process of upwelling in transferring nutrient-dense waters 

• Oceanic Conveyor Belts are important for nutrient and carbon dioxide cycles:
  • Surface waters are not nutrient- or carbon-dioxide-rich 
  • As they move through the conveyor belt, they become enriched with nutrients and carbon dioxide
• Solar radiation hits the Earth unequally. Ocean currents are important for distributing warmer and colder waters around the globe:
  • Heat energy transfers around the world as warmer water from the equator moves towards the poles and vice versa
  • Ocean currents play a major role in controlling our climate and weather