Global Ocean Circulation

The Earth receives most of its heat energy as a direct source from the sun. This is known as insolation. However this insolation is not evenly distributed around the globe [1,2]. Areas in the equatorial region receive a surplus of energy, and areas in the higher latitudes, have an energy deficit. This is caused mostly by both the curvature of the Earth, as well as its position on its axis[1].

The curvature of the Earth means that the solar insolation directed to the higher latitudes are required to cover a greater surface area than those at the equator. This is demonstrated in the figure below. Can you see how the yellow bars are having to spread further in the higher latitudes?

A globe with highlighted areas of solar insolation. Towards the centre of the Earth, the insolation covers a smaller surface than in the higher latitudes
Figure 1. Solar insolation distribution across Earth

Methods of Heat Transport

Thermal energy, just like any other type of energy, is continuously flowing from areas of high energy to areas of low[1,2]. Both atmospheric and oceanic processes allow the heat energy surplus from the equator to travel to higher latitudes where there is a heat deficiency.

Figure 2. Diagram of atmospheric circulation. NASA

In the atmosphere, heat can be convected and transported through a series of convection cells where the warmer air rises at the equator, and is then transported to the polar regions. Different atmospheric cells, in both the Northern and Southern Hemisphere are responsible for this flow of heat energy. These atmospheric cells are named the Hadley, Ferrel and Polar cells as illustrated in Figure 2.

This model of atmospheric circulation is very efficient. You can read more about these cells and the methods of atmospheric heat transport here.

Ocean Circulation and the Thermohaline Conveyor

Another method of heat transport is through ocean currents. The global oceans cover up to 71% of the Earth’s surface, and is able to retain heat for much longer than both the atmosphere. This means therefore that the oceans can redistribute heat around the globe very efficiently. Ocean currents are driven by different processes including wind speed and direction, coriolis forcings, and are also influenced by the presence of different landmasses [1,2,3].

There are different types of ocean currents, all of which have a different purpose. These sit at different depths of the ocean dependent on their temperature and salinity.

Warm ocean currents are those which flow at the surface of the ocean. They are able to do this because they are warmer and have a higher salinity and thus are less dense. These warmer currents transport heat from the equator to the polar regions. Cold ocean currents are different. These flow at lower depths as they are much cooler and are mixed with a greater amount of freshwater from the melting ice and thus are more dense and sink. There is a constant flow of warm currents flowing to higher latitudes where they cool and sink in downwelling areas, and flow back to the equator (Figure 3). This process occurs all around the globe and is part of a big system known as the Thermohaline Conveyor which travels all through the world’s oceans[2,3].

Ocean Circulation pattern flowing around the globe
Figure 3. a diagram displaying the Thermohaline Conveyor. Source WikiComms

Ocean Circulation around Antarctica

Thermohaline conveyor from a polar perspective showing the warmer and cooler currents around Antarctica
Figure 4: The Antarctic Circumpolar Current (ACC) and its connection to the wider Thermohaline Conveyor

Ocean circulation plays a large role in Antarctica’s complex climate system with the Antarctic Circumpolar Current (ACC) being the most important. The ACC is part of the thermohaline conveyor and is a complex system of interacting currents from surrounding oceans.

The ACC helps maintain the cold climate of Antarctica by limiting the amount of meridional heat transport to the continent. Because of the nature of Antarctica’s position, the ACC is able to flow all around the continent without interactions from other landmasses. This also makes the ACC unique compared to other ocean currents. The ACC is mostly driven by dominant westerlies, enabling it to flow around Antarctica in a clockwise direction [4].

Like other ocean currents, the ACC is characterised with areas of downwelling, where the denser water sinks, exchanging both heat and gasses to lower depths, and also areas of upwelling [4]. These areas of upwelling are very important for biological life as it enables nutrients to reach the surface and facilitate phytoplankton blooms making an important part of the Antarctic food chain.

Ocean Currents StoryMap

For more information about the global heat distribution, ocean and atmospheric circulation, as well as interactive activities relating to global ocean currents, check out this teaching resource from the Royal Holloway Geography Department’s TeacherHub page. This teaching resource includes a StoryMap produced for UK KS5 students, and a lecture.

References

1. NASA Earth Observatory ‘Climate and Earth’s Energy Budget’ Available at: https://earthobservatory.nasa.gov/features/EnergyBalance

2. Seidov, D. (2009). Heat Transport, Oceanic and Atmospheric. In: Gornitz, V. (eds) Encyclopedia of Paleoclimatology and Ancient Environments. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-4411-3_102

3. Bennett, M.R. (2022). Earth’s External Heat Engine. In: Our Dynamic Earth: A Primer . Springer, Cham. https://doi.org/10.1007/978-3-030-90351-0_4

4. Toggweiler, J. R., and Russell, J. (2008). Ocean circulation in a warming climate. Nature, 451(7176), 286–288.

About

I am Laura Boyall, a PhD student in the Department of Geography at Royal Holloway University of London. My PhD research focuses on reconstructing past climate using different statistical methods and computer models to help us understand more about the predictability of the climate system.

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