Ocean Circulation and the Earth’s Climate
- Fiona Sridjaja
- Jan 7, 2023
- 4 min read
Updated: Sep 27, 2023
Introduction
The oceans covers a large area of the earth, making up to 71 percent of the planet’s surface, which is a total of 930 million square kilometres and holds a total volume of 1,400 million cubic kilometres, unevenly distributed across the planet. It is also largely responsible for the earth’s climate and in sustaining life here on earth. The five major oceans are the Pacific Ocean, the Atlantic Ocean, the Arctic Ocean, the Indian Ocean and the Antarctic Ocean (also called the Southern Ocean). On the other hand, seas are smaller and of little depth compared to oceans and have some surrounding land. This includes the Caribbean, Baltic, Bering and Mediterranean.
What is Ocean Circulation?
Ocean circulation is when large amounts of water that move together, transport heat, marine organisms, nutrients, dissolved gases (for example oxygen and carbon dioxide) and pollutants across and below the earth’s surface. As a result, our climate and ecosystems, no matter how discrete, are all affected by the patterns of ocean circulation. Ocean circulation patterns are regulated by atmospheric circulation patterns, differences in sunlight absorbed with latitude, the Earth’s rotation and the different water densities present due to temperature and salinity.
Additionally, there are different types of ocean currents which make up the circulation pattern. For example, surface currents and deep ocean currents (thermohaline circulation). Surface currents occur because of wind acting on the surface of the water, and their direction and magnitude is caused by the placement of continents all over the world. These result in vertical and horizontal surface currents. Horizontal surface currents are short term and occur within a smaller area. For example, rip currents, longshore currents, tidal currents. Moreover, vertical water currents bring cold, nutrient concentrated water to the surface through an upwelling. As well as force water which has lower density and higher temperature through a downwelling, causing it to condense and sink. These (horizontal and vertical water currents) form a cycle which affects the physical, biological and chemical characteristics of the ocean and eventually the Earth’s climate.
On the other hand, thermohaline circulation occurs deeper in the ocean, and its currents move up and down the water column. It is produced by different densities of water masses, where the heavier sinks and the lighter rises. Due to the different densities of water caused by its temperature and salinity, it is also called the thermohaline circulation (where: therme = heat, halos = salt). This can be seen where warmer and less salty waters are less dense. Therefore, the thermohaline circulation process is described when the wind transports warmer surface waters from the equator to the poles. The warmer surface waters will then cool down and potentially freeze, leaving more salt in the remaining water, thus the water becomes even more dense. The cold, highly saline waters around the poles will sink and distribute throughout the bottom towards the equator, until it gradually ascends back to the surface, where the cycle repeats. This global scale circulation system also known as the global conveyor belt, (which includes the thermohaline circulation and surface currents) takes about 1000 years for the water to circulate around the world’s water basins.
How does Ocean Circulation affect the Earth’s Climate?
The Earth’s climate is influenced by the heat transported by the ocean currents. Heat from the atmosphere and some of the sunlight are absorbed by water on the Earth’s surface and stored in the oceans as heat, therefore increasing its temperature. As ocean currents move, they carry along with it stored heat, which is then radiated again, affecting the regional climate and air temperatures across the earth where the currents flow. Thus, as ocean currents follow a certain pattern, different regions have specific climate patterns which are in accordance with the flow of heat by circulating waters. This helps to regulate and stabilise the Earth’s climate and distribute solar radiation on the Earth’s surface. For example, the Gulf Stream in the Atlantic Ocean from near the equator travels to Europe bringing heat, therefore making Europe warmer than other regions at the same latitude.
Ocean Circulation and Climate Change
Human activities (such as agriculture, deforestation, burning of fossil fuels and increasing pollutants and waste) increase the amount of greenhouse gases released into the atmosphere and therefore increases the Earth’s overall temperature. This may disrupt wind circulation and ocean circulation, which in turn may affect global climate. Firstly, changes in air temperature differences across the equator and the poles, may affect atmospheric circulation patterns which in turn influence ocean circulation patterns (surface currents). Furthermore, there may also be a disruption in the thermohaline circulation. This is because an increase in air temperature and water temperature also increases the melting of ice at the poles which releases freshwater and reduces salinity and density of water at the poles. Therefore, as time goes by, less of the dense waters at the poles will sink and formation of sea ice may decrease, slowing down the transport of heat from the global conveyor belt. Thus, altering weather and climate across the globe.
Resources
Oceans of the World by Susan Wells
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