What Drives Deep Ocean Currents?

Thermohaline circulation is a fundamental phenomenon in global ocean circulation that is part of what drives deep ocean currents. It results from variations in seawater temperature and salinity, something represented in its name. Thermo- refers to temperature and -haline refers to the amount of salt. This complex system of ocean currents develops due to differences in water density, which are, in turn, influenced by temperature and salinity.

The salinity of seawater comes from various processes. These include the dissolution of mineral salts from the Earth's crust and the release of ions during the formation of ice in polar regions. Temperature also affects the density of water, as colder water is denser than warmer water.

Thermohaline circulation begins with the formation of dense deep water in the polar regions. Water cools significantly in these areas and becomes saltier due to the freezing process of ice. This dense water is known as deep water or bottom water. It sinks to the ocean floor and flows toward equatorial regions, establishing a deep circulation current within the ocean.

As this deep water moves toward the equator, it gradually warms and mixes with the less dense layers of water. Subsequently, it rises towards the surface, completing the thermohaline circulation cycle. Rising water in equatorial regions allows the transfer of heat from the depths of the ocean to the surface, influencing regional and global climate patterns. Due to this flow and return of ocean currents, it is known as the global ocean conveyor belt.

Learn more about the basics of ocean currents with our article on what are sea waters or ocean waters?

As we have established, deep ocean currents are driven by the thermohaline circulation cycle. This cycle operates in several stages, beginning in the polar regions and culminating in the equatorial zones. We are going to explain what the key steps of this process:

1. Formation of dense water in polar regions: especially in the Arctic and Antarctic oceans, surface water form polar regions cools considerably during winter. As the temperature drops, the water becomes denser. Additionally, the freezing process of ice releases salt into the ocean, increasing the salinity of the water. Learn more about these areas with our article on what are polar ecosystems.
   
   
2. Sinking of dense water: the water of the oceans becomes dense due to its lower temperature and higher salinity. Once it is sufficiently dense, it tends to sink towards the deeper layers of the ocean. This sinking marks the beginning of the deep circulation ocean current.
   
   
3. Flow of deep water towards equatorial regions: once subsidence has occurred, deep water begins its journey from the polar regions towards the equatorial areas through deep ocean currents. This flow is driven by the difference in density between deep water and less dense layers in other parts of the ocean.
   
   
4. Warming and mixing in equatorial regions: as deep water moves toward equatorial areas, it encounters less dense layers of water and gradually warms. The resulting mixing between deep and less dense water layers occurs in these equatorial regions.
   
   
5. Rising to the surface: now less dense due to warming, water rises to the surface in equatorial regions. This rise completes the thermohaline circulation cycle.
   
   
6. Heat transfer to the atmosphere: the rise of water to the surface brings with it heat from the depths of the ocean. This process influences regional climate patterns and heat transfer into the atmosphere.
   
   
7. Restart of the cycle: once on the surface, the newly ascended water can return to the polar regions through surface currents, where it cools again and a new cycle of thermohaline circulation begins. This makes a return of ocean currents from a very similar path to before.

This thermohaline circulation cycle contributes significantly to the global distribution of heat and nutrient transport. These are what is being carried on the global ocean conveyor belt. For this reason, it plays and important role in climate regulation and marine biodiversity.

If the thermohaline circulation were to stop, deep ocean currents would stop. Even if it did not stop, significant changes to thermohaline circulation would cause equally significant changes to deep ocean currents. The potential impacts of such changes would be the following:

• Changes in regional climate: if disrupted or significantly weakened, regions that depend on this circulation could experience abrupt climate changes. For example, coastal areas that benefit from heat transport from the equator to high latitudes could become colder.
  
  
• Variations in sea levels: thermohaline circulation is also linked to circulation patterns that affect sea levels in different regions. Alterations in this circulation could cause changes in the distribution of water masses, which in turn would affect sea levels in certain areas.
  
  
• Impact on marine biodiversity: deep ocean currents resulting from thermohaline circulation are necessary for the distribution of nutrients and marine life. Disrupting this flow would affect nutrient availability in various regions. In turn, this would have consequences for marine biodiversity and food webs.
  
  
• Modification in precipitation patterns: the thermohaline circulation also influences atmospheric patterns. Changes in this system could have repercussions on the distribution of rainfall. By affecting heat transfer between the ocean and the atmosphere, variations in precipitation patterns could arise. This can dramatically affect climatic conditions on land.
  
  
• Slowing carbon sequestration: the ocean absorbs large amounts of atmospheric carbon dioxide. Disrupting thermohaline circulation would slow this process, causing implications for climate change by increasing the amount of CO2 in the atmosphere.
  
  
• Extreme weather events: changes in ocean circulation can influence the frequency and intensity of extreme weather events, such as hurricanes and typhoons. Alterations in heat transfer between the ocean and the atmosphere could affect the formation and intensification of these phenomena. Learn more about what is a hurricane and how it forms.

Although we have analyzed the consequences of stopping the thermohaline circulation, this scenario is actually highly unlikely in the short term. However, research suggests that changes in this circulation could occur in response to human-accelerated climate change.

Now you know by what the world's deep ocean currents are driven, you can learn more about ocean current dynamics with our article on whether the Atlantic and Pacific Oceans mix with each other.

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