What is Thermohaline Circulation?
The ocean’s currents flow over 100 meters above the ocean’s surface, as well as several thousands of meters below the surface. Some contributing factors to the flow of these currents include the density of the water, which is directly controlled by its temperature and salt concentration, however environmental conditions including evaporation, sea ice formation, precipitation, runoff and ice-melt can affect these properties of the water sources1.
Additionally, the primary driving factor of thermohaline circulation (THC) is high-latitude cooling, which is when the oceans at the coldest regions of the world reach their highest density and cause convective mixing and subsequent circulation of the water.
Circulation in the Atlantic Ocean
While Atlantic meridonial overturning circulation (AMOC) and THC are two terms that are often used interchangeable, the AMOC differs as it involves the total circulation in the latitude depth plane that, in its name alone, does not provide any information on the driving factors of its circulation, whereas the driving factor of THC is implied in its name as directly related to the creation and destruction of buoyancy.
The AMOC describes a system of ocean currents that exists in the North Atlantic and can be characterized by a movement of warm water towards the north, high salt concentrations in the water present at the upper layers of the Atlantic, as well as a flow of colder water towards the south by the deepest parts of this vast ocean2.
By transporting heated water from the Southern Hemisphere, this ocean circulation system absorbs a substantial amount of heat from these areas and transports it to higher latitudes for release into the atmosphere. The AMOC therefore play a crucial role in maintaining the climate of nearby coastal areas, however any external variations, such as an increased amount of greenhouse gases, can have deleterious effects on the full warming of the atmosphere.
The Affect of Global Warming
Over the past 2.6 million years, changes in the circulation of the Atlantic Ocean have resulted in some of the most rapid climate shifts, which have affected not only the North Atlantic regions and surrounding areas, but also areas around the globe. These changes have been primarily attributed to rising global temperatures, as warmer water reduces the ability of the natural circulation within oceans, particularly the AMOC, to cool.
For example, ice sheets and glaciers that are present in Greenland and other Arctic areas are melting, which is pouring excessive amounts of water into nearby oceans thereby decreasing the salinity and density of these bodies of water3.
In an effort to monitor any potential changes in the AMOC, researchers over the last ten years have been equipped with the tools capable of periodically measuring the AMOC. A recent study published in Nature utilized a novel high-resolution climate model to identify a characteristic sea-surface temperature (SST) fingerprint, which provides information on the reduced AMOC as it is related to rising carbon dioxide (CO2) levels within the atmosphere2.
In their work, the researchers successfully connected the extreme warming currently taking place along the northeastern coast of the United States and subsequent cooling of the southern areas towards the Gulf Stream to slowing down the AMOC.
Additionally, the researchers reconstructed a time evolution of the AMOC based on data that has been collected from 1870-2016. From this data, it was determined that around 1990 the AMOC reached a minimum until a peak value was reached in the early 2000s until it began to decline again to its current state, which is estimated to be the lowest circulation rates that have existed in over 1,000 years.
The weakening of the AMOC has been speculated as a causative factor in the rising summer temperatures that have been measured across Europe, as well as the increased amount of storms that have been taking place across this continent. Researchers anticipate that if the AMOC continues to weaken over the next several decades, that drastic changes in the hydrological cycle, as well as a dramatic loss in sea-ice, will follow.
- “The Thermohaline Ocean Circulation” – Stefan Rahmstorf
- Caesear, L., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature. DOI: 10.1038/s41586-018-0006-5.
- “Global warming is slowing Atlantic ocean circulation, study finds” – CBS News