Our world is driven by currents of all kinds. From air currents that influence the weather, to convection currents beneath the Earth which are responsible for continental drift; our world’s processes are dominated by currents. The ocean is no exception.

Perhaps the most important of all of Earth’s processes is thermohaline circulation, which is responsible for the movement of all deep-ocean currents. Our planet has been interconnected long before globalisation and for that, we have thermohaline circulation to thank.

What is Thermohaline Circulation?

Thermohaline circulation (THC) plays a major part in large-scale ocean circulation. THC is driven primarily by global density gradients resulting from surface temperature and salinity fluctuations. The name comprises of thermo-, referring to temperature, and ­-haline, referring to salinity, both of which determine the density of sea water.

In conjunction with the faster wind-driven surface currents, density-driven currents are responsible for the transport of nutrients, heat, and other materials across the entire global ocean.

The movement of deep-water is significantly slower than its wind-driven shallow-water counterparts. Typically moving only one centimetre per second, the NOAA estimates that the THC turns over all the deep water in the ocean every 600 years.

Due to its role in circulating the entire ocean and thus transporting a number of vital components, the THC has been commonly referred to as the ‘ocean conveyor belt’.

The term ‘THC’ is sometimes used when referring to the Atlantic Meridional Overturning Circulation (AMOC). However, the AMOC is more well-defined and includes wind effects on circulation as opposed to solely considering the effects of density.

As such, when exploring the importance of the THC on a global level, examining the AMOC will give us a more holistic view.

So, what drives the AMOC?

Deep water formation

One of the key components of driving the AMOC is the formation of deep-water. The poles are the main sources, each producing a mass of dense, deep water that propels ocean circulation.

North Atlantic Deep Water

North Atlantic Deep Water (NADW) is the layer of salty water found at depths of 1500-3500 m in the Atlantic. This water is characterised by high oxygen and low nutrient concentrations and plays a key role in the AMOC.

Upon arrival into the North Atlantic, warm surface waters from the tropics are cooled by the wind and low air temperatures of the northern hemisphere. In addition to cooling, evaporation rates are increased as a result of wind movement; evaporation removes water molecules but leaves salt behind and thus increases the salinity of the remaining water mass.

These processes take place in the Nordic Seas, the Labrador Sea, and the Mediterranean Sea. And so, the NADW is formed, and begins its journey through the ocean basins and across the Atlantic abyssal plains.

Antarctic Bottom Water

The formation of Antarctic Bottom Water (AABW) takes us to the other side of the planet: the Southern Ocean.

Strong katabatic winds from the Antarctic continent blows away newly formed sea ice and forms areas of open water surrounded by sea ice, called polynyas. Direct contact with the sub-zero Antarctic temperatures induces the formation of new sea ice.

A polynya off the Antarctic Coast. Nasa Earth Observatory

When sea water freezes, the salt components are expelled and hence the salinity of the remaining water increases. This phenomenon is known as ‘brine rejection’ and plays a central role in the formation of AABW.

Due to the increased salinity and decreased volume, the remaining seawater has significantly increased in density and in turn, sinks and flows north and east. This water mass is known as the AABW and is formed in the Weddell and Ross Seas.

The density of the AABW is so great in fact, that it flows beneath the NADW, in varying directions depending on the point of origin.

Now we have seen the origin of the deep water masses that are responsible for the global conveyor belt. But where does the story go from here?

The Deep Water Journey

The entire AMOC is driven by the formation of dense deep-water masses in the poles as well as the formation of fresher, warmer waters in the tropics. The upwelling of deep water in warmer tropical waters is the process that underlies the movement of the entire conveyor belt.

After forming and then sinking in the North Atlantic, the NADW begins its journey across the Atlantic basin. It flows through the Antarctic Ocean Basin around South Africa where it is divided into two routes: one which passes through the Indian Ocean and another which travels past Australia and into the Pacific.

Along both routes, the cold and salty NADW is drawn along by the movement of warm, fresh surface waters. This causes a vertical exchange between the denser waters below and the lighter waters above – a process known as overturning.

Thermohaline circulation. Britannica

As a result of the constant outflow of deep, dense water, the Atlantic sea level is on average 40 centimetres lower than in the Pacific as well as being more saline. These differences in ocean structure generates a large-scale, albeit slow, movement of warm and fresh Pacific water through the Indonesian Archipelago to replace the cold and salty AABW. This is known as ‘haline forcing’.

This water continues its journey northwards across the South Atlantic, through the warm Gulf Stream until reaching Greenland, where it once again sinks and becomes NADW. Hence, the thermohaline circulation is complete.

A New Ice Age

As is to be expected with most natural processes, thermohaline circulation faces some severe consequences of climate change.

Rising atmospheric temperatures as a result of increasing greenhouses gases has diminished the ability for the North Atlantic Surface to form cold water. In this way, one of the driving forces of the AMOC has been weakened.

Assessments undertaken by the Intergovernmental Panel on Climate Change (IPCC) have concluded that the AMOC is “very likely” to weaken over the 21st century, as greenhouse gas emissions increase and global temperatures continue to rise.

On the contrary to what we all imagine when we think of climate change, the weakening of the AMOC will result in decreasing ambient temperatures in the northern hemisphere. This is due to the reduced heat input, brought from the tropics via the Gulf Stream.

If the AMOC were to completely shut down, as some scientists once predicted, the northern hemisphere would be plunged into an ice age. Current predictions believe an AMOC shutdown in the 21st century to be “very unlikely – though still plausible”.

A weakened AMOC also holds the potential for extreme regional sea level rise which has the potential to destroy countless homesteads as well as greatly impact local ecosystems.

On a more positive note, the weakening ocean circulation has also been linked to a decline in Atlantic major hurricane frequency.

Conclusions

Thermohaline circulation plays a tremendous role in Earth’s climate regulation and supports an overwhelming amount of life across the planet.

By transporting heat to the northern hemisphere via the Gulf Stream, THC singlehandedly facilitates life in a great portion of the planet.

In addition to this, by influencing the rate of sea ice formation in the poles, THC indirectly influences a myriad of other aspects of our climate system such the reflection of solar radiation, or albedo.

All in all, our planet could not function without thermohaline circulation. Despite this, anthropogenic activity continually poses a threat on the functioning of ocean circulation which in turn could spell doom for us all.

Written by Lucas King