Thermohaline circulation

Thermohaline circulation is a massive, global system of ocean currents. It is often called the "global conveyor belt" or the ocean conveyor belt. This name was coined by the climate scientist Wallace Smith Broecker. The term "thermohaline" comes from two different factors: "thermo," meaning temperature, and "haline," meaning salt content. Together, these two factors determine the density of seawater. Because density changes how water moves, this circulation acts as a giant engine for the planet. It transports heat, energy, and mass, such as dissolved gases and solids, around the entire Earth.

The mechanism of this circulation relies on density gradients. Water becomes denser as it gets colder or saltier. In the North Atlantic, high winds and evaporation play a major role. Evaporation removes fresh water and leaves salt behind, making the surface water very salty. This region is also quite cool. The combination of high salinity and low temperature makes the water heavy enough to sink. This sinking water forms the North Atlantic Deep Water (NADW). It then flows south through deep ocean basins.

In the Southern Ocean, a different process drives deep water formation. Strong katabatic winds blow across the Antarctic ice shelves. These winds push newly formed sea ice away from the coast, creating open areas called polynyas. In these open areas, the ocean loses heat to the cold air. As sea ice forms, it undergoes a process called brine rejection. The ice crystals prefer to freeze pure water, leaving behind a very salty, cold liquid called brine. This heavy brine sinks to the bottom, creating Antarctic Bottom Water (AABW). This water is even denser than the NADW, so it flows beneath it.

This global system is divided into two major limbs. The first is the Atlantic Meridional Overturning Circulation (AMOC), which is centered in the North Atlantic. The second is the Southern Ocean Meridional Circulation (SMOC), located near Antarctica. Because 90% of humans live in the Northern Hemisphere, scientists have researched the AMOC extensively. However, the SMOC is just as important for the global climate. These two systems are connected through extensive mixing between ocean basins. This mixing reduces density differences and turns the oceans into one single, connected system.

Research into these deep currents has a long history. In the 19th century, oceanographers began suggesting that heat could drive deep currents. In 1908, Johan Sandström proved this at a research station in Bornö. He showed that currents occur when heating happens at a greater depth than cooling. In the 1920s, scientists expanded this by adding the role of salinity. Later, in 1960, Henry Stommel and Arnold B. Arons established the modern understanding of this circulation. They identified how distinct water masses move through the abyss.

The scale of this circulation is immense. The oldest waters in the system can take approximately 1,000 years to complete their journey. These old waters eventually rise to the surface through a process called upwelling. While most upwelling happens in the Southern Ocean, some scientists believe the bulk occurs in the North Pacific. To track these movements, the UK-US RAPID programme has been collecting data since 2004. It uses current meters and subsea cables to measure the strength of the AMOC at 26.5°N.

Changes to this circulation could significantly impact Earth's climate. Climate change is causing ice sheets to melt, which adds fresh water to the ocean. This fresh water dilutes the salt, making the water less dense and harder to sink. If the circulation slows down or collapses, it could reach a "tipping point." A collapse of the AMOC could cause much lower temperatures in Europe and faster sea level rise on the North American east coast. It could also lead to more droughts and fewer nutrients for marine ecosystems.