Thermohaline circulation

Thermohaline circulation (THC) is a part of the large-scale ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes.^[1][2] The name thermohaline is derived from thermo-, referring to temperature, and haline, referring to salt content—factors which together determine the density of sea water.

Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling and sinking en-route to higher latitudes - eventually becoming part of the North Atlantic Deep Water - before flowing into the ocean basins.^[3] While the bulk of thermohaline water upwells in the Southern Ocean, the oldest waters (with a transit time of approximately 1000 years) upwell in the North Pacific;^[4] extensive mixing takes place between the ocean basins, reducing the difference in their densities, forming the Earth's oceans a global system.^[3] The water in these circuits transport energy - as heat - and mass - as dissolved solids and gases - around the globe. Consequently, the state of the circulation greatly impacts the climate of Earth.

The thermohaline circulation is often referred to as the ocean conveyor belt, great ocean conveyor, or "global conveyor belt" - a term coined by climate scientist, Wallace Smith Broecker.^[5][6] It is also known as the meridional overturning circulation, or MOC; a name used to signify that circulation patterns caused by temperature and salinity gradients are not necessarily part of a single global circulation. This is due, in part, to the difficulty in separating parts of the circulation driven by temperature and salinity from those affected by factors such as wind and tidal force.^[7]

This global circulation comprises two major "limbs;" the Atlantic meridional overturning circulation (AMOC) centered in the north Atlantic Ocean, and the Southern Ocean overturning circulation, or Southern Ocean meridional circulation (SMOC) located near Antarctica. Since 90% of the human population occupies the Northern Hemisphere,^[8] more extensive research has been undertaken on the AMOC, however the SMOC is of equal importance to the global climate. Evidence suggests both circulations are slowing due to climate change in line with increasing rates of dilution from melting ice sheets - critically affecting the salinity of Antarctic bottom water.^[9][10] In addition, the potential for outright collapse of either circulation to a much weaker state exemplifies tipping points in the climate system. If either hemisphere experiences collapse of its circulation, the likelihood of proplonged dry spells and droughts would increase as precipitation decreases, while the other hemisphere will become wetter. Marine ecosystems are then more likely to receive fewer nutrients and experience greater ocean deoxygenation. In the Northern Hemisphere, the collapse of AMOC would lead to substantially lower temperatures in many European countries, while the east coast of North America is predicted to see accelerated sea level rise. The collapse of these circulations is generally accepted to be more than a century away, and may only occur in the event of rapid and high sea-temperature increases. However, these projections are marked by significant uncertainty.^[10][11]