Thermohaline circulation (THC) is a part of the large-scale ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. 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. 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; extensive mixing takes place between the ocean basins, reducing the difference in their densities, forming the Earth's oceans a global system. 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. 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. 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, 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. 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 prolonged 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.
History of research
It has long been known that wind drives ocean currents, but only at the surface. In the 19th century, some oceanographers suggested that the convection of heat could drive deeper currents. In 1908, Johan Sandström performed a series of experiments at a Bornö Marine Research Station which proved that the currents driven by thermal energy transfer can exist, but require that "heating occurs at a greater depth than cooling". Normally, the opposite occurs, because ocean water is heated from above by the Sun and becomes less dense, so the surface layer floats on the surface above the cooler, denser layers, resulting in ocean stratification. However, wind and tides cause mixing between these water layers, with diapycnal mixing caused by tidal currents being one example. This mixing is what enables the convection between ocean layers, and thus, deep water currents. In the 1920s, Sandström's framework was expanded by accounting for the role of salinity in ocean layer formation. Salinity is important because like temperature, it affects water density. Water becomes less dense as its temperature increases and the distance between its molecules expands, but more dense as the salinity increases, since there is a larger mass of salts dissolved within that water. Further, while fresh water is at its most dense at 4 °C, seawater only gets denser as it cools, up until it reaches the freezing point. That freezing point is also lower than for fresh water due to salinity, and can be below −2 °C, depending on salinity and pressure.
Structure
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