How does the thermohaline circulation work ?

In the ocean, all the water masses have different physical properties, defining their density. The most important factors impacting the water density are the salinity and the temperature. The lower the salinity and the temperature, the denser the water mass. These density difference makes the water masses move and create currents (figure).

In the tropical areas, the water masses are warmer. Furthermore, the sea surface evaporation is important, and consequently, the surface water masses are saltier. When these masses arrive to the poles, their temperature decrease and the melting ice caps are adding fresh water, making these water masses even denser. At high latitude, the densest bodies of water are plunging beneath the least dense and these denser bodies of water circulate at depth, beneath the less dense bodies of water, which remain on the surface. The thermohaline circulation is of much of importance for the climate system, as it has an influence on the heat transport and repartition at the Earth surface. 

These water masses, with different properties (e.g. salinity, temperature, nutrient content or ion concentrations) also have a strong influence on the marine life and marine productivity. For example, in some places, the cold and rich in nutrients bottom water is brought to the sea surface due to the wind configuration. This is called an upwelling and this important nutrient supply from the depths generally leads to an important biodiversity development.

The continental configuration at the Earth surface has an influence on the currents dynamics. For example, from Neogene to present, the main tectonic change that has affected the Atlantic Ocean is the closure of the Panama Isthmus, between 13 Ma and 2.6 Ma (Bartoli et al., 2005; Lunt et al., 2008; O’Dea et al., 2016). This affected the oceanic circulation, currents position and exchange between the Pacific and the Atlantic as well as ocean atmosphere feedback mechanisms, and so, climate in general since the late Miocene-early Pliocene (Haug and Tiedemann, 1998; Lear et al., 2003; Maier-Reimer and Mikolajewicz, 1990; Billups et al., 1999; Haug et al., 2001). For these reasons, the closure of the Panama Isthmus is also at the origin of the intensification of the Northern hemisphere glaciation (NHG) (appearance of perennial ice caps in the North hemisphere) (Bartoli et al., 2005; Lunt et al., 2008a). These currents are also under the influence of the glacial-Interglacial cycles and orbital cycles during the Cenozoic (Kleiven et al., 2003; Rühlemann et al., 2004, 1999).

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January 2025

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