Thermohaline circulation is a slow, deep movement of ocean water that contributes to global climate and ocean health. It is triggered by the temperature and salinity of water, with the Atlantic Ocean being denser than the Indian and Pacific Oceans. The cycle begins in the Pacific Ocean and can take thousands of years to complete. Global warming may impact the circulation, potentially causing localized cooling in the northern hemisphere.
The thermohaline circulation is a very slow and extremely deep movement of water in the world’s oceans. A complete cycle can take thousands of years to complete from start to finish, with the cycle contributing greatly to the mixing of the world’s oceans. Thermohaline circulation appears to be linked to global climate and ocean health, as it mixes dissolved gases and minerals in water and can impact global temperatures as well.
When broken down into its component roots, thermohaline refers to the temperature (thermo-) and dissolved salts (-haline) in ocean water. These salts are known as halides. The temperature and the alinity of the water have an impact on its density, and this triggers the thermohaline circulation. The colder the water, the denser it is, and the saltier the water, the higher the density. Due to thermohaline circulation, the Atlantic Ocean is denser than the Indian and Pacific Oceans, making it slightly shallower.
The cycle begins around the equator in the Pacific Ocean when surface currents such as the jet stream slowly drive water westward. This water is warm and not excessively salty, making it light enough to float on the ocean’s surface. As the water heads west, it slowly makes its way into the North Atlantic, where it cools due to ambient temperature. Also, salinity increases, through evaporation.
This combination of factors causes the water to slowly sink to the ocean floor. It also slowly begins to move south, as it is pushed by more water sinking in from the surface current. Some of the thick, cold water settles in deep ocean basins, but most slowly circulates south before moving east, around the Horn of Africa, and back to where it originated; some of the water slips around Australia to emerge in the South Pacific, while the other part of the deep ocean current meanders through the Indian Ocean. As the water flows, it gets hotter and less salty, until it backs up into the Pacific only to start the cycle all over again.
Some people have expressed concern about the impact of global warming on thermohaline circulation. There is some concern that an increase in glacial melt could lower the alinity of the ocean in the North Atlantic, causing the cycle to shut down. This can lead to a localized cooling in the northern hemisphere, which could have serious repercussions on the organisms that live there.
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