Thermohaline circulation (THC)
Thermohaline circulation (THC) is a crucial component of the ocean's global circulation, driven by variations in water density caused by temperature and salinity. In polar regions, surface water cools and increases in salinity due to sea-ice formation, leading to dense water that sinks to the ocean depths. This process creates distinct water masses, such as North Atlantic deep water (NADW) and Antarctic bottom water (AABW), which play significant roles in the movement of ocean water across vast distances. THC acts like a conveyor belt, transporting warm surface water from the equator to the poles and returning colder water to lower latitudes over an extensive timescale of about a thousand years.
The significance of THC extends to climate change, as it redistributes heat and influences global climate patterns. Scientists are concerned that an influx of freshwater from melting glaciers or increased precipitation could destabilize this circulation, potentially leading to cooling in the Northern Hemisphere and other ecological impacts, such as reduced phytoplankton productivity and more frequent El Niño events. Though there is ongoing debate about changes in THC intensity, its role in climate regulation and ocean health remains vital. Understanding THC is essential for comprehending broader climatic shifts and their implications for the planet.
Subject Terms
Thermohaline circulation (THC)
Definition
Thermohaline circulation (THC) is the large-scale circulation of water in the ocean basins, driven by the dense deep water of the polar regions. The density of ocean water is a function of both temperature and salinity. Water increases in density in the polar regions, as surface water cools and its salinity is increased through sea-ice formation. (Salt excluded from sea ice increases the salt content of polar liquid water.) A column of water becomes unstable when surface water has a higher density than deeper water, causing the high-density surface water to sink until it reaches a depth at which it is neutrally buoyant. At that depth, the water will flow horizontally throughout the oceans.
High-density surface water that forms in the Labrador and Greenland Seas sinks to become North Atlantic deep water (NADW), while the high-density water that forms in the Weddell and Ross Seas sinks to become Antarctic bottom water (AABW). Other water masses that sink to intermediate depths form in the Mediterranean Sea (Mediterranean intermediate water) and along the edges of the Antarctic Circumpolar Current (Antarctic intermediate water); yet other water masses downwell in the centers of the subtropical gyres. NADW flows southward in the Atlantic Ocean, until it mixes with north-flowing AABW in the extreme southern Atlantic Ocean. Once mixed, these two masses form Pacific-Indian common water (PICO).
AABW flows north through the South Atlantic Ocean and can be detected as far north as the equator. After forming in the southern polar regions, deep water circulates around Antarctica and then flows northward into the Indian and Pacific Oceans. Although the mechanisms are not yet fully explained, it is thought that deep water ultimately returns to the surface as diffuse flow over a large area of the North Pacific Ocean. The net transport of warm surface water to the polar regions to replace surface water that sinks to become deep water is brought about by wind-driven surface currents.
Thermohaline circulation is sometimes used synonymously with meridional overturning circulation (MOC), although this is not technically correct. THC refers to a global circulation pattern linking surface- and deep-ocean circulation, while MOC refers to deep circulation in the Atlantic Ocean.
Significance for Climate Change
THC can be thought of as a large conveyer belt that transports surface water and heat from the equator to the North and South Poles, with a return flow of cold, deep water to the equator along the ocean bottom. THC is driven by water sinking to the ocean depths in a few distinct locations. After sinking, this water returns to the surface via diffuse flow over a broad geographic region in the North Pacific Ocean. THC is relatively slow: A single cycle takes approximately one thousand years.
Because THC redistributes large amounts of heat on the Earth, it is an important facet of global climate and climate change. In particular, global warming could lead to an increase in the freshwater into the North Atlantic by melting glaciers or increasing precipitation and river flow. An increase in freshwater flux to the polar regions would lower the density of polar surface water, stabilizing water columns and preventing the surface water from sinking into the ocean depths. An influx of surface freshwater in the Greenland Sea would also serve to block the northward flow of the Gulf Stream, a surface current that brings heat and ocean water to the polar regions.
It has been hypothesized that a shutdown of THC would lead to localized cooling of the Northern Hemisphere. In 2005, scientists from Britain’s National Oceanography Centre presented data to suggest that THC in the Atlantic had slowed during the late twentieth century. These data were subsequently challenged by other scientists, who presented other data sets that showed no such slowing. Despite the controversy regarding whether slowing is occurring, computer models consistently indicate that a shutdown of THC could lead to decreased warming or even cooling in the Northern Hemisphere. Geologic evidence suggests that the Younger Dryas, a time of that lasted from 12,800 to 11,500 years ago, may have been caused by THC collapse due to a large flux of freshwater from the emptying Glacial Lake Agassiz into the North Atlantic. In addition to climate changes, a shutdown of THC would have other important consequences, including the formation of anoxic conditions in large portions of the world oceans, reduction or collapse of phytoplankton productivity, and more frequent and severe El Niño events.
Bibliography
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