In the deep ocean, a deep circulation completely different from the surface circulation covers the entire Earth. This circulation regulates Earth's climate on a scale of hundreds to thousands of years.
Basic Knowledge
Deep circulation (thermohaline circulation): Whereas surface currents are driven by wind, deep circulation is driven by density differences caused by water temperature and salinity. It is a large-scale circulation caused by the sinking of high-density seawater.
Main regions where deep water forms:
・North Atlantic (Greenland〜Labrador Sea): Warm currents (the Gulf Stream and North Atlantic Current) flow northward, release heat, cool, become high-density, and sink.
・Southern Ocean (Weddell Sea and other areas): Cold Antarctic air and sea-ice formation increase salinity and density, causing water to sink.
Deep water flows slowly through the deep sea and rises to the surface in the Pacific and Indian oceans over hundreds to thousands of years. This huge conveyor-belt-like flow is called the Atlantic Meridional Overturning Circulation (AMOC) or the global conveyor belt.
Deep circulation distributes heat, CO₂, oxygen, and nutrients throughout the oceans. It has been suggested that if polar freshwater (from melting glaciers) increases because of climate change, density will decrease, making deep-water formation more difficult and possibly weakening the circulation.
Deep circulation (thermohaline circulation) (driven by density differences from water temperature and salinity; a scale of hundreds to thousands of years)
Deep water (high density, low temperature, and high oxygen; formed in the North Atlantic and Southern Ocean)
Global conveyor belt (a global circulation connecting the world's oceans through deep circulation)
AMOC (Atlantic Meridional Overturning Circulation; helps maintain Europe's warm climate)
Upwelling (the phenomenon in which deep water rises to the surface; transports nutrients and creates excellent fishing grounds)
Further Exploration
If deep circulation weakens, Europe's climate may change greatly. Europe's current warm climate (approximately 10℃ warmer than the east coast of North America at the same latitude) exists because the Gulf Stream and North Atlantic Current flow northward and release large amounts of heat. If the AMOC weakens, cooling comparable to that of North America could occur. The film "The Day After Tomorrow" is an exaggerated science-fiction depiction of this idea.
Deep water also dissolves CO₂ from the atmosphere and helps trap it on the ocean floor. The oceans absorb approximately one-third of the CO₂ emitted by human activities, but rising seawater temperatures lower the solubility of CO₂ and may reduce the amount absorbed.
In deep-seafloor ecosystems, chemosynthetic ecosystems that do not depend on photosynthesis exist around hydrothermal vents and cold seeps. The unique environment in which high-temperature hydrothermal water erupts from chimneys is also attracting attention as a reference example in the search for life in space.
- Deep circulation = driven by density differences from water temperature and salinity
- Deep-water formation regions = North Atlantic and Southern Ocean
- Deep-circulation period = hundreds to thousands of years
- Global conveyor belt = connects the world's oceans
- The oceans absorb approximately one-third of CO₂
- If the AMOC weakens, Europe becomes colder
Points to Note
① Surface currents are driven by wind, whereas deep circulation is driven by density differences (water temperature and salinity). Their driving forces differ. ② Deep circulation is an extremely slow flow lasting hundreds to thousands of years (surface currents are at least several cm/s, whereas deep water flows at approximately several mm/s〜cm/s). ③ In relation to climate change, saying "the circulation stops = catastrophic cooling" is scientifically exaggerated. Use the expression "the circulation may weaken."
Practice
- What force drives deep circulation? Compare it with surface currents.
- Name the two main regions where deep water forms.
- Explain the effect of deep circulation on Europe's climate.