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is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
beneath warmer or fresher water or cold air beneath warm air. It is the sinking
limb of a convection cell
A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are density differences within a body of liquid or gas. The convection usually requires a gravitational field but in microgravity experiments, thermal convection has been observed without gravitational effects...
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
. The sinking of cold lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
at subduction zones is another example of downwelling in plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
Downwelling occurs at anti-cyclonic places within the ocean where warm rings are spinning clockwise creating surface convergence.