Downwelling is the downward movement of a fluid parcel and its properties (e.g., salinity, temperature, pH) within a larger fluid. It is closely related to

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...

, the upward movement of fluid. While downwelling is most commonly used to describe an oceanic process, it's also used to describe a variety of Earth phenomena. This includes mantle dynamics, air movement, and movement in freshwater systems (e.g., large

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

s). This article will focus on oceanic downwelling and its important implications for

ocean circulation An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, ...

and biogeochemical cycles. Two primary mechanisms transport water downward:

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

forcing and wind-driven

Ekman transport Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman transport is a component of wind-driven ocean current. Ekman transport occurs w ...

(i.e., Ekman pumping). Downwelling has important implications for

marine life Marine life, sea life or ocean life is the collective ecological communities that encompass all aquatic animals, aquatic plant, plants, algae, marine fungi, fungi, marine protists, protists, single-celled marine microorganisms, microorganisms ...

. Surface water generally has a lower

nutrient A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

content compared to deep water due to

primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through ...

using nutrients in the

photic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...

. Surface water is, however, high in oxygen compared to the deep ocean due to

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

and air-sea gas exchange. When water is moved downwards, oxygen is pumped below the surface, where it is used by decaying organisms. Downwelling events are accompanied by low primary production in the surface ocean due to a lack of nutrient supply from below.

Mechanisms

Buoyancy

Buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

-forced downwelling, often termed

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

, is the deepening of a water parcel due to a change in the density of that parcel. Density changes in the surface ocean are primarily the result of

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

, heating, cooling, or the introduction and mixing of an alternate water or salinity source, such as river input or

brine rejection Brine rejection is a process that occurs when salty water freezes. The salts do not fit in the crystal structure of water ice, so the salt is expelled. Since the oceans are salty, this process is important in nature. Salt rejected by the forming ...

. Notably, convection is the driving force behind global thermohaline circulation. For a water parcel to move downward, the density of that parcel must increase; therefore, evaporation, cooling, and brine rejection are the processes that control buoyancy-forced downwelling.

Wind-driven Ekman transport

is the net mass transport of the ocean surface resulting from wind stress and the

Coriolis force In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motio ...

. As wind blows across the ocean surface, it causes a frictional force that drags the uppermost surface water along with it. Due to the Earth's rotation, these surface currents develop at 45° to the wind direction. However, compounding frictional forces cause the net transport across the

Ekman layer Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman transport is a component of wind-driven ocean current. Ekman transport occurs ...

to be 90° to the right of wind stress in the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

and 90° to the left in the Southern Hemisphere. Ekman transport piles up water between the

trade winds The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere ...

and

westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes (about ...

in subtropical gyres, or near the shore during coastal downwelling. The increased mass of surface water creates high-pressure zones that push water downward. It can also create long convergence zones during sustained winds to create Langmuir circulation.

Buoyancy-forced downwelling

is lost through cooling,

, and

through

sea ice Sea ice arises as seawater freezes. Because ice is less density, dense than water, it floats on the ocean's surface (as does fresh water ice). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world' ...

formation. Buoyancy loss occurs on many spatial and temporal scales. In the open ocean, there are regions where cooling and

mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...

deepening occurs at night, and the ocean re-stratifies during the day. On annual cycles, widespread cooling begins in the fall, and convective mixed layer deepening can reach hundreds of meters into the ocean interior. In comparison, the wind-driven mixed layer depth is limited to 150 m. Large evaporation events can cause

; however,

latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process—usually a first-order phase transition, like melting or condensation. ...

loss associated with evaporation is usually dominant and in the winter, this process drives

Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...

deep water formation. In select locations - Greenland Sea,

Labrador Sea The Labrador Sea (; ) is an arm of the North Atlantic Ocean between the Labrador Peninsula and Greenland. The sea is flanked by continental shelf, continental shelves to the southwest, northwest, and northeast. It connects to the north with Baffi ...

Weddell Sea The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha C ...

, and

Ross Sea The Ross Sea is a deep bay of the Southern Ocean in Antarctica, between Victoria Land and Marie Byrd Land and within the Ross Embayment, and is the southernmost sea on Earth. It derives its name from the British explorer James Clark Ross who ...

- deep convection (>1000 m) ventilates ( oxygenates) most of the deep water of the global ocean and drives the

thermohaline circulation Thermohaline circulation (THC) is a part of the large-scale Ocean current, ocean circulation driven by global density gradients formed by surface heat and freshwater fluxes. The name ''thermohaline'' is derived from ''wikt:thermo-, thermo-'', r ...

Wind-forced downwelling

Subtropical gyres

Subtropical gyres act on the largest scale that we observe downwelling. Winds to the north and south of each ocean basin blow opposite each other such that

moves water toward the basin's center. This movement piles up water, creating a high-pressure zone in the center of the gyre, low pressure on the borders, and deepens the

. The water in this zone would diffuse outward if the planet weren't spinning. However, because of the

, the water rotates clockwise in the Northern Hemisphere and counterclockwise in the southern, creating a gyre. While it spins, the rotating high-pressure zone forces water downward, resulting in downwelling. Typical downwelling rates associated with ocean gyres are on the order of 10’s of meters per year.

Coastal downwelling

Coastal downwelling occurs when winds blow parallel to the shore. With such winds, Ekman transport directs water movement towards or directly away from the shore. If Ekman transport moves water towards the shore, the shoreline acts as a barrier causing surface water to pile up onshore. The piled-up water is forced downwards, pumping warm,

-poor, oxygenated water below the mixed layer.

Langmuir circulation

Langmuir circulation develops from the wind, which, through Ekman transport, creates alternating zones of convergence and divergence at the ocean surface. In convergent zones, marked by long strips of floating debris accumulation, such as the Great Pacific Garbage Patch, coherent vortices transport surface waters to the base of the mixed layer develop. Also, direct wind stirring and current shear at the base of the mixed layer can create instabilities and

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

that further mix properties within and at the base.

Association with other ocean features

Eddies

Meso- (>10-100's km) and submesoscale (<1-10 km) eddies are ubiquitous features of the upper ocean. Eddies have either a cyclonic ( cold-core) or

anticyclonic A high-pressure area, high, or anticyclone, is an area near the surface of a planet where the atmospheric pressure is greater than the pressure in the surrounding regions. Highs are middle-scale meteorological features that result from interpl ...

( warm-core) rotation. Warm-core eddies are characterized by

rotation that directs surface waters inward, creating high sea surface temperature and height. The high central hydrostatic pressure maintained by this rotation causes the downwelling of water and the depression of

isopycnal Isopycnals are layers within the ocean that are stratified based on their densities and can be shown as a line connecting points of a specific density or potential density on a graph. Isopycnals are often displayed graphically to help visualize ...

s - surfaces of constant density (see Eddy pumping) at scales of hundreds of meters per year. The typical result is a deeper surface layer of warm water often characterized by low

. Warm-core eddies play multiple important roles in biogeochemical cycling and air-sea interactions. For example, these eddies are seen to decrease ice formation in the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

due to their high sea surface temperatures. It has also been observed that air-sea fluxes of carbon dioxide decrease at the center of these eddies and that temperature was the leading cause of this inhibited flux. Warm-core eddies transport oxygen into the ocean interior (below the photic zone) which supports respiration. Although compounds such as oxygen are transported into the deep ocean, there is an observed decrease in carbon export in warm-core eddies due to intensified stratification at their center. Such stratification inhibits the mixing of nutrient-rich waters to the surface where they could fuel primary production. In this case, since primary production stays low, carbon export potential remains low.

Fronts and filaments

Ocean fronts are formed by the horizontal convergence of dissimilar water masses. They can develop at regions of

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

input marked by horizontal density gradients due to salinity and temperature differences or the stretching and elongation of rotating flows. Submesoscale fronts and filaments are formed by ocean current interactions and flow instabilities. They are regions that connect the surface layer and the ocean interior. These regions are characterized by horizontal

gradients < 10 km in scale, caused by sloping isopycnals. Two primary mechanisms transport surface waters to depth: the adiabatic tilting and relaxation of these isopycnals, and along-isopycnal flow or subduction. These mechanisms can transport surface properties, such as

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

, below the mixed layer and assist in

carbon sequestration Carbon sequestration is the process of storing carbon in a carbon pool. It plays a crucial role in Climate change mitigation, limiting climate change by reducing the amount of Carbon dioxide in Earth's atmosphere, carbon dioxide in the atmosphe ...

through the

biological pump The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven Carbon sequestration, sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sedim ...

. Numerical models predict vertical velocities at submesoscale fronts on the order of 100 m/day. However, vertical velocities over 1000 m/day have been observed using ocean floats. These observations are rare because ship-based sensors do not have sufficient accuracy to measure vertical velocities.

Variability

Downwelling trends differ between

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

s and can be associated with variations in wind strength and changing

season A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's axial tilt, tilted orbit around the Sun. In temperat ...

s. In some areas, coastal downwelling is a seasonal event pushing

-depleted waters towards the shore. The relaxation or reversal of

-favorable winds creates periods of downwelling as waters pile up along the coast. Temperature differences and wind patterns are seasonal in

temperate In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...

latitudes, creating highly variable upwelling and downwelling conditions. For example, in fall and winter along the

Pacific Northwest The Pacific Northwest (PNW; ) is a geographic region in Western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common ...

coast in the United States, southerly winds in the

Gulf of Alaska The Gulf of Alaska ( Tlingit: ''Yéil T'ooch’'') is an arm of the Pacific Ocean defined by the curve of the southern coast of Alaska, stretching from the Alaska Peninsula and Kodiak Island in the west to the Alexander Archipelago in the ...

and

California Current The California Current () is a cold water Pacific Ocean ocean current, current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. It is considered an ...

system create downwelling-favorable conditions, transporting offshore water from the south and west towards the coast. These downwelling events tend to last for days and can be associated with winter storms and contribute to low levels of

observed during fall and winter. In contrast, during the "spring transition" at the end of the downwelling season and the beginning of the upwelling season is marked by the presence of cold, nutrient-rich, upwelled water at the coast, which stimulates high levels of primary production. In contrast to seasonally variable temperate regions, downwelling is relatively steady at the poles as cold air decreases the temperature of salty water transported by gyres from the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

. During the neutral and

La Niña LA most frequently refers to Los Angeles, the second most populous city in the United States of America. La, LA, or L.A. may also refer to: Arts and entertainment Music *La (musical note), or A, the sixth note *"L.A.", a song by Elliott Smit ...

phases of the El Niño Southern Oscillation ( ENSO), steady easterly

in equatorial regions can cause water to pile up in the western Pacific. A weakening of these trade winds can create downwelling

Kelvin wave A Kelvin wave is a wave in the ocean, a large lake or the atmosphere that balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-d ...

s, which propagate along the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

in the eastern Pacific. Series of Kelvin waves associated with anomalously warm sea surface temperatures in the eastern Pacific can be a predecessor to an

El Niño EL, El or el may refer to: Arts and entertainment Fictional entities * El, a character from the manga series ''Shugo Chara!'' by Peach-Pit * Eleven (''Stranger Things'') (El), a fictional character in the TV series ''Stranger Things'' * El, fami ...

event. During the El Niño phase of ENSO, the disruption of trade winds causes ocean water to pile up off the western coast of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

. This shift is associated with a decrease in upwelling and may enhance coastal downwelling.

Effects on ocean biogeochemistry

Biogeochemical cycling related to downwelling is constrained by the location and frequency at which this process occurs. The majority of downwelling, as described above, occurs in polar regions as deep and bottom water formation or in the center of subtropical gyres. Bottom and deep water formation in the

(

) and North

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

(

Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...

Labrador Labrador () is a geographic and cultural region within the Canadian province of Newfoundland and Labrador. It is the primarily continental portion of the province and constitutes 71% of the province's area but is home to only 6% of its populatio ...

, Norwegian, and

s) is a major contributor towards the removal and sequestration of anthropogenic

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

dissolved organic carbon Dissolved organic carbon (DOC) is the fraction of organic carbon Operational definition, operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometre, micrometers. The fraction remain ...

(DOC), and dissolved oxygen. Dissolved gas

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

is greater in cold water allowing for increased gas concentrations. The Southern Ocean alone has been shown to be the most important high-latitude region controlling pre-industrial atmospheric carbon dioxide by

general circulation model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms for ...

simulations. Circulation of water into the Antarctic deep-water formation region is one of the main factors drawing carbon dioxide into the surface oceans. The other is the

, which is typically limited by

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

in the Southern Ocean in areas with high

s and low

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

( HNLC). DOC can become entrained during bottom and deep water formation which is a large portion of biogenic carbon export. It is thought that the export of DOC is up to 30% of the biogenic carbon that makes it into the deep ocean. The intensity of the DOC flux to depth relies on the strength of winter convection, which also affects the microbial food web, causing variations in the DOC exported to depth. Dissolved oxygen is also downwelled at bottom and deep water formation sites, contributing to elevated dissolved oxygen concentrations below 1000 meters. Subtropical gyres are typically limited in macro and micro nutrients such as

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

, and iron; resulting in picophytoplankton communities that have low nutrient requirements. This is in part due to consistent downwelling, which transports nutrients away from the photic zone. These

oligotroph An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates o ...

ic areas are thought to be sustained by rapid

nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...

which could leave little carbon remaining that could be sequestered. The dynamics of picophytoplankton's role in carbon cycling in subtropical gyres is poorly understood and is being actively researched. Areas with the highest primary productivity play significant roles in biogeochemical cycling of carbon and nitrogen. Downwelling can either alleviate or induce anoxic conditions, depending on the initial conditions and location. Sustained periods of upwelling can cause deoxygenation which is relieved by a downwelling event transporting dissolved oxygen back down to depths.

Anoxic Anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved ox ...

conditions can also result from persistent downwelling after an

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in fresh water or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompass ...

of high-biomass

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

s. The accumulation of dinoflagellates and other forms of biomass nearshore due to downwelling will eventually cause nutrient depletion and mortality of organisms. As the biomass decays, oxygen becomes depleted by

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

ic bacteria, inducing anoxic conditions.

References

External links

Wind-Driven Surface Currents: Upwelling and Downwelling Background
{{Physical oceanography Oceanography Meteorological phenomena