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Eddy pumping is a component of
mesoscale eddy In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid beh ...
-induced vertical motion. It is a physical mechanism through which vertical motion is created from variations in an eddy's rotational strength.
Cyclonic In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...
(
Anticyclonic An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from abov ...
) eddies lead primarily to
upwelling Upwelling is an 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 surface water. The nutr ...
(
downwelling Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the ''sinking'' limb of a convecti ...
) in the Northern Hemisphere and vice versa in the Southern hemisphere. It is a key mechanism driving biological and biogeochemical processes in the ocean such as
algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...
s and the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
.


The mechanism

Eddies have a re-stratifying effect, which means they tend to organise the water in layers of different density. These layers are separated by surfaces called
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. The re-stratification of the
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 ...
is strongest in regions with large horizontal density gradients, known also as “fronts”, where the geostrophic shear and potential energy provide an energy source from which
baroclinic In fluid dynamics, the baroclinity (often called baroclinicity) of a stratified fluid is a measure of how misaligned the gradient of pressure is from the gradient of density in a fluid. In meteorology a baroclinic flow is one in which the densi ...
and symmetric instabilities can grow. Below the mixed layer, a region of rapid density change (or
pycnocline A pycnocline is the Cline (hydrology), cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effec ...
) separates the upper and lower water, hindering vertical transport. Eddy pumping is a component of mesoscale eddy-induced vertical motion. Such vertical motion is caused by the deformation of the pycnocline. It can be conceptualised by assuming that ocean water has a density surface with mean depth averaged over time and space. This surface separates the upper ocean, corresponding to the
euphotic zone The photic zone, 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 proc ...
, from the lower,
deep ocean The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...
. When an eddy transits through, such density surface is deformed. Dependent on the phases of the lifespan of an eddy this will create vertical perturbations in different direction. Eddy lifespans are divided in formation, evolution and destruction. Eddy-pumping perturbations are of three types: * Cyclones * Anticyclones * Mode-water eddies


Eddy-centric approach

Mode-water eddies have a complex density structure. Due to their shape, they cannot be distinguished from regular anticyclones in an eddy-centric (focused on the core of the eddy) analysis based on sea level height. Nonetheless, eddy pumping induced vertical motion in the euphotic zone of mode-water eddies is comparable to cyclones. For this reasons, only the cyclonic and anticyclonic mechanisms of eddy-pumping perturbations are explained.


Conceptual explanation based on sea-surface level

An intuitive description of this mechanism is what is defined as eddy-centric-analysis based on sea-surface level. In the Northern hemisphere, anticlockwise rotation in cyclonic eddies creates a divergence of horizontal surface currents due to the
Coriolis effect In physics, the Coriolis force is an inertial or fictitious 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 ...
, leading to a dampened water surface. To compensate the inhomogeneity of surface elevation, isopycnal surfaces are uplifted toward the euphotic zone and incorporation of deep ocean, nutrient-rich waters can occur.


Physical explanation

Conceptually, eddy pumping associates the vertical motion in the interior of eddies to temporal changes in eddy relative vorticity. The vertical motion created by the change in
vorticity In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wit ...
is understood from the characteristics of the water contained in the core of the eddy. Cyclonic eddies rotate anticlockwise (clockwise) in the Northern (Southern) hemisphere and have a cold core. Anticyclonic eddies rotate clockwise (anticlockwise) in the Northern (Southern) hemisphere and have a warm core. The
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
and
salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
difference between the eddy core and the surrounding waters is the key element driving vertical motion. While propagating in horizontal direction, Cyclones and anticyclones “bend” the pycnocline upwards and downwards, respectively, induced by this temperature and salinity discrepancy. The extent of the vertical perturbation of the density surface inside the eddy (compared to the mean ocean density surface) is determined by the changes in rotational strength (relative vorticity) of the eddy. Ignoring horizontal advection in the density conservation equation, the density changes due to changes in vorticity can be directly related to vertical transport. This assumption is coherent with the idea of vertical motion occurring at the eddy centre, in correspondence to variations of a perfectly circular flow. 0=+\nabla\cdot (\rho\textbf) \longrightarrow =-\rho w Through such mechanism eddy pumping generates upwelling of cold, nutrient rich deep waters in cyclonic eddies and downwelling of warm, nutrient poor, surface water in anticyclonic eddies.


Dependency on the phase of lifespan

Eddies weaken over time due to kinetic energy
dissipation In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form to a ...
. As eddies form and intensify, the mechanisms mentioned above will strengthen and, as an increase in relative vorticity generates perturbations of the isopycnal surfaces, the pycnocline deforms. On the other hand, when eddies have aged and carry low
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
, their vorticity diminishes and leads to eddy destruction. Such process opposes to eddy formation and intensification, as the pycnocline will return to its original position prior to the eddy-induced deformation. This means that the pycnocline will uplift in anticyclones and compress in cyclones, leading to upwelling and downwelling, respectively.


Eddy pumping characteristics

The direction of vertical motion in cyclonic and anticyclonic eddies is independent of the hemisphere. Observed vertical velocities of eddy pumping are in the order of one meter per day. However, there are regional differences. In regions where kinetic energy is higher, such as in the
Western boundary current Boundary currents are ocean currents with dynamics determined by the presence of a coastline, and fall into two distinct categories: western boundary currents and eastern boundary currents. Eastern boundary currents Eastern boundary currents are ...
, eddies are found to generate stronger vertical currents than eddies in
open ocean The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
.


Limitations

When describing vertical motion in eddies it is important to note that eddy pumping is only one component of a complex mechanism. Another important factor to take into account, especially when considering ocean-wind interaction, is the role played by eddy-induced
Ekman pumping 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 ...
. Some other limitations of the explanation above are due to the idealised, quasi circular linear dynamical response to perturbations that neglects the vertical displacement that a particle can experience moving along a sloping neutral surface. Vertical motion in eddies is a fairly recent research topic that still presents limitations in the theory both due to complexity and lack of sufficient observations. Nonetheless, the one presented above is a simplification that helps explain partially the important role that eddies play in biological productivity, as well as their biogeochemical role in the carbon cycle.


Biological impact

Recent findings suggest that mesoscale eddies are likely to play a key role in nutrient transport, such as the spatial distribution of
chlorophyll Chlorophyll (also chlorophyl) 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 allow plants to a ...
concentration, in the open ocean. Lack of knowledge on the impact of eddy activity is however still notable, as eddies’ contribution has been argued not to be sufficient to maintain the observed primary production through nitrogen supply in parts of the subtropical gyre. Although the mechanisms through which eddies shape ecosystems are not yet fully understood, eddies transport nutrients through a combination of horizontal and vertical processes. Stirring and trapping relate to nutrient transport, whereas eddy pumping, eddy-induced
Ekman pumping 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 ...
, and eddy impacts on mixed-layer depth variate nutrient. Here, the role played by eddy pumping is discussed. Cyclonic eddy pumping drives new primary production by lifting nutrient-rich waters into the euphotic zone. Complete utilisation of the upwelled nutrients is guaranteed by two main factors. Firstly, biological uptake takes place in timescales that are much shorter than the average lifetime of eddies. Secondly, because the nutrient enhancement takes place in the eddy's interior, isolated from the surrounding waters, biomass can accumulate until upwelled nutrients are fully consumed.


Main examples

Evidence of the biological impacts of eddy pumping mechanism is present in various publications based on observations and modelling of multiple locations worldwide. Eddy-centric chlorophyll anomalies have been observed in the
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...
region and off the west coast of
British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...
(
Haida eddies Haida Eddies are episodic, clockwise rotating ocean eddies that form during the winter off the west coast of British Columbia’s Haida Gwaii and Alaska’s Alexander Archipelago. These eddies are notable for their large size, persistence, and freq ...
), as well as eddy-induced enhanced biological production in the Weddell-Scotia Confluence 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 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
, in the northern
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 east, ...
, in the
South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Phil ...
, in the
Bay of Bengal The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line between ...
, in the
Arabian Sea The Arabian Sea ( ar, اَلْبَحرْ ٱلْعَرَبِيُّ, Al-Bahr al-ˁArabī) is a region of the northern Indian Ocean bounded on the north by Pakistan, Iran and the Gulf of Oman, on the west by the Gulf of Aden, Guardafui Channel ...
and in the north-western Alboran Sea, to name a few. Estimations of the eddy pumping in the
Sargasso Sea The Sargasso Sea () is a region of the Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. It is distinguished from other parts of the Atlantic Ocean by its charac ...
resulted in a flux between 0.24 and 0.5 nitrogen \frac . These quantities have been deemed sufficient to sustain a rate of new primary production consistent with estimates for this region. On a wider ecological scale, eddy-driven variations in productivity influence the trade-off between
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...
larval survival and the abundance of predators. These concepts partially explain mesoscale variations in the distribution of larval
bluefin tuna Bluefin tuna is a common name used to refer to several species of tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, th ...
,
sailfish The sailfish is one or two species of marine fish in the genus ''Istiophorus'', which belong to the family Istiophoridae ( marlins). They are predominantly blue to gray in colour and have a characteristically large dorsal fin known as the ...
,
marlin Marlins are fish from the family Istiophoridae, which includes about 10 species. A marlin has an elongated body, a spear-like snout or bill, and a long, rigid dorsal fin which extends forward to form a crest. Its common name is thought to deri ...
,
swordfish Swordfish (''Xiphias gladius''), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordfis ...
, and other species. Distributions of adult fishes have also been associated with the presence of cyclonic eddies. Particularly, higher abundances of bluefin tuna and cetaceans in the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...
and blue marlin in the proximity of
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
are linked to cyclonic eddy activities. Such spatial patterns extend to seabirds spotted in the vicinities of eddies, including great frigate birds in the
Mozambique Channel The Mozambique Channel (french: Canal du Mozambique, mg, Lakandranon'i Mozambika, pt, Canal de Moçambique) is an arm of the Indian Ocean located between the Southeast African countries of Madagascar and Mozambique. The channel is about long ...
and
albatross Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Pacifi ...
,
tern Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consists of e ...
s, and
shearwater Shearwaters are medium-sized long-winged seabirds in the petrel family Procellariidae The family Procellariidae is a group of seabirds that comprises the fulmarine petrels, the gadfly petrels, the diving petrels, the prions, and the shearwa ...
s in the South
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
.


North Atlantic Algal Bloom

The North Sea is an ideal basin for the formation of
algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...
s or
spring bloom The spring bloom is a strong increase in phytoplankton abundance (i.e. stock) that typically occurs in the early spring and lasts until late spring or early summer. This seasonal event is characteristic of temperate North Atlantic, sub-polar, and ...
s due to the combination of abundant nutrients and intense Arctic winds that favour the mixing of waters. Blooms are important indicators of the health of a marine ecosystem. Springtime phytoplankton blooms have been thought to be initiated by seasonal light increase and near-surface stratification. Recent observations from the sub-polar North Atlantic experiment and biophysical models suggest that the bloom may be instead resulting from an eddy-induced stratification, taking place 20 to 30 days earlier than it would occur by seasonal changes. These findings revolutionise the entire understanding of spring blooms. Moreover, eddy pumping and eddy-induced Ekman pumping have been shown to dominate late-bloom and post-bloom biological fields.


Biogeochemistry

Phytoplankton absorbs CO_2 through photosynthesis. When such organisms die and sink to the seafloor, the carbon they absorbed gets stored in the deep ocean through what is known as the
biological pump The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...
. Recent research has been investigating the role of eddy pumping and more in general, of vertical motion in mesoscale eddies in the carbon cycle. Evidence has shown that eddy pumping-induced upwelling and downwelling may play a significant role in shaping the way that carbon is stored in the ocean. Despite the fact that research in this field is only developing recently, first results show that eddies contribute less than 5% of the total annual export of phytoplankton to the ocean interior.


Plastic pollution

Eddies play an important role in the sea surface distribution of
microplastics Microplastics are fragments of any type of plastic less than in length, according to the U.S. National Oceanic and Atmospheric Administration (NOAA) and the European Chemicals Agency. They cause pollution by entering natural ecosystems from a v ...
in the ocean. Due to their convergent nature, anticyclonic eddies trap and transport microplastics at the sea surface, along with nutrients, chlorophyll and zooplankton. In the North Atlantic subtropical gyre, the first direct observation of sea surface concentrations of microplastics between a cyclonic and an anticyclonic mesoscale eddy has shown an increased accumulation in the latter. Accumulation of microplastics has environmental impacts through its interaction with the biota. Initially buoyant plastic particles (between 0.01 and 1 mm) are submerged below the climatological mixed layer depth mainly due to
biofouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
. In regions with very low productivity, particles remain within the upper part of the mixed layer and can only sink below it if a spring bloom occurs.


See also

*
Algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...
- a rapid increase or accumulation in the population of algae in freshwater o marine water systems *
Baroclinic instability In fluid dynamics, the baroclinity (often called baroclinicity) of a stratified fluid is a measure of how misaligned the gradient of pressure is from the gradient of density in a fluid. In meteorology a baroclinic flow is one in which the densi ...
- fluid dynamical instability of fundamental importance in the atmosphere and ocean *
Ekman pumping 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 ...
- Ekman Pumping is the component of Ekman transport that results in areas of downwelling due to the convergence of water *
Haida Eddies Haida Eddies are episodic, clockwise rotating ocean eddies that form during the winter off the west coast of British Columbia’s Haida Gwaii and Alaska’s Alexander Archipelago. These eddies are notable for their large size, persistence, and freq ...
- episodic, clockwise rotating ocean eddies that form during the winter off the west coast of British Columbia *
Mesoscale ocean eddies In fluid dynamics, an eddy is the swirling of a fluid and the reverse current (water), current created when the fluid is in a Turbulence, turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream ...
- Swirling in the ocean created by its turbulent nature *
Spring bloom The spring bloom is a strong increase in phytoplankton abundance (i.e. stock) that typically occurs in the early spring and lasts until late spring or early summer. This seasonal event is characteristic of temperate North Atlantic, sub-polar, and ...
– Strong increase in phytoplankton abundance that typically occurs in the early spring


References

{{reflist Water physics