Irminger Rings (IRs) are mesoscale (15-50 kilometers) ocean
eddies
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 ...
that are formed off the West coast of
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...
and travel southwestwards through the
Labrador Sea
The Labrador Sea (French: ''mer du Labrador'', Danish: ''Labradorhavet'') 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, ...
. Most IRs are
anti-cyclonic (clockwise 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 as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
).
There is considerable interest in researching IRs, because they have been hypothesized to influence
deep convection in the Labrador sea, and therefore the formation of deep water.
Basin overview
The
Irminger Current
The Irminger Current is a north Atlantic ocean current setting westward off the southwest coast of Iceland. It is composed of relatively warm and saline waters from the eastern North Atlantic that are fed by the North Atlantic Drift. The Irminger ...
(IC) is a branch of the
North Atlantic Drift
The North Atlantic Current (NAC), also known as North Atlantic Drift and North Atlantic Sea Movement, is a powerful warm western boundary current within the Atlantic Ocean that extends the Gulf Stream northeastward.
The NAC originates from where ...
(NAD) that flows westward from
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
. Because of its
Atlantic
The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...
origins, IC waters are relatively warm and saline compared to the cold, fresh water of the
East Greenland Current
The East Greenland Current (EGC) is a cold, low- salinity current that extends from Fram Strait (~80N) to Cape Farewell (~60N). The current is located off the eastern coast of Greenland along the Greenland continental margin. The current cuts th ...
(EGC) originating from the Greenland Sea. Off the East coast of Greenland, the IC and the EGC meet and "combine" after rounding
Cape Farewell to form the heavily
stratified
Stratification may refer to:
Mathematics
* Stratification (mathematics), any consistent assignment of numbers to predicate symbols
* Data stratification in statistics
Earth sciences
* Stable and unstable stratification
* Stratification, or st ...
current system known as the
West Greenland Current
The West Greenland Current is a weak cold water current that flows to the north along the west coast of Greenland. The current results from the movement of water flowing around the southernmost point of Greenland caused by the East Greenland Curr ...
(WGC). The top layer of the WGC is 200 meters deep and consists of fresh EGC water. The layer below, from 200 to 700 meters, consists of salty Irminger Water (IW).
Irminger Ring formation
![Qualitative map of the Labrador sea currents](https://upload.wikimedia.org/wikipedia/commons/c/cf/Qualitative_map_of_the_Labrador_sea_currents._With_the_area_of_maximum_EKE_marked._With_Depth_contours.png)
The topography of the Greenland coast steepens rapidly between 60° and 62°N,
near
Cape Desolation
Cape Desolation ( da, Kap Desolation), also known as 'Cape Brill', is a headland in southwest Greenland in the Kujalleq municipality
A municipality is usually a single administrative division having corporate status and powers of self-gove ...
. This steep slope can induce instabilities in the WGC, leading to the formation of Irminger rings. It is unclear whether these instabilities are mainly
barotropic
In fluid dynamics, a barotropic fluid is a fluid whose density is a function of pressure only. The barotropic fluid is a useful model of fluid behavior in a wide variety of scientific fields, from meteorology to astrophysics.
The density of most ...
or
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 ...
,
with contradicting outcomes between models.
Barotropic instabilities can be created by a large horizontal shear in the current. The sudden change in topography causes the geostrophic contours of the flow to converge, which increases the vertical extent and a decreases the width of the WGC.
The resulting horizontal shears are sufficient to create barotropic instability.
Baroclinic instability is induced by the large horizontal density gradient in the WGC near the bottom.
The misalignment of surfaces of equal pressure and density induces a vertical velocity gradient. The energy of the baroclinic instability is proportional to the potential energy of the environmental flow related to the vertical shear of the current.
Both barotropic and baroclinic instabilities generate
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 ...
leading to eddies called Irminger Rings. Associated with the formation of IRs is an increase in
Eddy Kinetic Energy (EKE).
IRs are not the only type of eddy spawned around the Labrador Sea. Convective events in the interior Labrador sea create steep density gradients. The associated baroclinic instability gives rise to Convective Eddies (CEs)
(20-30 kilometers in diameter) that are more vertically homogeneous. In addition, weak instabilities in the WGC and LC along the West Greenland and Labrador coast spawn Boundary Current Eddies (BCEs).
Irminger Ring characteristics
Irminger Rings are mostly anticyclonic eddies with surface-intensified currents ranging from 30 to 80 cm/s in magnitude.
The
Rossby number
The Rossby number (Ro), named for Carl-Gustav Arvid Rossby, is a dimensionless number used in describing fluid flow. The Rossby number is the ratio of inertial force to Coriolis force, terms , \mathbf \cdot \nabla \mathbf, \sim U^2 / L and \Omeg ...
of IRs is between 0.1 and 0.5.
Since IRs are shed off the WGC, their vertical structure is similar to the WGC. The upper layer of IRs consists of freshwater, originating from the EGC. Below the upper layer is the relatively warm and saline IW. IRs are also regularly found to have secondary cores at depths between 1–1.5 km related to an enhanced downward
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 ...
depression.
Due to the contribution of IW, IRs are less dense and therefore more
buoyant
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
than typical water at the same depth.
Both the freshwater and IW layer have a steep vertical density gradient, which results in strongly
stratified
Stratification may refer to:
Mathematics
* Stratification (mathematics), any consistent assignment of numbers to predicate symbols
* Data stratification in statistics
Earth sciences
* Stable and unstable stratification
* Stratification, or st ...
IRs. The freshwater layer is found to be the largest contributor to Irminger ring stratification.
Over the lifetime of IRs, the stratification decreases as the upper layer becomes saltier and the lower layer becomes fresher. During winter, the freshwater layer often erodes, which also drastically reduces the stratification.
Propagation
The main mode of propagation of IRs is in southwestward direction
with an approximate speed of 5 cm/s.
Modelled IRs roughly follow the 3000 meter depth
isobath
Bathymetry (; ) is the study of underwater depth of ocean floors (''seabed topography''), lake floors, or river floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water de ...
.
IRs have a typical lifetime of a few months.
Models find that IRs are prone to decay during winters with large convection events, but some survive up to 2 years.
IRs that spawn in the south are likely to live long enough to reach the deep basin of the Labrador Sea, while IRs spawned further north are more likely to be disrupted by
Boundary Currents (BCs).
Temporal variability
IR production increases during winter, due to the EKE maximum associated with higher WGC velocities.
During fall the core of IRs has been measured to be warmer (1.9°C) and saltier (0.07 psu saltier) than in spring. This is theorized to be a response to the seasonal cycle of IW, which reaches the highest current velocities in fall.
On interannual timescales, the
Arctic Oscillation
The Arctic oscillation (AO) or Northern Annular Mode/Northern Hemisphere Annular Mode (NAM) is a weather phenomenon at the Arctic pole north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The s ...
influences the formation of IRs. If the Arctic Oscillation is its positive phase, this leads to stronger currents in the WGC and other boundary currents. The larger WGC current increases the available EKE for IR generation.
Influence on Labrador Sea deep convection
The Labrador Sea is one of the few places in the ocean where
deep convection occurs.
Due to the cyclonic large scale flow and high latitude positioning, the stratification in the Labrador Sea is usually weak.
Deep convection events can occur during winter, if the cooling in the top layer is large enough to create a higher density in the top layer than the water below. As a consequense of this unstable stratification, large scale vertical mixing can be induced,
which creates a deep
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 ...
. The homogeneous water mass that is formed during deep convection is called
Labrador Sea Water
Labrador Sea Water is an intermediate water mass characterized by cold water, relatively low salinity compared to other intermediate water masses, and high concentrations of both oxygen and anthropogenic tracers.Pickart, Robert S., et al. "Is Labr ...
(LSW). LSW is a source of
North Atlantic Deep Water
North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning circulation) of the world's oceans involves the flow of warm surface waters from the ...
,
which is essential for the
Atlantic Meridional Overturning Circulation
The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward fl ...
. Deep convection also allows for mixing of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
into the deep ocean.
Variations in the magnitude of deep convection are large, and can be up to 2000 meter depth.
After a convective event, the Labrador Sea gradually restratifies during spring. The extent of this restratification influences the variability of future convective events.
Suppression of deep convection
Due to the long lifetime of Irminger rings, some reach the convective area in the interior
Labrador Sea
The Labrador Sea (French: ''mer du Labrador'', Danish: ''Labradorhavet'') 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, ...
.
Since IRs are highly stratified and buoyant, they enhance the stratification of the Labrador Sea.
Consequently, Irminger Rings suppress deep convection in the Labrador Sea, which decreases Labrador Sea Water production. Specifically, IRs limit the area of deep convection in the North.
Although IRs are more abundant during the positive phase of the Arctic Oscillation, this doesn't lead to reduced deep convection since the positive Arctic Oscillation phase simultaniously enhances deep convection.
Restratification
In addition to suppressing deep convection, IRs enhance restratification after convective events.
The extent of IR-induced restratification is not clear. Possibly, IRs contribute to restratification only rarely and not on an annual basis.
Convective Eddies (CEs) and Boundary Current Eddies (BCEs) also enhance restratification in the Labrador Sea. The relative contribution of IRs, CEs and BCEs to restratification is disputed. Some modelling studies find that IRs resupply more heat after a convective event than CEs and BCEs,
while others find that CEs
or BCEs
are the main contributor. This variation can be explained partly by inter-model differences in position of the convective area in the Labrador Sea.
Influence of Labrador Sea deep convection on Irminger Rings
Some interannual variability of IRs is related to the intensity of convective events, as more intense deep convection produces higher density Labrador Sea Water. This in turn causes a greater density gradient between the sea and the buoyant West Greenland Current, which positively correlates with eddy fluxes.
Although Irminger Rings decrease the production of LSW by suppressing deep convection, LSW can also be produced by IRs. During deep convection events, vertical mixing can take place inside long lived IRs that have reached the convective area. The typical extent of IR convective vertical mixing is between 100 and 700 meters deep, but can be up to 1300 meter during large convective events.
This is almost as deep as in the rest of the convective area.
In an ocean model, LSW was produced during this mixing by Irminger Rings that lived over 2 years.
References
{{reflist, refs=
[{{cite journal , last1=Lilly , first1=Jonathan M. , last2=Rhines , first2=Peter B. , last3=Schott , first3=Friedrich , last4=Lavender , first4=Kara , last5=Lazier , first5=John , last6=Send , first6=Uwe , last7=D’Asaro , first7=Eric , title=Observations of the Labrador Sea eddy field , journal=Progress in Oceanography , date=1 October 2003 , volume=59 , issue=1 , pages=75–176 , doi=10.1016/j.pocean.2003.08.013]
[{{cite journal , last1=de Jong , first1=M. F. , last2=Bower , first2=A. S. , last3=Furey , first3=H. H. , title=Two Years of Observations of Warm-Core Anticyclones in the Labrador Sea and Their Seasonal Cycle in Heat and Salt Stratification , journal=Journal of Physical Oceanography , date=1 February 2014 , volume=44 , issue=2 , pages=427–444 , doi=10.1175/JPO-D-13-070.1, bibcode=2014JPO....44..427D ]
Currents of the Atlantic Ocean
Labrador Sea