Irminger Rings

Last updated

Irminger Rings (IRs) are mesoscale (15-50 kilometers) ocean eddies that are formed off the West coast of Greenland and travel southwestwards through the Labrador Sea. [1] Most IRs are anti-cyclonic (clockwise in the Northern Hemisphere). [2] 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. [3]

Contents

Map showing the mean velocity for 2020 and 2021 and depth contours of the Irminger and Labrador seas. Cape desolation marks the location where most irminger rings are formed. The 3000m depth contour is the contour Irminger rings tend to follow when they propagate. The bathymetry data is obtained from the General Bathymetric Chart of the Ocean (GEBCO). The current data is obtained from the CMEMs database. Map of the Labrador & Irminger seas. With depth contours and current data.png
Map showing the mean velocity for 2020 and 2021 and depth contours of the Irminger and Labrador seas. Cape desolation marks the location where most irminger rings are formed. The 3000m depth contour is the contour Irminger rings tend to follow when they propagate. The bathymetry data is obtained from the General Bathymetric Chart of the Ocean (GEBCO). The current data is obtained from the CMEMs database.

Basin overview

The Irminger Current (IC) is a branch of the North Atlantic Drift (NAD) that flows westward from Iceland. Because of its Atlantic origins, IC waters are relatively warm and saline compared to the cold, fresh water of the East Greenland Current (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 current system known as the West Greenland Current (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). [6]

Irminger Ring formation

Qualitative map of the currents in the Labrador sea. The red area marks an area with relatively high eddy kinetic energy. Qualitative map of the Labrador sea currents. With the area of maximum EKE marked. With Depth contours.png
Qualitative map of the currents in the Labrador sea. The red area marks an area with relatively high eddy kinetic energy.

The topography of the Greenland coast steepens rapidly between 60° and 62°N, [8] near Cape Desolation. 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 [9] or baroclinic, [10] with contradicting outcomes between models. [3]

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. [11] The resulting horizontal shears are sufficient to create barotropic instability. [3]

Baroclinic instability is induced by the large horizontal density gradient in the WGC near the bottom. [3] 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 leading to eddies called Irminger Rings. Associated with the formation of IRs is an increase in Eddy Kinetic Energy (EKE). [3] 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) [9] [3] (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). [11] [3]

Irminger Ring characteristics

Irminger Rings are mostly anticyclonic eddies with surface-intensified currents ranging from 30 to 80 cm/s in magnitude. [2] The Rossby number of IRs is between 0.1 and 0.5. [12] [2] 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 depression. [2] Due to the contribution of IW, IRs are less dense and therefore more buoyant than typical water at the same depth. [6] Both the freshwater and IW layer have a steep vertical density gradient, which results in strongly stratified IRs. The freshwater layer is found to be the largest contributor to Irminger ring stratification. [6] 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. [6]

Propagation

The main mode of propagation of IRs is in southwestward direction [13] [8] with an approximate speed of 5 cm/s. [2] Modelled IRs roughly follow the 3000 meter depth isobath. [6] IRs have a typical lifetime of a few months. [6] Models find that IRs are prone to decay during winters with large convection events, but some survive up to 2 years. [6] [9] 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). [6]

Temporal variability

IR production increases during winter, due to the EKE maximum associated with higher WGC velocities. [2] 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. [14]

On interannual timescales, the Arctic Oscillation 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. [3]

Influence on Labrador Sea deep convection

The Labrador Sea is one of the few places in the ocean where deep convection occurs. [15] Due to the cyclonic large scale flow and high latitude positioning, the stratification in the Labrador Sea is usually weak. [15] 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 consequence of this unstable stratification, large scale vertical mixing can be induced, [6] which creates a deep mixed layer. The homogeneous water mass that is formed during deep convection is called Labrador Sea Water (LSW). LSW is a source of North Atlantic Deep Water, [6] which is essential for the Atlantic Meridional Overturning Circulation. Deep convection also allows for mixing of oxygen and carbon dioxide into the deep ocean. [6] Variations in the magnitude of deep convection are large, [16] and can be up to 2000 meter depth. [9] After a convective event, the Labrador Sea gradually restratifies during spring. The extent of this restratification influences the variability of future convective events. [8]

Suppression of deep convection

Due to the long lifetime of Irminger rings, some reach the convective area in the interior Labrador Sea. [6] [8] Since IRs are highly stratified and buoyant, they enhance the stratification of the Labrador Sea. [6] 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. [3] 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 simultaneously enhances deep convection. [3]

Restratification

In addition to suppressing deep convection, IRs enhance restratification after convective events. [8] The extent of IR-induced restratification is not clear. Possibly, IRs contribute to restratification only rarely and not on an annual basis. [3] 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, [8] while others find that CEs [3] or BCEs [9] are the main contributor. This variation can be explained partly by inter-model differences in position of the convective area in the Labrador Sea. [3]

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. [17]

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. [6] This is almost as deep as in the rest of the convective area. [6] In an ocean model, LSW was produced during this mixing by Irminger Rings that lived over 2 years. [6]

Related Research Articles

<span class="mw-page-title-main">North Atlantic Deep Water</span> Deep water mass formed in the North Atlantic Ocean

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation of the world's oceans involves the flow of warm surface waters from the southern hemisphere into the North Atlantic. Water flowing northward becomes modified through evaporation and mixing with other water masses, leading to increased salinity. When this water reaches the North Atlantic it cools and sinks through convection, due to its decreased temperature and increased salinity resulting in increased density. NADW is the outflow of this thick deep layer, which can be detected by its high salinity, high oxygen content, nutrient minima, high 14C/12C, and chlorofluorocarbons (CFCs).

<span class="mw-page-title-main">Downwelling</span> Process of accumulation and sinking of higher density material beneath lower density material

Downwelling is the downward movement of a fluid parcel and its properties within a larger fluid. It is closely related to upwelling, the upward movement of fluid.

<span class="mw-page-title-main">Baroclinity</span> Measure of misalignment between the gradients of pressure and density in a fluid

In fluid dynamics, the baroclinity 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 density depends on both temperature and pressure. A simpler case, barotropic flow, allows for density dependence only on pressure, so that the curl of the pressure-gradient force vanishes.

<span class="mw-page-title-main">Physical oceanography</span> Study of physical conditions and processes within the ocean

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

<span class="mw-page-title-main">Surface layer</span> Layer of a turbulent fluid affected by interaction with a surface

The surface layer is the layer of a turbulent fluid most affected by interaction with a solid surface or the surface separating a gas and a liquid where the characteristics of the turbulence depend on distance from the interface. Surface layers are characterized by large normal gradients of tangential velocity and large concentration gradients of any substances transported to or from the interface.

<span class="mw-page-title-main">East Greenland Current</span> Current from Fram Strait to Cape Farewell off the eastern coat of Greenland

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 through the Nordic Seas and through the Denmark Strait. The current is of major importance because it directly connects the Arctic to the Northern Atlantic, it is a major contributor to sea ice export out of the Arctic, and it is a major freshwater sink for the Arctic.

Parameterization in a weather or climate model is a method of replacing processes that are too small-scale or complex to be physically represented in the model by a simplified process. This can be contrasted with other processes—e.g., large-scale flow of the atmosphere—that are explicitly resolved within the models. Associated with these parameterizations are various parameters used in the simplified processes. Examples include the descent rate of raindrops, convective clouds, simplifications of the atmospheric radiative transfer on the basis of atmospheric radiative transfer codes, and cloud microphysics. Radiative parameterizations are important to both atmospheric and oceanic modeling alike. Atmospheric emissions from different sources within individual grid boxes also need to be parameterized to determine their impact on air quality.

<span class="mw-page-title-main">Loop Current</span> Ocean current between Cuba and Yucatán Peninsula

A parent to the Florida Current, the Loop Current is a warm ocean current that flows northward between Cuba and the Yucatán Peninsula, moves north into the Gulf of Mexico, loops east and south before exiting to the east through the Florida Straits and joining the Gulf Stream. The Loop Current is an extension of the western boundary current of the North Atlantic subtropical gyre. Serving as the dominant circulation feature in the Eastern Gulf of Mexico, the Loop Currents transports between 23 and 27 sverdrups and reaches maximum flow speeds of from 1.5 to 1.8 meters/second.

<span class="mw-page-title-main">Eddy (fluid dynamics)</span> Swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime

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 behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle. This phenomenon is naturally observed behind large emergent rocks in swift-flowing rivers.

<span class="mw-page-title-main">Atlantic meridional overturning circulation</span> System of surface and deep currents in the Atlantic Ocean

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 flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean, although the extent of overturning in the Labrador Sea is disputed. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.

<span class="mw-page-title-main">Labrador Sea Water</span> Water mass formed by convective processes in the Labrador Sea

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. It is formed by convective processes in the Labrador Sea located between Greenland and the northeast coast of the Labrador Peninsula. Deep convection in the Labrador Sea allows colder water to sink forming this water mass, which is a contributor to the upper layer of North Atlantic Deep Water. North Atlantic Deep Water flowing southward is integral to the Atlantic Meridional Overturning Circulation. The Labrador Sea experiences a net heat loss to the atmosphere annually.

<span class="mw-page-title-main">Haida Eddies</span>

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 frequent recurrence. Rivers flowing off the North American continent supply the continental shelf in the Hecate Strait with warmer, fresher, and nutrient-enriched water. Haida eddies are formed every winter when this rapid outflow of water through the strait wraps around Cape St. James at the southern tip of Haida Gwaii, and meets with the cooler waters of the Alaska Current. This forms a series of plumes which can merge into large eddies that are shed into the northeast Pacific Ocean by late winter, and may persist for up to two years.

<span class="mw-page-title-main">Atlantification of the Arctic</span>

Atlantification is the increasing influence of Atlantic water in the Arctic. Warmer and saltier Atlantic water is extending its reach northward into the Arctic Ocean. The Arctic Ocean is becoming warmer and saltier and sea-ice is disappearing as a result. The process can be seen on the figure on the far right, where the sea surface temperature change in the past 50 years is shown, which is up to 5 degrees in some places. This change in the Arctic climate is most prominent in the Barents Sea, a shallow shelf sea north of Scandinavia, where sea-ice is disappearing faster than in any other Arctic region, impacting the local and global ecosystem.

Open ocean convection is a process in which the mesoscale ocean circulation and large, strong winds mix layers of water at different depths. Fresher water lying over the saltier or warmer over the colder leads to the stratification of water, or its separation into layers. Strong winds cause evaporation, so the ocean surface cools, weakening the stratification. As a result, the surface waters are overturned and sink while the "warmer" waters rise to the surface, starting the process of convection. This process has a crucial role in the formation of both bottom and intermediate water and in the large-scale thermohaline circulation, which largely determines global climate. It is also an important phenomena that controls the intensity of the Atlantic Meridional Overturning Circulation (AMOC).

A baroclinic instability is a fluid dynamical instability of fundamental importance in the atmosphere and ocean. It can lead to the formation of transient mesoscale eddies, with a horizontal scale of 10-100 km. In contrast, flows on the largest scale in the ocean are described as ocean currents, the largest scale eddies are mostly created by shearing of two ocean currents and static mesoscale eddies are formed by the flow around an obstacle (as seen in the animation on eddy. Mesoscale eddies are circular currents with swirling motion and account for approximately 90% of the ocean's total kinetic energy. Therefore, they are key in mixing and transport of for example heat, salt and nutrients.

Thermohaline staircases are patterns that form in oceans and other bodies of salt water, characterised by step-like structures observed in vertical temperature and salinity profiles; the patterns are formed and maintained by double diffusion of heat and salt. The ocean phenomenon consists of well-mixed layers of ocean water stacked on top of each other. The well-mixed layers are separated by high-gradient interfaces, which can be several meters thick. The total thickness of staircases ranges typically from tens to hundreds of meters.

Eddy saturation and eddy compensation are phenomena found in the Southern Ocean. Both are limiting processes where eddy activity increases due to the momentum of strong westerlies, and hence do not enhance their respective mean currents. Where eddy saturations impacts the Antarctic Circumpolar Current (ACC), eddy compensation influences the associated Meridional Overturning Circulation (MOC).

<span class="mw-page-title-main">Faroe-Bank Channel overflow</span> Overflow current from Nordic Seas towards North Atlantic Ocean

Cold and dense water from the Nordic Seas is transported southwards as Faroe-Bank Channel overflow. This water flows from the Arctic Ocean into the North Atlantic through the Faroe-Bank Channel between the Faroe Islands and Scotland. The overflow transport is estimated to contribute to one-third of the total overflow over the Greenland-Scotland Ridge. The remaining two-third of overflow water passes through Denmark Strait, the Wyville Thomson Ridge (0.3 Sv), and the Iceland-Faroe Ridge (1.1 Sv).

Eddy pumping is a component of mesoscale eddy-induced vertical motion in the ocean. It is a physical mechanism through which vertical motion is created from variations in an eddy's rotational strength. Cyclonic (Anticyclonic) eddies lead primarily to upwelling (downwelling) 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 blooms and the carbon cycle.

<span class="mw-page-title-main">Lofoten Vortex</span> Physical oceanographic feature

The Lofoten Vortex, also called Lofoten Basin Vortex or Lofoten Basin Eddy, is a permanent oceanic anticyclonic eddy, located in the northern part of the Norwegian Sea, off the coast of the Lofoten archipelago. It was documented for the first time in the 1970s.

References

  1. ↑ Femke de Jong, M.; Bower, Amy S.; Furey, Heather; Lilly, Jonathan M. (April 2013). Two years of IR observations offshore of the West Greenland Shelf. EGU General Assembly 2013, held 7–12 April 2013 in Vienna, Austria. Bibcode:2013EGUGA..15.1601F. EGU2013–1601.
  2. 1 2 3 4 5 6 Lilly, Jonathan M.; Rhines, Peter B.; Schott, Friedrich; Lavender, Kara; Lazier, John; Send, Uwe; D’Asaro, Eric (1 October 2003). "Observations of the Labrador Sea eddy field". Progress in Oceanography. 59 (1): 75–176. doi:10.1016/j.pocean.2003.08.013.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 13 Rieck, Jan K.; Böning, Claus W.; Getzlaff, Klaus (August 2019). "The Nature of Eddy Kinetic Energy in the Labrador Sea: Different Types of Mesoscale Eddies, Their Temporal Variability, and Impact on Deep Convection". Journal of Physical Oceanography. 49 (8): 2075–2094. Bibcode:2019JPO....49.2075R. doi: 10.1175/JPO-D-18-0243.1 . ISSN   0022-3670. S2CID   195545808.
  4. ↑ "Global bathymetric chart of the ocean".
  5. ↑ "CMEMs data base". Copernicus.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Pennelly, Clark; Myers, Paul G. (2022-02-01). "Tracking Irminger Rings' properties using a sub-mesoscale ocean model". Progress in Oceanography. 201: 102735. Bibcode:2022PrOce.20102735P. doi:10.1016/j.pocean.2021.102735. ISSN   0079-6611. S2CID   245452158.
  7. ↑ Jong, M. F. de; Bower, A. S.; Furey, H. H. (1 February 2014). "Two Years of Observations of Warm-Core Anticyclones in the Labrador Sea and Their Seasonal Cycle in Heat and Salt Stratification". Journal of Physical Oceanography. 44 (2): 427–444. Bibcode:2014JPO....44..427D. doi: 10.1175/JPO-D-13-070.1 . hdl: 10161/9577 .
  8. 1 2 3 4 5 6 Gelderloos, Renske; Katsman, Caroline A.; Drijfhout, Sybren S. (2011-11-01). "Assessing the Roles of Three Eddy Types in Restratifying the Labrador Sea after Deep Convection". Journal of Physical Oceanography. 41 (11): 2102–2119. Bibcode:2011JPO....41.2102G. doi: 10.1175/JPO-D-11-054.1 . ISSN   0022-3670.
  9. 1 2 3 4 5 Chanut, Jérôme; Barnier, Bernard; Large, William; Debreu, Laurent; Penduff, Thierry; Molines, Jean Marc; Mathiot, Pierre (2008-08-01). "Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification". Journal of Physical Oceanography. 38 (8): 1617–1643. Bibcode:2008JPO....38.1617C. doi: 10.1175/2008JPO3485.1 . ISSN   0022-3670. S2CID   54850302.
  10. ↑ Bracco, Annalisa; Pedlosky, Joseph; Pickart, Robert S. (1 September 2008). "Eddy Formation near the West Coast of Greenland". Journal of Physical Oceanography. 38 (9): 1992–2002. Bibcode:2008JPO....38.1992B. doi:10.1175/2008JPO3669.1. hdl: 1912/4059 .
  11. 1 2 Eden, Carsten; Böning, Claus (2002-12-01). "Sources of Eddy Kinetic Energy in the Labrador Sea". Journal of Physical Oceanography. 32 (12): 3346–3363. Bibcode:2002JPO....32.3346E. doi: 10.1175/1520-0485(2002)032<3346:SOEKEI>2.0.CO;2 . ISSN   0022-3670.
  12. ↑ Lilly, Jonathan M.; Rhines, Peter B. (1 February 2002). "Coherent Eddies in the Labrador Sea Observed from a Mooring". Journal of Physical Oceanography. 32 (2): 585–598. Bibcode:2002JPO....32..585L. doi: 10.1175/1520-0485(2002)032<0585:CEITLS>2.0.CO;2 .
  13. ↑ Rykova, Tatiana; Straneo, Fiammetta; Lilly, Jonathan M.; Yashayaev, Igor (1 May 2009). "Irminger Current Anticyclones in the Labrador Sea observed in the hydrographic record, 1990–2004". Journal of Marine Research. 67 (3): 361–384. doi:10.1357/002224009789954739. hdl: 1912/3114 .
  14. ↑ de Jong, M. F.; Bower, A. S.; Furey, H. H. (1 February 2014). "Two Years of Observations of Warm-Core Anticyclones in the Labrador Sea and Their Seasonal Cycle in Heat and Salt Stratification". Journal of Physical Oceanography. 44 (2): 427–444. Bibcode:2014JPO....44..427D. doi: 10.1175/JPO-D-13-070.1 . hdl: 10161/9577 .
  15. 1 2 Helen Jones. "Open-ocean Deep Convection". puddle.mit.edu. Retrieved 2022-03-24.
  16. ↑ Riser, Stephen C.; Freeland, Howard J.; Roemmich, Dean; Wijffels, Susan; Troisi, Ariel; Belbéoch, Mathieu; Gilbert, Denis; Xu, Jianping; Pouliquen, Sylvie; Thresher, Ann; Le Traon, Pierre-Yves (February 2016). "Fifteen years of ocean observations with the global Argo array". Nature Climate Change. 6 (2): 145–153. Bibcode:2016NatCC...6..145R. doi:10.1038/nclimate2872. hdl: 10508/10334 . ISSN   1758-6798.
  17. ↑ Jong, M. F. de; Bower, A. S.; Furey, H. H. (2016-06-01). "Seasonal and Interannual Variations of Irminger Ring Formation and Boundary–Interior Heat Exchange in FLAME". Journal of Physical Oceanography. 46 (6): 1717–1734. Bibcode:2016JPO....46.1717D. doi:10.1175/JPO-D-15-0124.1. ISSN   0022-3670.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.