The Lofoten

Vortex In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...

, also called Lofoten Basin Vortex or Lofoten Basin Eddy, is a permanent oceanic

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 ...

eddy Eddy may refer to: * Eddy (surname), surname used by descendants of a number of English, Irish and Scottish families * Eddy (given name), male given name * Eddy (fluid dynamics), the swirling of a fluid and the reverse current created when the fl ...

, located in the northern part of the

Norwegian Sea The Norwegian Sea (; ; ) is a marginal sea, grouped with either the Atlantic Ocean or the Arctic Ocean, northwest of Norway between the North Sea and the Greenland Sea, adjoining the Barents Sea to the northeast. In the southwest, it is separate ...

, off the coast of the Lofoten archipelago. It was documented for the first time in the 1970s. Due to the presence of the permanent vortex, the Lofoten basin features a localised area with high levels of

sea surface temperature Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...

and

kinetic energy. The local currents inside the vortex and the strong

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 ...

observed during winter generate a hot spot rich in nutrients, affecting the surrounding marine biology. Moreover, due to its extraordinary persistence and location, the Lofoten Vortex is likely to influence the dense water formation in the region.

Physical properties

The Lofoten Basin is a well-defined

topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary scienc ...

depression of about 3250 m depth, situated between the Norwegian continental slope in the east, the Vøring Plateau and the

Helgeland Helgeland is the most southerly Districts of Norway, district in Northern Norway. Generally speaking, Helgeland refers to the part of Nordland county that is located south of the Arctic Circle. It is bordered in the north by the Saltfjellet moun ...

Ridge in the south and southwest, and the Mohn Ridge in the northwest. The complex

bathymetric Bathymetry (; ) is the study of underwater depth of ocean floors ('' seabed topography''), river floors, or lake floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The first recorded evidence of water ...

structure is pivotal to locate the two major

ocean current 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, sh ...

s of the basin: * the Norwegian Atlantic Slope Current (NwASC), which flows south-north all along the continental shelf of Norway; * the Norwegian Atlantic Front Current (NwAFC), which streams almost parallel to the slope current, but down the west side of the Vøring Plateau and then along the Mohn Ridge. These currents play a key role in the mechanisms that guarantee the persistence of the Lofoten Vortex. The vortex has been localised in the deepest part of the Lofoten Basin. It is an apparent permanent anticyclonic eddy, whose persistence has been documented in the past years by shipborne,

Seaglider The Seaglider™ is a deep-diving Autonomous Underwater Vehicle (AUV) designed for missions lasting many months and covering thousands of miles. In military applications the Seaglider is more commonly referred to as an Unmanned Underwater Vehicle ...

s and

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

measurements. The estimated

radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...

of the vortex is 15–20 km and presents a 1200 m thick core of Atlantic Water (warm and saline) swirling at velocities that reach 0.8 m/s at 600–800 m depth. The velocity structure is similar to a

Rankine vortex The Rankine vortex is a simple mathematical model of a vortex in a viscous fluid. It is named after its discoverer, William John Macquorn Rankine. The vortices observed in nature are usually modelled with an irrotational (potential or free) vor ...

, characterised by a slow, outward decrease in

azimuth An azimuth (; from ) is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system. Mathematically, the relative position vector from an observer ( origin) to a point ...

al velocities.

RAFOS float RAFOS floatsThe RAFOS system
T. Rossby D. Dorson J. Fontaine ...

s trapped in the core of the Lofoten Vortex revealed that the vortex centre travelled 1850 km in 15 months, with an average drifting speed of 1 to 5 km/day, but with peaks reaching 15 km/day. From these measurements, a general downslope and

counter-clockwise Two-dimensional rotation can occur in two possible directions or senses of rotation. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands relative to the observer: from the top to the right, then down and then to ...

movement of the vortex around the deepest part of the Lofoten Basin is detected. From

salinity Salinity () is the saltiness or amount of salt (chemistry), 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 dimensio ...

and temperature profiles it is possible to see that the internal

hydrography Hydrography is the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers, as well as with the prediction of their change over time, for the primary ...

of the vortex is characterised by a doubly convex lens structure. The doming

isotherms A contour line (also isoline, isopleth, isoquant or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensi ...

, upward at around 200 m and downward at approximately 600 m depth, are visible in a vertical mean temperature profile. This structure reveals that the deep Lofoten Basin is a major

site in the

Nordic Seas The Nordic Seas are located north of Iceland and south of Svalbard. They have also been defined as the region located north of the Greenland-Scotland Ridge and south of the Fram Strait-Spitsbergen-Norway intersection. Known to connect the North P ...

, specifically in winter: inside the Lofoten Vortex, the Atlantic Water penetrates up to 800 m depth, much deeper than in any other location of the same sea. Strong seasonality characterises the

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

profile of the vortex: during summer a double core structure is detectable, with a shallow

pycnocline A pycnocline is the 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 effect, and tides caused ...

created by the stratification of surface water heated by the sun. On the other hand, in winter the surface's cooling creates strong convection that homogenises the density profile and deepens the pycnocline up to 1200 m depth.

Surface signature and tracking

The Lofoten Vortex has two surface features that are useful for its detection. First, on a

(SST) map, the vortex is recognisable as a negative SST anomaly. The cold-core surface signature, however, cannot be consistently detected by satellites' records, therefore it is not usually addressed as a reliable tracking method. Being an anticyclonic structure, the Lofoten Vortex can instead be analysed as a positive

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

anomaly (SLA). Satellites measuring SLA highlighted the persistent existence of the vortex in 83% of the available datasets, with a lifetime spanning from 90 days up to more than one year. The longest registered vortex lasted for two years, from May 2002 to April 2004. It is necessary to point out that in the period between two identified vortices, it is uncertain if the vortex disappeared or was simply not detected. That could be the case when the vortex adopts a submesoscale structure, with a non-detectable SLA.

Mechanisms sustaining the Lofoten Vortex

Two mechanism has been identified so far, both playing an important role in the formation and the sustainment of the Lofoten Vortex: * Merging of smaller anticyclones generated by the NwASC. * Wintertime convection.

Anticyclonic merging

Anticyclones' genesis region

The Lofoten vortex is situated in the deepest parts of the topographic depression of the Lofoten Basin. Due to the instabilities of the basin’s eastern boundary current (NwASC), cyclones and anticyclones are shed off from the east. As shown by experiments in a

rotating tank A rotating tank is a device used for fluid dynamics experiments. Typically cylinders filled with water on a rotating platform, the tanks can be used in various ways to simulate the atmosphere or ocean The ocean is the body of salt water t ...

cyclone 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 ant ...

s climb upslope in an anticyclonic spiral relative to the centre of a

seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...

, and

anticyclone 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 ...

s will descend towards the centre of a bottom depression in a cyclonic spiral. Likewise, the anticyclones released from the Norwegian Atlantic Slope Current (NwASC) spiral counterclockwise towards the deepest part of the Lofoten Basin. Some of the anticyclones terminate within the basin, whilst the longer-lasting trajectories (of 3–6 months) are traced back to the slope region related to the elevated

kinetic energy. The anticyclonic source region can be divided into two areas of generation, following different paths into the basin: * The southernmost track lies right off the 3200 m

isobath A bathymetric chart is a type of isarithmic map that depicts the submerged bathymetry and physiographic features of ocean and sea bottoms. Their primary purpose is to provide detailed depth contours of ocean topography as well as provide the si ...

and follows a direct route into the basin; * The other paths travel along curved trajectories and then move cyclonically along outer routes, roughly tracing the 3000 m isobath. As the anticyclones from the northernmost-lying sources have a longer travel time, they are subject to seasonal variability and are exposed to longer cooling periods, leaving them denser (than the vortices taking the direct route from the boundary current). When the boundary current is stronger, it is more unstable and will shed more eddies toward the west. In terms of seasonality, all the paths from the boundary current to the vortex show a maximum transport over the course of autumn and winter, with a maximum in January–February, and a minimum during spring and summer. Among the tracks, the one in the middle (between the southernmost and the northernmost source regions) has the maximum transport and yields a less pronounced seasonal variability. In the time-mean, a distinct maximum transport of approximately 2.5 Sv is found near 69.2° N.

Merging process

When the anticyclones have propagated southwestward from the east and into the trough of the Lofoten Basin, they establish a quasi-stationary vortex surrounded by cyclones and weaker anticyclones. These weaker anticyclonic vortices occasionally merge with the stronger Lofoten Vortex. The merging process is difficult to detect, but they are estimated to be 4-7 mergers a year with two seasonal peaks in late winter and in autumn, and a minimum in early winter. During a merging event, a vertical alignment between the light core anticyclones and the denser Lofoten Vortex occurs, creating a double-core. The lighter vortices may encounter the Lofoten Vortex at various depths, but on lighter isopycnals. Over the course of an alignment, the core is subject to a massive vertical compression of about 100 m or more. In accordance with conservation of

potential vorticity In fluid mechanics, potential vorticity (PV) is a quantity which is proportional to the dot product of vorticity and stratification. This quantity, following a parcel of air or water, can only be changed by diabatic or frictional processes. I ...

, there is a rapid and substantial increase in anticyclonic spin. The maximum increase in

vorticity In continuum mechanics, vorticity is a pseudovector (or axial vector) 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 an ...

is often found at 600–700 m depth, indicating that the lower core is most frequently squeezed. The negative vortices may also only partially merge. In this case, the core interacting with the Lofoten Vortex is situated on a shallower

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 ...

. Moving closer to the Lofoten Vortex, a vertical alignment is initiated, but not completed. The cores have started to compress, but no connection between them is established. After separating again, the vortices are left intensified.

Wintertime convection

Meteorological events above the centre of the Lofoten Basin play a vital role in the development of deep wintertime

that mixes water down to a mean wintertime mixing depth of 600 m. An inverse vertical distribution of

causes the sinking water to be warmer than the underlying one. This results in formation of a localised, vertically homogeneous, positive thermohaline anomaly in the intermediate and upper parts of the deep layer. The increase in the thermohaline anomaly in winter and spring is accompanied by the deepening and shrinking of the vortex to the

Rossby radius of deformation In atmospheric dynamics and physical oceanography, the Rossby radius of deformation is the length scale at which rotational effects become as important as buoyancy or gravity wave effects in the evolution of the flow about some disturbance. For ...

of about 10 km. Conversely, the Lofoten Vortex expands to 5-7 times the Rossby radius of deformation during summer. Warming causes the doubly convex internal structure to “separate” from the surface, decreasing the depth of the lower convection boundary and increasing the horizontal scale of the vortex

Effect on dense water formation

The Norwegian Atlantic Current (NwAC) is considered the northern branch of the Atlantic Meridional Overturning Circulation (AMOC). Its two limbs, the NwASC and the NwAFC, carry warm and saline Atlantic Water (AW) poleward to the

Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...

. Most of the dense water production in the Nordic Seas takes place on the east side of the Mohn Ridge system, in the Lofoten Basin. The AW, travelling northward, progressively loses heat to the atmosphere. Through the eddy activity of the Lofoten Basin and the persistence of the Lofoten Vortex, the residence time of the warm water is lengthened, leading to additional cooling of the AW before it reaches the Arctic Ocean. Due to heat loss, the temperature decreases and the light-to-dense water transformation takes place.