Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas

Richards, Clark; Straneo, Fiammetta

doi:10.1175/jpo-d-14-0238.1

Cited by 44 publications

(56 citation statements)

References 53 publications

(95 reference statements)

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“…A destabilizing buoyancy flux ( B 0 < 0) leads to convection whereas a stabilizing buoyancy flux strengthens the surface stratification. The surface buoyancy flux B 0 which depends on the heat and freshwater fluxes can be written as [ Richards and Straneo , ]

B_{0} = \frac{g α}{ρ_{0} C_{p}} (|, Q_{s} + Q_{l} + Q_{R}) + \frac{g β normalS normalS normalS}{ρ_{0}} (|, P - E),

where

g = 9.8

m s −2 is the acceleration due to gravity,

α

and

β

are the thermal expansion and haline contraction coefficients for seawater, ρ 0 = 1027 kg m −3 is the reference density,

C_{p}

is the specific heat for seawater, and SSS is the sea surface salinity (measured in practical salinity scale). The terms Q s , Q l , and Q R are the sensible, latent, and net radiative heat fluxes ( Q R is the sum of the surface net longwave radiation Q lw and the surface net shortwave radiation Q sw ), and P and E are the precipitation and evaporation, respectively.…”

Section: Methodssupporting

confidence: 93%

“…The 2012–2013 and 2014–2015 winters, however, lose more buoyancy than in 2013–2014, with the cumulative buoyancy loss of 0.64 m 2 s −2 (in 2012 and 2014), compared to 0.57 m 2 s −2 in 2013. For comparison, Richards and Straneo [] reported December–January–February average buoyancy flux of −6.0 (±0.7) × 10 −8 m 2 s −3 in LB, and −3.8 (±0.5) × 10 −8 m 2 s −3 in the entire Nordic Seas, using ERA Interim data between 1979 and 2012. In this location, precipitation typically exceeds evaporation.…”

Section: Environmental Conditions During Missionsmentioning

confidence: 98%

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The Lofoten Basin eddy: Three years of evolution as observed by Seagliders

Bosse

Fer

et al. 2017

JGR Oceans

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The Lofoten Basin in the Norwegian Sea is an area where the warm Atlantic Water is subject to the greatest heat losses anywhere in the Nordic Seas. A long‐lived, deep, anticyclonic eddy is located in the central part of the basin (the Lofoten Basin Eddy, LBE). Here we use observations from Seagliders, collected between July 2012 and July 2015, to describe LBE in unprecedented detail. The missions were designed to sample LBE repeatedly, allowing for multiple realizations of radial sections across the eddy. LBE has a mean radius of 18 ± 4 km and propagates cyclonically with a mean speed of approximately 3–4 cm s−1. The anticyclonic azimuthal peak velocity varies between 0.5 and 0.7 m s−1, located between 700 and 900 m depth. The average contribution of geostrophy in the cyclogeostrophic balance is 44%. The relative vorticity of the core is close to the local Coriolis parameter. The evolution of core water properties shows substantial interannual variability, influenced by surface buoyancy flux and advection of anomalous low‐salinity near‐surface waters that may affect the vertical extent of winter convection. A comparison of the eddy properties to those inferred from automated tracking of satellite altimeter observations shows that the location of eddy center is successfully detected to within one half eddy radius, but vorticity is underestimated and the radius overestimated, each approximately by a factor of 2, because of excessive smoothing relative to the small eddy radius.

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B_{0} = \frac{g α}{ρ_{0} C_{p}} (|, Q_{s} + Q_{l} + Q_{R}) + \frac{g β normalS normalS normalS}{ρ_{0}} (|, P - E),

where

g = 9.8

m s −2 is the acceleration due to gravity,

α

and

β

are the thermal expansion and haline contraction coefficients for seawater, ρ 0 = 1027 kg m −3 is the reference density,

C_{p}

Section: Methodssupporting

confidence: 93%

Section: Environmental Conditions During Missionsmentioning

confidence: 98%

The Lofoten Basin eddy: Three years of evolution as observed by Seagliders

Bosse

Fer

et al. 2017

JGR Oceans

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show abstract

“…The deepest penetration of Atlantic Water (1200 m depth) is found within the ACEs located in the Lofoten Vortex region. The cooler surface water associated with some ACE profiles in the western Lofoten Basin (Figure b) compared to that in CEs can be linked to the enhanced heat loss in the ACEs [ Richards and Straneo , ].…”

Section: Discussionmentioning

confidence: 99%

Quantifying mesoscale eddies in the Lofoten Basin

Raj¹,

Johannessen

Eldevik

et al. 2016

JGR Oceans

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The Lofoten Basin is the most eddy rich region in the Norwegian Sea. In this paper, the characteristics of these eddies are investigated from a comprehensive database of nearly two decades of satellite altimeter data (1995–2013) together with Argo profiling floats and surface drifter data. An automated method identified 1695/1666 individual anticyclonic/cyclonic eddies in the Lofoten Basin from more than 10,000 altimeter‐based eddy observations. The eddies are found to be predominantly generated and residing locally. The spatial distributions of lifetime, occurrence, generation sites, size, intensity, and drift of the eddies are studied in detail. The anticyclonic eddies in the Lofoten Basin are the most long‐lived eddies (>60 days), especially in the western part of the basin. We reveal two hotspots of eddy occurrence on either side of the Lofoten Basin. Furthermore, we infer a cyclonic drift of eddies in the western Lofoten Basin. Barotropic energy conversion rates reveals energy transfer from the slope current to the eddies during winter. An automated colocation of surface drifters trapped inside the altimeter‐based eddies are used to corroborate the orbital speed of the anticyclonic and cyclonic eddies. Moreover, the vertical structure of the altimeter‐based eddies is examined using colocated Argo profiling float profiles. Combination of altimetry, Argo floats, and surface drifter data is therefore considered to be a promising observation‐based approach for further studies of the role of eddies in transport of heat and biomass from the slope current to the Lofoten Basin.

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“…The bimonthly supply of ∼10 EJ (EJ is exa Joule or 10 18 J) each tells us that over a 1 year period about 60 EJ is brought into the region of the LBE. The circular area required to lose this heat to the atmosphere assuming an annual average of 80

{normalW / normalm}^{2}

[ Richards and Straneo , ] has a radius of 87 km; close to 2 times the size of the LBE. However, given that the LBE has a deep mixed layer and can maintain a high heat loss throughout the winter the effective heat loss rate may be greater.…”

Section: Discussionmentioning

confidence: 99%

On the long‐term stability of the Lofoten Basin Eddy

2016

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In recent years, several studies have identified an area of intense anticyclonic activity about 500 km straight west of the Lofoten Islands at 70°N in the northern Norwegian Sea. Now recognized as the coherent Lofoten Basin Eddy (LBE), it is maintained by a supply of anticyclonic eddies that break away from the Norwegian Atlantic Current. Here we show from ship‐based surveys of its velocity field that it is quite stable with a central core in solid body rotation ∼1000 m deep, ∼8 km radius, and a relative vorticity close to its theoretical limit –f. The surveys also show the LBE typically has a >60 km radius with maximum swirl velocities at 17–20 km radius. From the velocity field, we estimate the dynamic height amplitude at the surface to be about ∼0.21 ± 0.03 dyn. m. Second, altimetry from the last 20 years shows the extremum in sea surface height relative to the surrounding waters to be about the same, 0.2 dyn. m. Third, a float trapped in the LBE for many months reveals a clear cyclonic wandering of the eddy over the deepest parts of the basin. Last, three hydrographic sections from the 1960s show the dynamic height signal to be virtually the same then as it is now. From these observations, we conclude that the LBE is a permanent feature of the Nordic Seas and plays a central role in maintaining the pool of warm water in the western Lofoten Basin.

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Observations of Water Mass Transformation and Eddies in the Lofoten Basin of the Nordic Seas

Cited by 44 publications

References 53 publications

The Lofoten Basin eddy: Three years of evolution as observed by Seagliders

The Lofoten Basin eddy: Three years of evolution as observed by Seagliders

Quantifying mesoscale eddies in the Lofoten Basin

On the long‐term stability of the Lofoten Basin Eddy

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