Atlantic water flow through the Faroese Channels

Hansen, Bogi; Poulsen, Turið; Larsen, Karin Margretha Húsgarð; Hátún, Hjálmar; Østerhus, Svein; Darelius, Elin; Berx, Barbara; Quadfasel, Detlef; Jochumsen, Kerstin

doi:10.5194/os-13-873-2017

Cited by 10 publications

(9 citation statements)

References 31 publications

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“…They also recognized that a fraction of the Faroe Current turns south along the eastern slope of the Faroe Plateau. This flow, now called the South Faroe Current (Hansen et al, ), retroflects and joins the Slope Current although there is some evidence that a variable fraction continues around the southern tip of the Faroe Plateau (Rossby et al, ; Rossby & Flagg, ). However, what complicates the picture of the FSC is that the Slope Current can break away from the slope, perhaps due to eddies originating from north of the Faroes and advected into the FSC by the South Faroe Current, to form large nearly stationary meanders that can span the channel and thus pool large amounts of North Atlantic water in the FSC (Chafik, ).…”

Section: Discussionmentioning

confidence: 99%

A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

et al. 2018

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The flow of warm salty water toward the Nordic Seas is of fundamental importance to the climate of central and northern Europe. In an effort to gain an improved quantitative assessment of these fluxes a program was started in 2008 to measure upper-ocean currents from the high-seas ferry Norröna, which operates out of the Faroes to Iceland and Denmark. The current measurements were made with an acoustic Doppler current profiler mounted in the Norröna's hull. Starting in fall of 2013 monthly deployments of Expendable BathyThermographs give comprehensive information on the temperature field. These velocity and temperature data can be combined to estimate mean volume and temperature fluxes (referenced to 0°C) for the two sections. Archived hydrographic data give us the corresponding salt transport. Thanks to an array of 12 tall moorings across the Blosseville Basin that measured currents, temperature, and salinity net transport of volume, temperature, and salt between Iceland and Greenland can also be estimated. By combining the velocity data from these three sections the strength of the Nordic Seas branch of the meridional overturning circulation is estimated to be 7.7 ± 0.8 Sv where 1 Sv = 10 6 m 3 /s. Imposing the constraint of zero net volume and salt flux, the corresponding heat and freshwater fluxes are estimated to be 264 ± 27 TW (1 TW = 10 12 W) and À0.104 ± 0.01 Sv, respectively. The uncertainties in heat and freshwater fluxes are largely governed by volume fluxes. The Norröna program is ongoing.Plain Language Summary The climate of central and northern Europe depends upon the flow of warm water into the northern North Atlantic and Nordic Seas. This study uses directly measured currents from the high-seas ferry Norröna in regular traffic between Denmark, the Faroes and Iceland to determine in great detail the flow of warm salty water into the Nordic Seas. An extensive array of current meters deployed between Iceland and Greenland covers all exchange with the Nordic Seas west of Iceland. During its transit through the Nordic Seas the warm and salty North Atlantic water loses its heat to the atmosphere, sinks, and eventually spills back across the sills between Greenland, Iceland, the Faroes, and Scotland into the deep North Atlantic. As these waters sink, they entrain cooled water from the Labrador and Irminger Seas. Without this overflow of dense water from the Nordic Seas the northern North Atlantic would become much cooler because of the cessation of warm water flow north to maintain the overturning process. Thus quantifying the flow of warm water into the Nordic Seas and tracking how it might change over time is one of the overarching questions regarding the climate of North Atlantic and surrounding landmasses.

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Section: Discussionmentioning

confidence: 99%

A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

et al. 2018

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“…This is partly from theoretical arguments and partly from observations of a strong and persistent bottom current downstream from the Western Valley that seems to have been generated by IF-overflow (Perkins et al, 1998;Olsen et al, 2016). Measurements within the Western Valley have not, however, shown any clear evidence of strong overflow (Perkins et al, 1998;Beaird et al, 2013); and based on a dedicated field experiment from August 2016 to May 2017, Hansen et al (2018) argue that the long-term average overflow transport through the Western Valley is less than 0.1 Sv.…”

Section: Iceland-faroe Ridge Overflow ("If-overflow")mentioning

confidence: 96%

Arctic Mediterranean exchanges: a consistent volume budget and trends in transports from two decades of observations

et al. 2019

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Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into this region through the Bering Strait (Pacific inflow) and through the passages across the Greenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modified waters leave the AM in several flow branches which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat and salt into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently the observational coverage has become sufficiently complete to allow an integrated assessment of the AM exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993 and 2015. The total AM import (oceanic inflows plus freshwater) is found to be 9.1 Sv (sverdrup, 1 Sv =106 m3 s−1) with an estimated uncertainty of 0.7 Sv and has the amplitude of the seasonal variation close to 1 Sv and maximum import in October. Roughly one-third of the imported water leaves the AM as surface outflow with the remaining two-thirds leaving as overflow. The overflow water is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but is still approximately two-thirds of modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s, and this enables us to estimate trends for the AM exchanges as a whole. At the 95 % confidence level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the meridional overturning circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflow through the Denmark Strait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

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“…The deepest part of the channel is deeper than 1000 m. Water of Atlantic origin usually fills the upper layers down to 400 -500 m across the whole channel, but a significant fraction originally crossed the ridge north of the Faroes, entered the Faroe Current, and bifurcated into the FSC, where it flows southwestwards along the Faroe side of the channel, Figure 3 (Helland- Hansen and Nansen, 1909;Meincke, 1978;Hátún, 2004, Berx et al, 2013. Most of this water is believed to recirculate within the channel and join the direct inflow continuing into the Norwegian Sea (Hansen et al, 2017).…”

Section: Faroe-shetland Atlantic Inflow (Fs-inflow)mentioning

confidence: 99%

Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

Østerhus¹,

Woodgate²,

Valdimarsson³

et al. 2018

Preprint

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Abstract. The Arctic Mediterranean (AM) is the collective name for the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Into this region, water enters through the Bering Strait (Pacific inflow) and through the passages across the Greenland-Scotland Ridge (Atlantic inflow) and then modified within the AM. The modified waters leave the AM in several flow branches, which are grouped into two different categories: (1) overflow of dense water through the deep passages across the Greenland-Scotland Ridge, and (2) outflow of light water – here termed surface outflow – on both sides of Greenland. These exchanges transport heat, salt, and other substances into and out of the AM and are important for conditions in the AM. They are also part of the global ocean circulation and climate system. Attempts to quantify the transports by various methods have been made for many years, but only recently, has the observational coverage become sufficiently complete to allow an integrated assessment of the AM-exchanges based solely on observations. In this study, we focus on the transport of water and have collected data on volume transport for as many AM-exchange branches as possible between 1993–2015. The total AM-import (oceanic inflows plus freshwater) is found to be 9.1 ± 0.7 Sv (1 Sv = 106 m3 s−1) and has a seasonal variation of amplitude close to 1 Sv and maximum import in October. Roughly one third of the imported water leaves the AM as surface outflow with the remaining two thirds leaving as overflow. The overflow is mainly produced from modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted into overflow, indicating a strong coupling between these two exchanges. The surface outflow is fed from the Pacific inflow and freshwater, but is still ~ 2/3rds from modified Atlantic water. For the inflow branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel), systematic monitoring of volume transport has been established since the mid-1990s and this allows us to estimate trends for the AM-exchanges as a whole. At the 95 % level, only the inflow of Pacific water through the Bering Strait showed a statistically significant trend, which was positive. Both the total AM-inflow and the combined transport of the two main overflow branches also showed trends consistent with strengthening, but they were not statistically significant. They do suggest, however, that any significant weakening of these flows during the last two decades is unlikely and the overall message is that the AM-exchanges remained remarkably stable in the period from the mid-1990s to the mid-2010s. The overflows are the densest source water for the deep limb of the North Atlantic part of the Meridional Overturning Circulation (AMOC), and this conclusion argues that the reported weakening of the AMOC was not due to overflow weakening or reduced overturning in the AM. Although the combined data set has made it possible to establish a consistent budget for the AM-exchanges, the observational coverage for some of the branches is limited, which introduces considerable uncertainty. This lack of coverage is especially extreme for the surface outflows through the Denmark Strait, the overflow across the Iceland-Faroe Ridge, and the inflow over the Scottish shelf. We recommend that more effort is put into observing these flows as well as maintaining the monitoring systems established for the other exchange branches.

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Atlantic water flow through the Faroese Channels

Cited by 10 publications

References 31 publications

A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

Arctic Mediterranean exchanges: a consistent volume budget and trends in transports from two decades of observations

Arctic Mediterranean Exchanges: A consistent volume budget and trends in transports from two decades of observations

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