scholarly journals A stable Faroe Bank Channel overflow 1995–2015

Ocean Science ◽  
2016 ◽  
Vol 12 (6) ◽  
pp. 1205-1220 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Hjálmar Hátún ◽  
Svein Østerhus
Keyword(s):  
World Ocean ◽  
Deep Ocean ◽  
Acoustic Doppler Current Profiler ◽  
Narrow Channel ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Positive Trend ◽  
The North ◽  
Current Profiler

Abstract. The Faroe Bank Channel (FBC) is the deepest passage across the Greenland–Scotland Ridge (GSR) and there is a continuous deep flow of cold and dense water passing through it from the Arctic Mediterranean into the North Atlantic and further to the rest of the world ocean. This FBC overflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which has recently been suggested to have weakened. From November 1995 to May 2015, the FBC overflow has been monitored by a continuous ADCP (acoustic Doppler current profiler) mooring, which has been deployed in the middle of this narrow channel. Combined with regular hydrography cruises and several short-term mooring experiments, this allowed us to construct time series of volume transport and to follow changes in the hydrographic properties and density of the FBC overflow. The mean kinematic overflow, derived solely from the velocity field, was found to be (2.2 ± 0.2) Sv (1 Sv  =  106 m3 s−1) with a slight, but not statistically significant, positive trend. The coldest part, and probably the bulk, of the FBC overflow warmed by a bit more than 0.1 °C, especially after 2002, increasing the transport of heat into the deep ocean. This warming was, however, accompanied by increasing salinities, which seem to have compensated for the temperature-induced density decrease. Thus, the FBC overflow has remained stable in volume transport as well as density during the 2 decades from 1995 to 2015. After crossing the GSR, the overflow is modified by mixing and entrainment, but the associated change in volume (and heat) transport is still not well known. Whatever effect this has on the AMOC and the global energy balance, our observed stability of the FBC overflow is consistent with reported observations from the other main overflow branch, the Denmark Strait overflow, and the three Atlantic inflow branches to the Arctic Mediterranean that feed the overflows. If the AMOC has weakened during the last 2 decades, it is not likely to have been due to its northernmost extension – the exchanges across the Greenland–Scotland Ridge.

scholarly journals A stable Faroe Bank Channel overflow 1995–2015

2016 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Hjálmar Hátún ◽  
Svein Østerhus
Keyword(s):  
Acoustic Doppler Current Profiler ◽  
Narrow Channel ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Positive Trend ◽  
The North ◽  
Denmark Strait ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Faroe Bank Channel is the deepest passage across the Greenland-Scotland Ridge (GSR), and through it, there is a continuous deep flow of cold and dense water passing from the Arctic Mediterranean into the North Atlantic and further to the rest of the World oceans. This FBC-overflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which has recently been suggested to have weakened. From November 1995 to May 2015, the FBC-overflow has been monitored by a continuous ADCP (Acoustic Doppler Current Profiler) mooring, which has been deployed in the middle of this narrow channel. Combined with regular hydrography cruises and several short-term mooring experiments, this allows us to construct time series of volume transport and to follow changes in the hydrographic properties and density of the FBC-overflow. The mean kinematic overflow, derived from the velocity field solely, was found to be (2.2 ± 0.2) Sv (1 Sv = 106 m3 s−1) with a slight, but not statistically significant, positive trend. The coldest part, and probably the bulk, of the FBC-overflow warmed by a bit more than 0.1 °C, especially after 2002. This warming was, however, accompanied by increasing salinities, which seem to have compensated for the temperature-induced density decrease. Thus, the FBC-overflow has remained stable in volume transport as well as density during the two decades from 1995 to 2015. This is consistent with reported observations from the other main overflow branch, the Denmark Strait overflow, and the three Atlantic inflow branches to the Arctic Mediterranean that feed the overflows. If the AMOC has weakened during the last two decades, it is not likely to have been due to its northernmost extension – the exchanges across the Greenland-Scotland Ridge.

scholarly journals Overflow of cold water across the Iceland-Faroe Ridge through the Western Valley

2018 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Steffen Malskær Olsen ◽  
Detlef Quadfasel ◽  
Kerstin Jochumsen ◽  
...  
Keyword(s):  
Satellite Altimetry ◽  
Cold Water ◽  
Large Fraction ◽  
Acoustic Doppler Current Profiler ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Direct Measurements ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Iceland-Faroe Ridge (IFR) is considered to be the third-most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1 Sv (106 m3 s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an Acoustic Doppler Current Profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be less than 0.03 Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic Water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1 Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1 Sv.

scholarly journals Overflow of cold water across the Iceland–Faroe Ridge through the Western Valley

Ocean Science ◽  
2018 ◽  
Vol 14 (4) ◽  
pp. 871-885 ◽  
Author(s):  
Bogi Hansen ◽  
Karin Margretha Húsgarð Larsen ◽  
Steffen Malskær Olsen ◽  
Detlef Quadfasel ◽  
Kerstin Jochumsen ◽  
...  
Keyword(s):  
Satellite Altimetry ◽  
Cold Water ◽  
Large Fraction ◽  
Acoustic Doppler Current Profiler ◽  
Atlantic Water ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Direct Measurements ◽  
Current Profiler ◽  
Meridional Overturning

Abstract. The Iceland–Faroe Ridge (IFR) is considered to be the third most important passage for dense overflow water from the Nordic Seas feeding into the lower limb of the Atlantic Meridional Overturning Circulation with a volume transport on the order of 1 Sv (106 m3 s−1). The Western Valley, which is the northernmost deep passage across the IFR, has been presumed to supply a strong and persistent overflow (WV-overflow), contributing a large fraction of the total overflow across the IFR. However, prolonged measurements of this transport are so far missing. In order to quantify the flow by direct measurements, three instrumental packages were deployed close to the sill of the Western Valley for 278 days (2016–2017) including an acoustic Doppler current profiler at the expected location of the overflow core. The average volume transport of WV-overflow during this field experiment was found to be (0.02±0.05) Sv. Aided by the observations and a two-layer hydraulic model, we argue that the reason for this low value is the inflow of warm Atlantic water to the Norwegian Sea in the upper layers suppressing the deep overflow. The link between deep and surface flows explains an observed relationship between overflow and sea level slope as measured by satellite altimetry. This relationship, combined with historical hydrographic measurements, allows us to conclude that the volume transport of WV-overflow most likely has been less than 0.1 Sv on average since the beginning of regular satellite altimetry in 1993. Our new direct measurements do not allow us to present an updated estimate of the total overflow across the IFR, but they indicate that it may well be considerably less than 1 Sv.

scholarly journals Halocline Catastrophes, Sea Ice, and Ocean Climate

1990 ◽  
Vol 14 ◽  
pp. 328-328 ◽  
Author(s):  
Knut Aagaard
Keyword(s):  
Sea Ice ◽  
Fresh Water ◽  
North Atlantic ◽  
World Ocean ◽  
Deep Ocean ◽  
Present Situation ◽  
The Arctic ◽  
The North ◽  
The Arctic Ocean ◽  
The North Atlantic

Rapid melting of continental ice during deglaciations has been hypothesized to shift the thermohaline circulation of the world ocean to a mode radically different from the one dominated by the North Atlantic, such as operates today. This scenario has been referred to as the halocline catastrophe. We consider here the freezing, transport, and melting of sea ice in the North Atlantic sector as a possible modern analog to such events.The rejection of salt during the freezing and subsequent development of sea ice results, by early summer, in ice with only 5–10% of its original salt content. Since sea ice several meters thick is typically formed annually in the polar regions, the distillation rates from freezing are fully comparable to those from evaporation in such highly evaporative basins as the Red Sea. If the ice is subsequently exported from its production area, then freezing and melting are the oceanic equivalent of the hydrologic cycle. In the Arctic, the major ice outflow from the 107 km2 of the Polar Basin occurs east of Greenland, where the exodus represents a fresh-water transport of at least 2800 km3 a−1. This is a discharge more than twice that of North America’s four largest rivers combined.The fresh water can subsequently be traced around the subpolar gyre of the North Atlantic, on its way transferring small but significant amounts of buoyancy into the interior convective gyres, e.g. in the Greenland Sea. The convection which occurs in these gyres under present climatic conditions makes them major ventilation and water mass formation sites for the deep world ocean, but because the density of sea water at constant pressure and low temperature is almost solely dependent on salinity, the convection is extremely sensitive to changes in the freshwater supply to the gyres. Small variations in the supply will be transferred into the deep ocean by convection, where they will be manifested by a cooling and freshening, such as has recently been observed in the deep North Atlantic. However, if the surface layers are freshened too much, cooling even to the freezing point will be insufficient to initiate convection. Instead, the convective gyres will be capped by a fresh-water lid, essentially what has been proposed in the halocline catastrophe scenarios. During such events, sea ice will form in the gyres, sometimes with disastrous consequences, as occurred north of Iceland during the late 1960s when, during the extreme years of 1965 and 1968, the entire north and east coasts of Iceland were enveloped by sea ice; at the same time, renewal to the north of the deep ocean waters diminished or ceased.We suggest that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection, and that we have in fact during the past several decades seen a small-scale analog to the halocline catastrophe proposed for past deglaciations. A major difference is that the present situation does not require dramatic increases in fresh-water flux to effect a capping of the convection; nor does it depend on deglaciation. Rather, very modest changes in the disposition of the fresh water presently carried by the East Greenland Current can alter or stop the convection; and the principal source of fresh water is sea ice, rather than glacial ice. The essence of the present situation is that the large fresh-water output from the Arctic Ocean, which is the distillate of freezing, passes perilously close to the very weakly stratified convective gyres; and that the stratification in these gyres is easily perturbed, either by variations in the discharge from the Arctic Ocean or by leaks or recirculation from the boundary current.

scholarly journals Halocline Catastrophes, Sea Ice, and Ocean Climate

1990 ◽  
Vol 14 ◽  
pp. 328
Author(s):  
Knut Aagaard
Keyword(s):  
Sea Ice ◽  
Fresh Water ◽  
North Atlantic ◽  
World Ocean ◽  
Deep Ocean ◽  
Present Situation ◽  
The Arctic ◽  
The North ◽  
The Arctic Ocean ◽  
The North Atlantic

Rapid melting of continental ice during deglaciations has been hypothesized to shift the thermohaline circulation of the world ocean to a mode radically different from the one dominated by the North Atlantic, such as operates today. This scenario has been referred to as the halocline catastrophe. We consider here the freezing, transport, and melting of sea ice in the North Atlantic sector as a possible modern analog to such events. The rejection of salt during the freezing and subsequent development of sea ice results, by early summer, in ice with only 5–10% of its original salt content. Since sea ice several meters thick is typically formed annually in the polar regions, the distillation rates from freezing are fully comparable to those from evaporation in such highly evaporative basins as the Red Sea. If the ice is subsequently exported from its production area, then freezing and melting are the oceanic equivalent of the hydrologic cycle. In the Arctic, the major ice outflow from the 107 km2 of the Polar Basin occurs east of Greenland, where the exodus represents a fresh-water transport of at least 2800 km3 a−1. This is a discharge more than twice that of North America’s four largest rivers combined. The fresh water can subsequently be traced around the subpolar gyre of the North Atlantic, on its way transferring small but significant amounts of buoyancy into the interior convective gyres, e.g. in the Greenland Sea. The convection which occurs in these gyres under present climatic conditions makes them major ventilation and water mass formation sites for the deep world ocean, but because the density of sea water at constant pressure and low temperature is almost solely dependent on salinity, the convection is extremely sensitive to changes in the freshwater supply to the gyres. Small variations in the supply will be transferred into the deep ocean by convection, where they will be manifested by a cooling and freshening, such as has recently been observed in the deep North Atlantic. However, if the surface layers are freshened too much, cooling even to the freezing point will be insufficient to initiate convection. Instead, the convective gyres will be capped by a fresh-water lid, essentially what has been proposed in the halocline catastrophe scenarios. During such events, sea ice will form in the gyres, sometimes with disastrous consequences, as occurred north of Iceland during the late 1960s when, during the extreme years of 1965 and 1968, the entire north and east coasts of Iceland were enveloped by sea ice; at the same time, renewal to the north of the deep ocean waters diminished or ceased. We suggest that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection, and that we have in fact during the past several decades seen a small-scale analog to the halocline catastrophe proposed for past deglaciations. A major difference is that the present situation does not require dramatic increases in fresh-water flux to effect a capping of the convection; nor does it depend on deglaciation. Rather, very modest changes in the disposition of the fresh water presently carried by the East Greenland Current can alter or stop the convection; and the principal source of fresh water is sea ice, rather than glacial ice. The essence of the present situation is that the large fresh-water output from the Arctic Ocean, which is the distillate of freezing, passes perilously close to the very weakly stratified convective gyres; and that the stratification in these gyres is easily perturbed, either by variations in the discharge from the Arctic Ocean or by leaks or recirculation from the boundary current.

scholarly journals The thermohaline structure of the North Atlantic waters in different phases of the Atlantic multidecadal oscillation

2019 ◽  
Vol 55 (6) ◽  
pp. 157-170
Author(s):  
N. A. Diansky ◽  
V. A. Bagatinsky
Keyword(s):  
North Atlantic ◽  
Thermohaline Circulation ◽  
Potential Temperature ◽  
Greenland Ice Sheet ◽  
Deep Ocean ◽  
Atlantic Multidecadal Oscillation ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
The North ◽  
The North Atlantic

The meridional structure of climatic trends and anomalies of potential temperature and salinity in the North Atlantic waters in different periods of the Atlantic Multidecadal Oscillation (AMO) in 19482017 are studied based on the EN4 and WOA2013 objective analyses data. An analysis of these different data sets allowed us to reveal almost identical patterns of variability of the thermohaline fields of the North Atlantic, which increases the reliability of the results. Long-term temperature and salinity trends simulated over the period 19482017 show that warming and salinization of water occur in the upper ~1 km layer of the North Atlantic. On the contrary, cooling and freshening of deep waters are observed, which is associated with the melting of the Greenland ice sheet, transport of fresher waters from the Arctic Ocean, and deepening of these cold and fresher waters into the deeper layers. Composite analysis of the zonally averaged temperature and salinity anomalies of the North Atlantic waters after removing the trends showed that in the warm AMO periods warming and salinization of waters are observed in the upper 1-km layer of the North Atlantic when compared to the cold periods based both on the EN4 and WOA2013 data. Below the 1-km layer, significant regions of cooling and freshening are observed; this distribution is more pronounced in the EN4 data. Analysis of the dynamics of zonally averaged temperature and salinity anomalies in the successive periods associated with the temporal variability of the AMO index revealed that these anomalies propagate along the zonally averaged meridional thermohaline circulation. To show this using the Institute of Numerical Mathematics Ocean Model (INMOM), the stream function of the Atlantic Meridional Overturning Circulation (AMOC) was simulated. It is shown that positive and negative anomalies of both temperature and salinity circulate along the water motion in the AMOC around its core, descending down into the deep ocean layers approximately at 60 N and ascending to the surface at 25 N, replacing each other with a period of about 60 years. It can be assumed that due to this process both the warm and cold phases of the AMO are formed.

scholarly journals Change in the North Atlantic circulation associated with the mid-Pleistocene transition

2018 ◽  
Vol 14 (11) ◽  
pp. 1639-1651 ◽  
Author(s):  
Gloria M. Martin-Garcia ◽  
Francisco J. Sierro ◽  
José A. Flores ◽  
Fátima Abrantes
Keyword(s):  
North Atlantic ◽  
Major Change ◽  
Water Masses ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
North Atlantic Current ◽  
The North ◽  
Oceanic Fronts ◽  
Surface Circulation ◽  
The North Atlantic

Abstract. The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20–MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37∘34.285′ N, 10∘7.562′ W; 2585 m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55∘ N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south–north, and east–west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.

Danian Mollusks from the Prince Creek Formation, Northern Alaska, and Implications for Arctic Ocean Paleogeography

1993 ◽  
Vol 67 (S35) ◽  
pp. 1-35 ◽  
Author(s):  
Louie Marincovich
Keyword(s):  
Arctic Ocean ◽  
World Ocean ◽  
Ocean Basin ◽  
The Arctic ◽  
The North ◽  
The World ◽  
The Arctic Ocean ◽  
Molluscan Fauna ◽  
Endemic Genera ◽  
Northern Alaska

The marine molluscan fauna of the Prince Creek Formation near Ocean Point, northern Alaska, is of Danian age. It is the only diverse and abundant Danian molluscan fauna known from the Arctic Ocean realm, and is the first evidence for an indigenous Paleocene shallow-water biota within a discrete Arctic Ocean Basin faunal province.A high percentage of endemic species, and two endemic genera, emphasize the degree to which the Arctic Ocean was geographically isolated from the world ocean during the earliest Tertiary. Many of the well-preserved Ocean Point mollusks, however, also occur in Danian faunas of the North American Western Interior, the Canadian Arctic Islands, Svalbard, and northwestern Europe, and are the basis for relating this Arctic Ocean fauna to that of the Danian world ocean.The Arctic Ocean was a Danian refugium for some genera that became extinct elsewhere during the Jurassic and Cretaceous. At the same time, this nearly landlocked ocean fostered the evolution of new taxa that later in the Paleogene migrated into the world ocean by way of the northeastern Atlantic. The first Cenozoic occurrences are reported for the bivalves Integricardium (Integricardium), Oxytoma (Hypoxytoma), Placunopsis, Tancredia (Tancredia), and Tellinimera, and the oldest Cenozoic records given for the bivalves Gari (Garum), Neilo, and Yoldia (Cnesterium). Among the 25 species in the molluscan fauna are four new gastropod species, Amauropsis fetteri, Ellipsoscapha sohli, Mathilda (Fimbriatella) amundseni, and Polinices (Euspira) repenningi, two new bivalve genera, Arcticlam and Mytilon, and 15 new bivalve species, Arcticlam nanseni, Corbula (Caryocorbula) betsyae, Crenella kannoi, Cyrtodaria katieae, Gari (Garum) brouwersae, Integricardium (Integricardium) keenae, Mytilon theresae, Neilo gryci, Nucula (Nucula) micheleae, Nuculana (Jupiteria) moriyai, Oxytoma (Hypoxytoma) hargrovei, Placunopsis rothi, Tancredia (Tancredia) slavichi, Tellinimera kauffmani, and Yoldia (Cnesterium) gladenkovi.

scholarly journals Effect of gas-transfer-velocity parameterization choice on CO<sub>2</sub> air–sea fluxes in the North Atlantic and European Arctic

2015 ◽  
Vol 12 (6) ◽  
pp. 2591-2616
Author(s):  
I. Wróbel ◽  
J. Piskozub
Keyword(s):  
North Atlantic ◽  
World Ocean ◽  
Software Tool ◽  
The Arctic ◽  
Gas Transfer ◽  
Transfer Velocity ◽  
Gas Transfer Velocity ◽  
The North ◽  
European Arctic ◽  
The North Atlantic

Abstract. The ocean sink is an important part of the anthropogenic CO2 budget. Because the terrestrial biosphere is usually treated as a residual, understanding the uncertainties the net flux into the ocean sink is crucial for understanding the global carbon cycle. One of the sources of uncertainty is the parameterization of CO2 gas transfer velocity. We used a recently developed software tool, FluxEngine, to calculate monthly net carbon air–sea flux for the extratropical North Atlantic, European Arctic as well as global values (or comparison) using several available parameterizations of gas transfer velocity of different dependence of wind speed, both quadratic and cubic. The aim of the study is to constrain the uncertainty caused by the choice of parameterization in the North Atlantic, a large sink of CO2 and a region with good measurement coverage, characterized by strong winds. We show that this uncertainty is smaller in the North Atlantic and in the Arctic than globally, within 5 % in the North Atlantic and 4 % in the European Arctic, comparing to 9 % for the World Ocean when restricted to functions with quadratic wind dependence and respectively 42, 40 and 67 % for all studied parameterizations. We propose an explanation of this smaller uncertainty due to the combination of higher than global average wind speeds in the North Atlantic and lack of seasonal changes in the flux direction in most of the region. We also compare the available pCO2 climatologies (Takahashi and SOCAT) pCO2 discrepancy in annual flux values of 8 % in the North Atlantic and 19 % in the European Arctic. The seasonal flux changes in the Arctic have inverse seasonal change in both climatologies, caused most probably by insufficient data coverage, especially in winter.

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