scholarly journals Variability of the Overflow Water Transport in the Western Subpolar North Atlantic, 1950–97

2006 ◽  
Vol 36 (3) ◽  
pp. 435-456 ◽  
Author(s):  
Dagmar Kieke ◽  
Monika Rhein
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Deep Water ◽  
Reference Level ◽  
Labrador Sea ◽  
The North ◽  
Irminger Sea ◽  
The North Atlantic ◽  
Baroclinic Transport ◽  
The Impact

Abstract One of the major topics in current field research is the question of whether or to what extent the North Atlantic Ocean is subject to changes in water mass transports, and how they are related to atmospheric phenomena like the North Atlantic Oscillation (NAO). Bottle and CTD data from the 1950s to 1990s are presented to reconstruct spatially and temporally the baroclinic contribution to the deep water transports in the western subpolar North Atlantic. The focus is on the two densest components of North Atlantic Deep Water: the Gibbs Fracture Zone Water (GFZW) and the Denmark Strait Overflow Water (DSOW). Direct velocity measurements in the considered time period are sparse. For this reason it was decided to calculate the geostrophic velocity relative to 1400 dbar. This level is located in the weakly stratified Labrador Sea Water. The combined baroclinic volume transport of GFZW and DSOW during the early 1990s was about 5 Sv (Sv ≡ 106 m3 s−1) in the Irminger Sea and 7–8 Sv in the Labrador Sea. Near the Flemish Cap, baroclinic transports reached 16–29 Sv. Because of the impact of the North Atlantic Current on the flow field resulting in steeply sloping isopycnals, the latter estimate is strongly dependent on the choice of the reference level, in contrast to other locations. Time series were obtained from data in the Irminger and Labrador Seas. In the Irminger Sea, the combined baroclinic transport of GFZW and DSOW increased from 4–5 Sv in the mid-1950s to 8–9 Sv in the 1980s, followed by a decrease to 5 Sv in the 1990s. In the Labrador Sea, the temporal variability was stronger (3–11 Sv), with interannual changes of 5–6 Sv. The importance of baroclinic transport variability is not easy to interpret. Results presented herein indicate that relations of the Irminger and Labrador Seas time series to the NAO remain ambiguous. Among other impacts the presence of eddies significantly affects the time series of baroclinic transport. Whether baroclinic variability represents the total variability of the flow (baroclinic and barotropic part) cannot be assessed without knowledge of the variability of the velocity field in the reference level.

Late Quaternary palaeo-oceanography of the Denmark Strait overflow pathway, South-East Greenland margin

1998 ◽  
Vol 180 ◽  
pp. 163-167
Author(s):  
Antoon Kuijpers ◽  
Jørn Bo Jensen ◽  
Simon R . Troelstra ◽  
And shipboard scientific party of RV Professor Logachev and RV Dana
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Deep Convection ◽  
Conveyor Belt ◽  
Water Masses ◽  
Labrador Sea ◽  
Greenland Sea ◽  
The North ◽  
Denmark Strait ◽  
The North Atlantic

Direct interaction between the atmosphere and the deep ocean basins takes place today only in the Southern Ocean near the Antarctic continent and in the northern extremity of the North Atlantic Ocean, notably in the Norwegian–Greenland Sea and Labrador Sea. Cooling and evaporation cause surface waters in the latter region to become dense and sink. At depth, further mixing occurs with Arctic water masses from adjacent polar shelves. Export of these water masses from the Norwegian–Greenland Sea (Norwegian Sea Overflow Water) to the North Atlantic basin occurs via two major gateways, the Denmark Strait system and the Faeroe– Shetland Channel and Faeroe Bank Channel system (e.g. Dickson et al. 1990; Fig.1). Deep convection in the Labrador Sea produces intermediate waters (Labrador Sea Water), which spreads across the North Atlantic. Deep waters thus formed in the North Atlantic (North Atlantic Deep Water) constitute an essential component of a global ‘conveyor’ belt extending from the North Atlantic via the Southern and Indian Oceans to the Pacific. Water masses return as a (warm) surface water flow. In the North Atlantic this is the Gulf Stream and the relatively warm and saline North Atlantic Current. Numerous palaeo-oceanographic studies have indicated that climatic changes in the North Atlantic region are closely related to changes in surface circulation and in the production of North Atlantic Deep Water. Abrupt shut-down of the ocean-overturning and subsequently of the conveyor belt is believed to represent a potential explanation for rapid climate deterioration at high latitudes, such as those that caused the Quaternary ice ages. Here it should be noted, that significant changes in deep convection in Greenland waters have also recently occurred. While in the Greenland Sea deep water formation over the last decade has drastically decreased, a strong increase of deep convection has simultaneously been observed in the Labrador Sea (Sy et al. 1997).

scholarly journals Dissolved iron in the North Atlantic Ocean and Labrador Sea along the GEOVIDE section (GEOTRACES section GA01)

2018 ◽  
Author(s):  
Manon Tonnard ◽  
Hélène Planquette ◽  
Andrew R. Bowie ◽  
Pier van der Merwe ◽  
Morgane Gallinari ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Sea Water ◽  
North Atlantic Ocean ◽  
Deep Convection ◽  
Labrador Sea ◽  
Intermediate Water ◽  
The North ◽  
Irminger Sea ◽  
The North Atlantic

Abstract. Dissolved Fe (DFe) samples from the GEOVIDE voyage (GEOTRACES GA01, May–June 2014) in the North Atlantic Ocean were analysed using a SeaFAST-picoTM coupled to an Element XR HR-ICP-MS and provided interesting insights on the Fe sources in this area. Overall, DFe concentrations ranged from 0.09 ± 0.01 nmol L−1 to 7.8 ± 0.5 nmol L−1. Elevated DFe concentrations were observed above the Iberian, Greenland and Newfoundland Margins likely due to riverine inputs from the Tagus River, meteoric water inputs and sedimentary inputs. Air-sea interactions were suspected to be responsible for the increase in DFe concentrations within subsurface waters of the Irminger Sea due to deep convection occurring the previous winter, that provided iron-to-nitrate ratios sufficient to sustain phytoplankton growth. Increasing DFe concentrations along the flow path of the Labrador Sea Water were attributed to sedimentary inputs from the Newfoundland Margin. Bottom waters from the Irminger Sea displayed high DFe concentrations likely due to the dissolution of Fe-rich particles from the Denmark Strait Overflow Water and the Polar Intermediate Water. Finally, the nepheloid layers were found to act as either a source or a sink of DFe depending on the nature of particles.

Identification and quantification of the North Atlantic Deep Water pathways

2021 ◽  
Author(s):  
Philippe Miron ◽  
Maria J. Olascoaga ◽  
Francisco J. Beron-Vera ◽  
Kimberly L. Drouin ◽  
M. Susan Lozier
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Sea Water ◽  
Lower Layer ◽  
North Atlantic Deep Water ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Western Boundary ◽  
The North ◽  
The North Atlantic

<p>The North Atlantic Deep Water (NADW) flows equatorward along the Deep Western Boundary Current (DWBC) as well as interior pathways and is a critical part of the Atlantic Meridional Overturning Circulation. Its upper layer, the Labrador Sea Water (LSW), is formed by open-ocean deep convection in the Labrador and Irminger Seas while its lower layers, the Iceland–Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW), are formed north of the Greenland–Iceland–Scotland Ridge.</p><p>In recent years, more than two hundred acoustically-tracked subsurface floats have been deployed in the deep waters of the North Atlantic.  Studies to date have highlighted water mass pathways from launch locations, but due to limited float trajectory lengths, these studies have been unable to identify pathways connecting  remote regions.</p><p>This work presents a framework to explore deep water pathways from their respective sources in the North Atlantic using Markov Chain (MC) modeling and Transition Path Theory (TPT). Using observational trajectories released as part of OSNAP and the Argo projects, we constructed two MCs that approximate the lower and upper layers of the NADW Lagrangian dynamics. The reactive NADW pathways—directly connecting NADW sources with a target at 53°N—are obtained from these MCs using TPT.</p><p>Preliminary results show that twenty percent more pathways of the upper layer(LSW) reach the ocean interior compared to  the lower layer (ISOW, DSOW), which mostly flows along the DWBC in the subpolar North Atlantic. Also identified are the Labrador Sea recirculation pathways to the Irminger Sea and the direct connections from the Reykjanes Ridge to the eastern flank of the Mid–Atlantic Ridge, both previously observed. Furthermore, we quantified the eastern spread of the LSW to the area surrounding the Charlie–Gibbs Fracture Zone and compared it with previous analysis. Finally, the residence time of the upper and lower layers are assessed and compared to previous observations.</p>

scholarly journals The silicon stable isotope distribution along the GEOVIDE section (GEOTRACES GA-01) of the North Atlantic Ocean

2018 ◽  
Vol 15 (18) ◽  
pp. 5663-5676 ◽  
Author(s):  
Jill N. Sutton ◽  
Gregory F. de Souza ◽  
Maribel I. García-Ibáñez ◽  
Christina L. De La Rocha
Keyword(s):  
Stable Isotope ◽  
Atlantic Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
North Atlantic Ocean ◽  
Water Masses ◽  
Labrador Sea ◽  
The North ◽  
The North Atlantic ◽  
Eastern North Atlantic

Abstract. The stable isotope composition of dissolved silicon in seawater (δ30SiDSi) was examined at 10 stations along the GEOVIDE section (GEOTRACES GA-01), spanning the North Atlantic Ocean (40–60∘ N) and Labrador Sea. Variations in δ30SiDSi below 500 m were closely tied to the distribution of water masses. Higher δ30SiDSi values are associated with intermediate and deep water masses of northern Atlantic or Arctic Ocean origin, whilst lower δ30SiDSi values are associated with DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly, the lowest δ30SiDSi values were observed in the deep and abyssal eastern North Atlantic, where dense southern-sourced waters dominate. The extent to which the spreading of water masses influences the δ30SiDSi distribution is marked clearly by Labrador Sea Water (LSW), whose high δ30SiDSi signature is visible not only within its region of formation within the Labrador and Irminger seas, but also throughout the mid-depth western and eastern North Atlantic Ocean. Both δ30SiDSi and hydrographic parameters document the circulation of LSW into the eastern North Atlantic, where it overlies southern-sourced Lower Deep Water. The GEOVIDE δ30SiDSi distribution thus provides a clear view of the direct interaction between subpolar/polar water masses of northern and southern origin, and allow examination of the extent to which these far-field signals influence the local δ30SiDSi distribution.

scholarly journals Interannual Variability in the Pathways of the North Atlantic Current over the Mid-Atlantic Ridge and the Impact of Topography

2008 ◽  
Vol 38 (1) ◽  
pp. 104-120 ◽  
Author(s):  
Amy S. Bower ◽  
Wilken-Jon von Appen
Keyword(s):  
Time Series ◽  
North Atlantic ◽  
Fracture Zone ◽  
Subpolar Gyre ◽  
North Atlantic Current ◽  
The North ◽  
Mid Atlantic Ridge ◽  
The North Atlantic ◽  
The Impact ◽  
Atlantic Current

Abstract Recent studies have indicated that the North Atlantic Ocean subpolar gyre circulation undergoes significant interannual-to-decadal changes in response to variability in atmospheric forcing. There are also observations, however, suggesting that the southern limb of the subpolar gyre, namely, the eastward-flowing North Atlantic Current (NAC), may be quasi-locked to particular latitudes in the central North Atlantic by fracture zones (gaps) in the Mid-Atlantic Ridge. This could constrain the current’s ability to respond to variability in forcing. In the present study, subsurface float trajectories at 100–1000 m collected during 1997–99 and satellite-derived surface geostrophic velocities from 1992 to 2006 are used to provide an improved description of the detailed pathways of the NAC over the ridge and their relationship to bathymetry. Both the float and satellite observations indicate that in 1997–99, the northern branch of the NAC was split into two branches as it crossed the ridge, one quasi-locked to the Charlie–Gibbs Fracture Zone (CGFZ; 52°–53°N) and the other to the Faraday Fracture Zone (50°–51°N). The longer satellite time series shows, however, that this pattern did not persist outside the float sampling period and that other branching modes persisted for one or more years, including an approximately 12-month time period in 2002–03 when the strongest eastward flow over the ridge was at ∼49°N. Schott et al. showed how northward excursions of the NAC can temporarily block the westward flow of the Iceland–Scotland Overflow Water through the CGFZ. From the 13-yr time series of surface geostrophic velocity, it is estimated that such blocking may occur on average 6% of the time, although estimates for any given 12-month period range from 0% to 35%.

First Records of the Gymnoblastic Hydroid, Ichthyocodium sarcotretis, on the Copepod, Sphyrion lumpi, from Redfish of the Northwest Atlantic

1973 ◽  
Vol 30 (11) ◽  
pp. 1655-1660 ◽  
Author(s):  
Wilfred Templeman
Keyword(s):  
Continental Shelf ◽  
North Atlantic ◽  
Labrador Sea ◽  
Northwest Atlantic ◽  
The North ◽  
Irminger Sea ◽  
The North Atlantic ◽  
Sebastes Mentella ◽  
Colonial Hydroid

The gymnoblastic colonial hydroid Ichthyocodium sarcotretis was found on the copepod Sphyrion lumpi on redfish from three areas of the northwest Atlantic: on copepods (three of ten) on deepwater redfish (Sebastes mentella) from about 200–500 m over great depths at the mouth of the Labrador Sea and, in bottom otter trawling, on copepods on S. mentella or possibly S. fasciatus from the Labrador Shelf (6 of 686) and the northeast Newfoundland Shelf (1 of 364). None were found on 492 S. lumpi from redfish taken on the continental shelf south and west of the northeast Newfoundland Shelf.The characteristics of the hydroid colonies and of their feeding polyps and reproductive hydranths and medusae are compared with published information on this hydroid found on the same hosts in the Irminger Sea and on the copepod, Sarcotretes scopeli, on the lantern fish, Benthosema glaciale, in the North Atlantic. The incidence of the hydroid on S. lumpi on redfish may possibly help in distinguishing Sebastes species in the northwest Atlantic.

scholarly journals Freshening of the Labrador Sea as a trigger for Little Ice Age development

2017 ◽  
Vol 13 (4) ◽  
pp. 317-331 ◽  
Author(s):  
Montserrat Alonso-Garcia ◽  
Helga (Kikki) F. Kleiven ◽  
Jerry F. McManus ◽  
Paola Moffa-Sanchez ◽  
Wallace S. Broecker ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Deep Water ◽  
Little Ice Age ◽  
Ice Age ◽  
Labrador Sea ◽  
Deep Water Formation ◽  
The North ◽  
Water Formation ◽  
The North Atlantic

Abstract. Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea ice can have a large impact on ocean circulation dynamics in the North Atlantic, modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29° N, 48.37° W; 3432 m water depth). IRD from the fraction 63–150 µm shows particularly high concentrations during the intervals  ∼  AD 1000–1100,  ∼  1150–1250,  ∼  1400–1450,  ∼  1650–1700 and  ∼  1750–1800. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling that occurred during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes. This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.

scholarly journals Dissolved iron in the North Atlantic Ocean and Labrador Sea along the GEOVIDE section (GEOTRACES section GA01)

2020 ◽  
Vol 17 (4) ◽  
pp. 917-943 ◽  
Author(s):  
Manon Tonnard ◽  
Hélène Planquette ◽  
Andrew R. Bowie ◽  
Pier van der Merwe ◽  
Morgane Gallinari ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Inductively Coupled Plasma ◽  
Sea Water ◽  
North Atlantic Ocean ◽  
Labrador Sea ◽  
Intermediate Water ◽  
The North ◽  
Irminger Sea ◽  
The North Atlantic

Abstract. Dissolved Fe (DFe) samples from the GEOVIDE voyage (GEOTRACES GA01, May–June 2014) in the North Atlantic Ocean were analyzed using a seaFAST-pico™ coupled to an Element XR sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) and provided interesting insights into the Fe sources in this area. Overall, DFe concentrations ranged from 0.09±0.01 to 7.8±0.5 nmol L−1. Elevated DFe concentrations were observed above the Iberian, Greenland, and Newfoundland margins likely due to riverine inputs from the Tagus River, meteoric water inputs, and sedimentary inputs. Deep winter convection occurring the previous winter provided iron-to-nitrate ratios sufficient to sustain phytoplankton growth and lead to relatively elevated DFe concentrations within subsurface waters of the Irminger Sea. Increasing DFe concentrations along the flow path of the Labrador Sea Water were attributed to sedimentary inputs from the Newfoundland Margin. Bottom waters from the Irminger Sea displayed high DFe concentrations likely due to the dissolution of Fe-rich particles in the Denmark Strait Overflow Water and the Polar Intermediate Water. Finally, the nepheloid layers located in the different basins and at the Iberian Margin were found to act as either a source or a sink of DFe depending on the nature of particles, with organic particles likely releasing DFe and Mn particle scavenging DFe.

scholarly journals Simulated Variability of the Circulation in the North Atlantic from 1953 to 2003

2008 ◽  
Vol 21 (19) ◽  
pp. 4919-4933 ◽  
Author(s):  
Julie Deshayes ◽  
Claude Frankignoul
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
Labrador Sea ◽  
Ekman Pumping ◽  
Subpolar Gyre ◽  
Dense Water ◽  
The North ◽  
Irminger Sea ◽  
Dense Water Formation ◽  
The North Atlantic

Abstract The variability of the circulation in the North Atlantic and its link with atmospheric variability are investigated in a realistic hindcast simulation from 1953 to 2003. The interannual-to-decadal variability of the subpolar gyre circulation and the Meridional Overturning Circulation (MOC) is mostly influenced by the North Atlantic Oscillation (NAO). Both circulations intensified from the early 1970s to the mid-1990s and then decreased. The monthly variability of both circulations reflects the fast barotropic adjustment to NAO-related Ekman pumping anomalies, while the interannual-to-decadal variability is due to the baroclinic adjustment to Ekman pumping, buoyancy forcing, and dense water formation, consistent with previous studies. An original characteristic of the oceanic response to NAO is presented that relates to the spatial patterns of buoyancy and wind forcing over the North Atlantic. Anomalous Ekman pumping associated with a positive NAO phase first induces a decrease of the southern subpolar gyre strength and an intensification of the northern subpolar gyre. The latter is reinforced by buoyancy loss and dense water formation in the Irminger Sea, where the cyclonic circulation increases 1–2 yr after the positive NAO phase. Increased buoyancy loss also occurs in the Labrador Sea, but because of the early decrease of the southern subpolar gyre strength, the intensification of the cyclonic circulation is delayed. Hence the subpolar gyre and the MOC start increasing in the Irminger Sea, while in the Labrador Sea the circulation at depth leads its surface counterpart. In this simulation where the transport of dense water through the North Atlantic sills is underestimated, the MOC variability is well represented by a simple integrator of convection in the Irminger Sea, which fits better than a direct integration of NAO forcing.

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