scholarly journals Kinematic Structure and Dynamics of the Denmark Strait Overflow from Ship-Based Observations

2020 ◽  
Vol 50 (11) ◽  
pp. 3235-3251
Author(s):  
Peigen Lin ◽  
Robert S. Pickart ◽  
Kerstin Jochumsen ◽  
G. W. K. Moore ◽  
Héðinn Valdimarsson ◽  
...  
Keyword(s):  
Water Mass ◽  
Significant Degree ◽  
Meridional Overturning Circulation ◽  
The North ◽  
East Greenland Current ◽  
Denmark Strait ◽  
Time Period ◽  
Entrainment Process ◽  
Meridional Overturning ◽  
Flow Components

AbstractThe dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 106 m3 s−1), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.

scholarly journals Lagrangian Perspective on the Origins of Denmark Strait Overflow

2020 ◽  
Vol 50 (8) ◽  
pp. 2393-2414
Author(s):  
Atousa Saberi ◽  
Thomas W. N. Haine ◽  
Renske Gelderloos ◽  
M. Femke de Jong ◽  
Heather Furey ◽  
...  
Keyword(s):  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Near Surface ◽  
The North ◽  
East Greenland Current ◽  
Nao Index ◽  
Denmark Strait ◽  
Meridional Overturning ◽  
In Situ Observations

AbstractThe Denmark Strait Overflow (DSO) is an important contributor to the lower limb of the Atlantic meridional overturning circulation (AMOC). Determining DSO formation and its pathways is not only important for local oceanography but also critical to estimating the state and variability of the AMOC. Despite prior attempts to understand the DSO sources, its upstream pathways and circulation remain uncertain due to short-term (3–5 days) variability. This makes it challenging to study the DSO from observations. Given this complexity, this study maps the upstream pathways and along-pathway changes in its water properties, using Lagrangian backtracking of the DSO sources in a realistic numerical ocean simulation. The Lagrangian pathways confirm that several branches contribute to the DSO from the north such as the East Greenland Current (EGC), the separated EGC (sEGC), and the North Icelandic Jet (NIJ). Moreover, the model results reveal additional pathways from south of Iceland, which supplied over 16% of the DSO annually and over 25% of the DSO during winter of 2008, when the NAO index was positive. The southern contribution is about 34% by the end of March. The southern pathways mark a more direct route from the near-surface subpolar North Atlantic to the North Atlantic Deep Water (NADW), and needs to be explored further, with in situ observations.

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 The Iceland-Faroe Slope Jet: a conduit for dense water toward the Faroe Bank Channel overflow

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Stefanie Semper ◽  
Robert S. Pickart ◽  
Kjetil Våge ◽  
Karin Margretha H. Larsen ◽  
Hjálmar Hátún ◽  
...  
Keyword(s):  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
Common Source ◽  
Overturning Circulation ◽  
Nordic Seas ◽  
Dense Water ◽  
The North ◽  
Denmark Strait ◽  
Meridional Overturning

Abstract Dense water from the Nordic Seas passes through the Faroe Bank Channel and supplies the lower limb of the Atlantic Meridional Overturning Circulation, a critical component of the climate system. Yet, the upstream pathways of this water are not fully known. Here we present evidence of a previously unrecognised deep current following the slope from Iceland toward the Faroe Bank Channel using high-resolution, synoptic shipboard observations and long-term measurements north of the Faroe Islands. The bulk of the volume transport of the current, named the Iceland-Faroe Slope Jet (IFSJ), is relatively uniform in hydrographic properties, very similar to the North Icelandic Jet flowing westward along the slope north of Iceland toward Denmark Strait. This suggests a common source for the two major overflows across the Greenland-Scotland Ridge. The IFSJ can account for approximately half of the total overflow transport through the Faroe Bank Channel, thus constituting a significant component of the overturning circulation in the Nordic Seas.

scholarly journals Changes in the geometry and strength of the Atlantic Meridional Overturning Circulation during the last glacial (20–50 ka)

2016 ◽  
Author(s):  
Pierre Burckel ◽  
Claire Waelbroeck ◽  
Yiming Luo ◽  
Didier Roche ◽  
Sylvain Pichat ◽  
...  
Keyword(s):  
Flow Rate ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20–50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). At the onset of Heinrich Stadial 2, the structure of the AMOC significantly changed. The deep Atlantic was probably directly affected by a southern sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.

scholarly journals Water mass transport variability to the North Icelandic shelf, 1994–2010

2012 ◽  
Vol 69 (5) ◽  
pp. 809-815 ◽  
Author(s):  
Steingrímur Jónsson ◽  
Héðinn Valdimarsson
Keyword(s):  
Mass Transport ◽  
Heat Transport ◽  
Water Mass ◽  
Atlantic Water ◽  
Hydrographic Data ◽  
The North ◽  
East Greenland Current ◽  
Denmark Strait ◽  
Irminger Current ◽  
Polar Water

Abstract Jónsson, S., and Valdimarsson, H. 2012. Water mass transport variability to the North Icelandic shelf, 1994–2010. – ICES Journal of Marine Science, 69: 809–815. In the Denmark Strait between Greenland and Iceland, the north-flowing warm, saline Atlantic Water (AW) of the Irminger Current meets the south-flowing cold, relatively fresh Polar Water (PW) of the East Greenland Current. A mixture of these two surface water masses then flows along the shelf north of Iceland. The mixture can vary from being almost pure AW to consisting, to a large extent, of PW. The relative quantities of each water mass to some extent determine the productivity and the living conditions on the shelf north of Iceland. The flow has been monitored with current meters on a section north of Iceland since 1994, and these measurements, together with hydrographic data, are used to study its structure and variability. The amount of AW carried by the flow is calculated along with the associated heat transport. In the period 1994–2010, the flow consisted on average of 68% of AW with a transport of 0.88 Sv and an associated heat transport of 24 TW. There is notable seasonal variation in the flow and strong interannual variability.

scholarly journals Changes in the geometry and strength of the Atlantic meridional overturning circulation during the last glacial (20–50 ka)

2016 ◽  
Vol 12 (11) ◽  
pp. 2061-2075 ◽  
Author(s):  
Pierre Burckel ◽  
Claire Waelbroeck ◽  
Yiming Luo ◽  
Didier M. Roche ◽  
Sylvain Pichat ◽  
...  
Keyword(s):  
Flow Rate ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. We reconstruct the geometry and strength of the Atlantic meridional overturning circulation during the Heinrich stadial 2 and three Greenland interstadials of the 20–50 ka period based on the comparison of new and published sedimentary 231Pa / 230Th data with simulated sedimentary 231Pa / 230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present-day North Atlantic deep water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic bottom water (AABW). Our results further show that during Heinrich stadial 2, the deep Atlantic was probably directly affected by a southern-sourced water mass below 2500 m depth, while a slow, southward-flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.

scholarly journals Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges

2021 ◽  
Author(s):  
Mattia Almansi ◽  
Thomas Haine ◽  
Renske Gelderloos ◽  
Robert Pickart
Keyword(s):  
North Atlantic ◽  
Potential Vorticity ◽  
Atlantic Meridional Overturning Circulation ◽  
Volume Transport ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Denmark Strait ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
New Evidence

<p><em>Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. </em><em>Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.</em></p>

Mechanisms linking the Labrador Sea with subtropical Atlantic overturning

2021 ◽  
Author(s):  
Yavor Kostov ◽  
Marie-José Messias ◽  
Helen Johnson ◽  
Herlé Mercier ◽  
David Marshall
Keyword(s):  
North Atlantic ◽  
Causal Chain ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Labrador Sea ◽  
State Estimate ◽  
The North ◽  
Denmark Strait ◽  
Meridional Overturning ◽  
Surface Buoyancy

<p>We analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea and variability in the subtropical Atlantic meridional overturning circulation (AMOC) in the ECCO ocean state estimate on inter-annual timescales. Our study highlights the importance of Lower North Atlantic Deep Water (LNADW) for the north-south connectivity in the Atlantic Ocean. We identify important mechanisms that allow the Labrador Sea to impact the southward transport of LNADW. We show that NAC plays an essential role in the export of buoyancy anomalies from the Labrador Sea – and it furthermore exerts a positive feedback that amplifies these upper ocean anomalies in the eastern subpolar gyre – before they reach the denser water masses along the lower limb of the AMOC. Our results also highlight the contribution of the western Labrador Sea for the surface uptake of tracers that penetrate the LNADW near Denmark Strait, which has implications for the redistribution of ocean heat anomalies.</p>

scholarly journals The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation

2016 ◽  
Vol 29 (3) ◽  
pp. 941-962 ◽  
Author(s):  
Thomas L. Delworth ◽  
Fanrong Zeng
Keyword(s):  
North Atlantic ◽  
Time Scale ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The North ◽  
Hemispheric Temperature ◽  
Meridional Overturning ◽  
The North Atlantic Oscillation ◽  
The Impact ◽  
Model Ocean

Abstract The impact of the North Atlantic Oscillation (NAO) on the Atlantic meridional overturning circulation (AMOC) and large-scale climate is assessed using simulations with three different climate models. Perturbation experiments are conducted in which a pattern of anomalous heat flux corresponding to the NAO is added to the model ocean. Differences between the perturbation experiments and a control illustrate how the model ocean and climate system respond to the NAO. A positive phase of the NAO strengthens the AMOC by extracting heat from the subpolar gyre, thereby increasing deep-water formation, horizontal density gradients, and the AMOC. The flux forcings have the spatial structure of the observed NAO, but the amplitude of the forcing varies in time with distinct periods varying from 2 to 100 yr. The response of the AMOC to NAO variations is small at short time scales but increases up to the dominant time scale of internal AMOC variability (20–30 yr for the models used). The amplitude of the AMOC response, as well as associated oceanic heat transport, is approximately constant as the time scale of the forcing is increased further. In contrast, the response of other properties, such as hemispheric temperature or Arctic sea ice, continues to increase as the time scale of the forcing becomes progressively longer. The larger response is associated with the time integral of the anomalous oceanic heat transport at longer time scales, combined with an increased impact of radiative feedback processes. It is shown that NAO fluctuations, similar in amplitude to those observed over the last century, can modulate hemispheric temperature by several tenths of a degree.

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