Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

2008 ◽  
Vol 38 (8) ◽  
pp. 1617-1643 ◽  
Author(s):  
Jérôme Chanut ◽  
Bernard Barnier ◽  
William Large ◽  
Laurent Debreu ◽  
Thierry Penduff ◽  
...  
Keyword(s):  
Deep Convection ◽  
Labrador Sea ◽  
Central Basin ◽  
Barotropic Instability ◽  
Boundary Current ◽  
The West ◽  
Simultaneous Generation ◽  
Near Surface ◽  
West Greenland ◽  
Convective Mixing

Abstract The cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (⅓°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective overturning event produces convective eddies (CEs) of 20–30 km in diameter, as observed and produced in more idealized models, with a distinct seasonal cycle of EKE peaking in April. Sensitivity experiments show that each of these eddy types plays a distinct role in the heat budget of the central Labrador Sea, hence in the convection cycle. As observed in nature, deep convective mixing is limited to areas where adequate preconditioning can occur, that is, to a small region in the southwestern quadrant of the central basin. To the east, west, and south, BCEs flux heat from the BCS at a rate sufficient to counteract air–sea buoyancy loss. To the north, this eddy flux alone is not enough, but when combined with the effects of Irminger Rings, preconditioning is effectively inhibited here too. Following a deep convective mixing event, the homogeneous convection patch reaches as deep as 2000 m and a horizontal scale on the order of 200 km, as has been observed. Both CEs and BCEs are found to play critical roles in the lateral mixing phase, when the patch restratifies and transforms into Labrador Sea Water (LSW). BCEs extract the necessary heat from the BCS and transport it to the deep convection site, where it fluxed into convective patches by CEs during the initial phase. Later in the phase, BCE heat flux maintains and strengthens the restratification throughout the column, while solar heating establishes a near-surface seasonal stratification. In contrast, IRs appear to rarely enter the deep convection region. However, by virtue of their control on the surface area preconditioned for deep convection and the interannual variability of the associated barotropic instability, they could have an important role in the variability of LSW.

scholarly journals Cyclonic eddies in the West Greenland boundary current system

2021 ◽  
Author(s):  
Astrid Pacini ◽  
Robert S. Pickart ◽  
Isabela A. Le Bras ◽  
Fiammetta Straneo ◽  
N. Penny Holliday ◽  
...  
Keyword(s):  
North Atlantic ◽  
Current System ◽  
Deep Convection ◽  
Labrador Sea ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Denmark Strait ◽  
First Time ◽  
Insight Into

<p>The Labrador Sea is an important site for deep convection, and the boundary current surrounding the Sea impacts the strength of this convection and the subsequent restratification. As part of the Overturning of the Subpolar North Atlantic Program, ten moorings have been maintained on the West Greenland shelf and slope that provide hourly, high-resolution renderings of the boundary current. These data reveal the presence and propagation of abundant mid-depth intensified cyclonic eddies, which have not previously been documented in the West Greenland boundary current system. This study quantifies these features and their structure and demonstrates that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. Using the mooring data, the statistics of these features are presented, a composite eddy is constructed, and the velocity and transport structure are described. A synoptic survey of the region captured two of these features, and provides further insight into their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea, and their presence, propagation, and transport must be accounted for in order to assess their contribution to the heat and freshwater budgets of the Labrador Sea interior.</p>

Assessing the Roles of Three Eddy Types in Restratifying the Labrador Sea after Deep Convection

2011 ◽  
Vol 41 (11) ◽  
pp. 2102-2119 ◽  
Author(s):  
Renske Gelderloos ◽  
Caroline A. Katsman ◽  
Sybren S. Drijfhout
Keyword(s):  
Time Scales ◽  
Deep Convection ◽  
Early Spring ◽  
Labrador Sea ◽  
Boundary Current ◽  
Key Factors ◽  
The West ◽  
Warm Core ◽  
Water Formation ◽  
Dense Water Formation

Abstract Restratification after deep convection is one of the key factors in determining the temporal variability of dense water formation in the Labrador Sea. In the subsurface, it is primarily governed by lateral buoyancy fluxes during early spring. The roles of three different eddy types in this process are assessed using an idealized model of the Labrador Sea that simulates the restratification season. The first eddy type, warm-core Irminger rings, is shed from the boundary current along the west coast of Greenland. All along the coastline, the boundary current forms boundary current eddies. The third type, convective eddies, arises directly around the convection area. In the model, the latter two eddy types are together responsible for replenishing 30% of the winter heat loss within 6 months. Irminger rings add another 45% to this number. The authors’ results thus confirm that the presence of Irminger rings is essential for a realistic amount of restratification in this area. The model results are compared to observations using theoretical estimates of restratification time scales derived for the three eddy types. The time scales are also used to explain contradicting conclusions in previous studies on their respective roles.

scholarly journals Mean Conditions and Seasonality of the West Greenland Boundary Current System near Cape Farewell

2020 ◽  
Vol 50 (10) ◽  
pp. 2849-2871
Author(s):  
Astrid Pacini ◽  
Robert S. Pickart ◽  
Frank Bahr ◽  
Daniel J. Torres ◽  
Andrée L. Ramsey ◽  
...  
Keyword(s):  
Current System ◽  
Labrador Sea ◽  
Western Boundary ◽  
Coastal Current ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Irminger Sea ◽  
West Greenland Current ◽  
Recirculation Gyre

AbstractThe structure, transport, and seasonal variability of the West Greenland boundary current system near Cape Farewell are investigated using a high-resolution mooring array deployed from 2014 to 2018. The boundary current system is comprised of three components: the West Greenland Coastal Current, which advects cold and fresh Upper Polar Water (UPW); the West Greenland Current, which transports warm and salty Irminger Water (IW) along the upper slope and UPW at the surface; and the Deep Western Boundary Current, which advects dense overflow waters. Labrador Sea Water (LSW) is prevalent at the seaward side of the array within an offshore recirculation gyre and at the base of the West Greenland Current. The 4-yr mean transport of the full boundary current system is 31.1 ± 7.4 Sv (1 Sv ≡ 106 m3 s−1), with no clear seasonal signal. However, the individual water mass components exhibit seasonal cycles in hydrographic properties and transport. LSW penetrates the boundary current locally, through entrainment/mixing from the adjacent recirculation gyre, and also enters the current upstream in the Irminger Sea. IW is modified through air–sea interaction during winter along the length of its trajectory around the Irminger Sea, which converts some of the water to LSW. This, together with the seasonal increase in LSW entering the current, results in an anticorrelation in transport between these two water masses. The seasonality in UPW transport can be explained by remote wind forcing and subsequent adjustment via coastal trapped waves. Our results provide the first quantitatively robust observational description of the boundary current in the eastern Labrador Sea.

Export of newly oxygenated Labrador Sea Water at 53N

2021 ◽  
Author(s):  
Jannes Koelling ◽  
Dariia Atamanchuk ◽  
Johannes Karstensen ◽  
Douglas W.R. Wallace
Keyword(s):  
Sea Water ◽  
Deep Convection ◽  
Sudden Change ◽  
Deep Ocean ◽  
Labrador Sea ◽  
Low Oxygen ◽  
Boundary Current ◽  
Convective Mixing ◽  
Future Work ◽  
Labrador Sea Water

<div> <p>Most of the life-sustaining oxygen found in the global deep ocean is supplied in one of only a handful of key regions around the globe, such as the Labrador Sea in the subpolar North Atlantic. Here, oxygen is supplied directly to the deep ocean during the formation of Labrador Sea Water (LSW), when convective mixing continuously brings low-oxygen deep water towards the surface and into contact with the atmosphere. The continuous exchange between the surface and deep ocean during convection can bring newly oxygenated waters as deep as 2000m. Although the associated oxygen uptake has been observed and quantified, and the resulting oxygen-rich water mass in the deep ocean is readily detected throughout the Atlantic Ocean, relatively little is known about the exact mechanisms and timing of its export out of the basin.</p> </div><div> <p>In this talk, we will present a novel dataset of oxygen sensors deployed within the boundary current at the exit of the Labrador Sea to investigate oxygen variability in the deep ocean. This is the first time that a continuous time series of oxygen has been collected in the boundary current of the Labrador Sea, with a total of 10 sensors deployed on 4 moorings from 2016 to 2020. The sensors at 600m depth show a sudden change in oxygen, temperature, and salinity in the spring, which we discuss in relation to deep convection in the interior. We also use data from Argo floats to analyse export pathways from the convection region to the location of the moorings. Our results give new insights into how the oxygen taken up in the central Labrador Sea subsequently spreads into the global deep ocean, and lay the basis for future work on quantifying variability of oxygen transport at the exit of the Labrador Sea.</p> </div>

scholarly journals Cyclonic eddies in the West Greenland Boundary Current System

2021 ◽  
Author(s):  
Astrid Pacini ◽  
Robert S. Pickart ◽  
Isabela A. Le Bras ◽  
Fiammetta Straneo ◽  
N.P. Holliday ◽  
...  
Keyword(s):  
Propagation Velocity ◽  
Current System ◽  
Labrador Sea ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Irminger Sea ◽  
Denmark Strait ◽  
Integral Role ◽  
Thick Lens

AbstractThe boundary current system in the Labrador Sea plays an integral role in modulating convection in the interior basin. Four years of mooring data from the eastern Labrador Sea reveal persistent mesoscale variability in the West Greenland boundary current. Between 2014 and 2018, 197 mid-depth intensified cyclones were identified that passed the array near the 2000 m isobath. In this study, we quantify these features and show that they are the downstream manifestation of Denmark Strait Overflow Water (DSOW) cyclones. A composite cyclone is constructed revealing an average radius of 9 km, maximum azimuthal speed of 24 cm/s, and a core propagation velocity of 27 cm/s. The core propagation velocity is significantly smaller than upstream near Denmark Strait, allowing them to trap more water. The cyclones transport a 200-m thick lens of dense water at the bottom of the water column, and increase the transport of DSOW in the West Greenland boundary current by 17% relative to the background flow. Only a portion of the features generated at Denmark Strait make it to the Labrador Sea, implying that the remainder are shed into the interior Irminger Sea, are retroflected at Cape Farewell, or dissipate. A synoptic shipboard survey east of Cape Farewell, conducted in summer 2020, captured two of these features which shed further light on their structure and timing. This is the first time DSOW cyclones have been observed in the Labrador Sea—a discovery that could have important implications for interior stratification.

scholarly journals Structure, variability, and dynamics of the West Greenland Boundary Current System

2022 ◽  
Author(s):  
◽  
Astrid Pacini
Keyword(s):  
Current System ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Small Scale ◽  
Western Boundary ◽  
Coastal Current ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Atlantic Origin

The ventilation of intermediate waters in the Labrador Sea has important implications for the strength of the Atlantic Meridional Overturning Circulation. Boundary current-interior interactions regulate the exchange of properties between the slope and the basin, which in turn regulates the magnitude of interior convection and the export of ventilated waters from the subpolar gyre. This thesis characterizes theWest Greenland Boundary Current System near Cape Farewell across a range of spatio-temporal scales. The boundary current system is composed of three velocity cores: (1) the West Greenland Coastal Current (WGCC), transporting Greenland and Arctic meltwaters on the shelf; (2) the West Greenland Current (WGC), which advects warm, saline Atlantic-origin water at depth, meltwaters at the surface, and newly-ventilated Labrador Sea Water (LSW); and (3) the Deep Western Boundary Current, which carries dense overflow waters ventilated in the Nordic Seas. The seasonal presence of the LSW and Atlantic-origin water are dictated by air-sea buoyancy forcing, while the seasonality of the WGCC is governed by remote wind forcing and the propagation of coastally trapped waves from East Greenland. Using mooring data and hydrographic surveys, we demonstrate mid-depth intensified cyclones generated at Denmark Strait are found offshore of the WGC and enhance the overflow water transport at synoptic timescales. Using mooring, hydrographic, and satellite data, we demonstrate that the WGC undergoes extensive meandering due to baroclinic instability that is enhanced in winter due to LSW formation adjacent to the current. This leads to the production of small-scale, anticyclonic eddies that can account for the entirety of wintertime heat loss within the Labrador Sea. The meanders are shown to trigger the formation of Irminger Rings downstream. Using mooring, hydrographic, atmospheric, and Lagrangian data, and a mixing model, we find that strong atmospheric storms known as forward tip jets cause upwelling at the shelfbreak that triggers offshore export of freshwater. This freshwater flux can explain the observed lack of ventilation in the eastern Labrador Sea. Together, this thesis documents previously unobserved interannual, seasonal, and synoptic-scale variability and dynamics within the West Greenland boundary current system that must be accounted for in future modeling.

scholarly journals Eddy Formation near the West Coast of Greenland

2008 ◽  
Vol 38 (9) ◽  
pp. 1992-2002 ◽  
Author(s):  
Annalisa Bracco ◽  
Joseph Pedlosky ◽  
Robert S. Pickart
Keyword(s):  
Vertical Structure ◽  
Labrador Sea ◽  
Boundary Current ◽  
Layer Model ◽  
The West ◽  
Current Configuration ◽  
Topographic Slope ◽  
Eddy Formation ◽  
Perturbation Energy ◽  
Quasigeostrophic Model

Abstract This paper extends A. Bracco and J. Pedlosky’s investigation of the eddy-formation mechanism in the eastern Labrador Sea by including a more realistic depiction of the boundary current. The quasigeostrophic model consists of a meridional, coastally trapped current with three vertical layers. The current configuration and topographic domain are chosen to match, as closely as possible, the observations of the boundary current and the varying topographic slope along the West Greenland coast. The role played by the bottom-intensified component of the boundary current on the formation of the Labrador Sea Irminger Rings is explored. Consistent with the earlier study, a short, localized bottom-trapped wave is responsible for most of the perturbation energy growth. However, for the instability to occur in the three-layer model, the deepest component of the boundary current must be sufficiently strong, highlighting the importance of the near-bottom flow. The model is able to reproduce important features of the observed vortices in the eastern Labrador Sea, including the polarity, radius, rate of formation, and vertical structure. At the time of formation, the eddies have a surface signature as well as a strong circulation at depth, possibly allowing for the transport of both surface and near-bottom water from the boundary current into the interior basin. This work also supports the idea that changes in the current structure could be responsible for the observed interannual variability in the number of Irminger Rings formed.

scholarly journals Role of Resolved and Parameterized Eddies in the Labrador Sea Balance of Heat and Buoyancy

2014 ◽  
Vol 44 (12) ◽  
pp. 3008-3032 ◽  
Author(s):  
Oleg A. Saenko ◽  
Frédéric Dupont ◽  
Duo Yang ◽  
Paul G. Myers ◽  
Igor Yashayaev ◽  
...  
Keyword(s):  
High Resolution ◽  
West Coast ◽  
Heat Budget ◽  
Deep Convection ◽  
Mesoscale Eddies ◽  
Global Ocean ◽  
Labrador Sea ◽  
Low Resolution ◽  
The West

Abstract Deep convection in the Labrador Sea is an important component of the global ocean ventilation. The associated loss of heat to the atmosphere from the interior of the sea is thought to be mostly supplied by mesoscale eddies, generated either remotely or as a result of convection itself—processes that are not resolved by low-resolution ocean climate models. The authors first employ a high-resolution (°) ocean model forced with high-resolution (33 km, 3 h) atmospheric fields to further elaborate on the role of mesoscale eddies in maintaining the balance of heat and buoyancy in the Labrador Sea. In general agreement with previous studies, it is found that eddies remove heat along the coast and supply it to the interior. Some of the eddies that are generated because of the barotropic instability off the west coast of Greenland are recaptured by the boundary current. In the region of deep convection, the convergence of heat and buoyancy by eddies significantly increases with the deepening of the winter mixed layer. In addition, the vertical eddy flux plays an important part in the heat budget of the upper Labrador Sea, accounting for up to half of the heat loss to the atmosphere north of 60°N. A low-resolution (1°) model with parameterized eddies is then applied to show that it does capture, qualitatively, the general structure of eddy buoyancy advection along the Labrador Current. However, the 1° model is deficient in this regard in the most eddy active region off the west coast of Greenland, although some improvements can be made by forcing it with the high-resolution atmospheric fields.

scholarly journals Effects of Wind on Convection in Strongly and Weakly Baroclinic Flows with Application to the Labrador Sea*

2002 ◽  
Vol 32 (9) ◽  
pp. 2603-2618 ◽  
Author(s):  
Fiammetta Straneo ◽  
Mitsuhiro Kawase ◽  
Robert S. Pickart
Keyword(s):  
Deep Convection ◽  
Labrador Sea ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Convective Mixing ◽  
Slantwise Convection ◽  
Horizontal Density Gradient ◽  
Surface Buoyancy ◽  
Convective Layer ◽  
Buoyancy Loss

Abstract Large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection. Here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean. In strongly baroclinic flows, convective mixing is influenced both by the background horizontal density gradient and by the across-front advection of buoyancy due to wind. The former process—known as slantwise convection—results in deeper convection, while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front. For the case of the Labrador Sea, wintertime winds act to destabilize the baroclinic Labrador Current causing a buoyancy removal roughly one-third as large as the air–sea buoyancy loss. Simulations using a nonhydrostatic numerical model, initialized and forced with observed fields from the Labrador Sea, show how the combination of wind and lateral gradients can result in significant convection within the current, in contrast with previous ideas. Though the advection of buoyancy due to wind in weakly baroclinic flows is negligible compared to the surface buoyancy removal typical of convective conditions, convective plumes are substantially deformed by wind. This deformation, and the associated across-front secondary circulation, are explained in terms of the vertical advection of wind-generated vorticity from the surface boundary layer to deeper depths. This mechanism generates vertical structure within the convective layer, contradicting the historical notion that properties become vertically homogenized during convection. For the interior Labrador Sea, this mechanism may be partly responsible for the vertical variability observed during convection, which modeling studies have until now failed to reproduce.

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