Labrador Sea Water Formation Rate and Its Impact on the Local Meridional Overturning Circulation

Abstract While it has generally been understood that the production of Labrador Sea Water (LSW) impacts the Atlantic meridional overturning circulation (MOC), this relationship has not been explored extensively or validated against observations. To explore this relationship, a suite of global ocean–sea ice models forced by the same interannually varying atmospheric dataset, varying in resolution from non-eddy-permitting to eddy-permitting (1°–1/4°), is analyzed to investigate the local and downstream relationships between LSW formation and the MOC on interannual to decadal time scales. While all models display a strong relationship between changes in the LSW volume and the MOC in the Labrador Sea, this relationship degrades considerably downstream of the Labrador Sea. In particular, there is no consistent pattern among the models in the North Atlantic subtropical basin over interannual to decadal time scales. Furthermore, the strong response of the MOC in the Labrador Sea to LSW volume changes in that basin may be biased by the overproduction of LSW in many models compared to observations. This analysis shows that changes in LSW volume in the Labrador Sea cannot be clearly and consistently linked to a coherent MOC response across latitudes over interannual to decadal time scales in ocean hindcast simulations of the last half century. Similarly, no coherent relationships are identified between the MOC and the Labrador Sea mixed layer depth or the density of newly formed LSW across latitudes or across models over interannual to decadal time scales.

Download Full-text

Breaking the Linkage Between Labrador Sea Water Production and Its Advective Export to the Subtropical Gyre

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0210.1 ◽

2016 ◽

Vol 46 (7) ◽

pp. 2169-2182 ◽

Cited By ~ 23

Author(s):

Sijia Zou ◽

M. Susan Lozier

Keyword(s):

Deep Water ◽

Sea Water ◽

Water Masses ◽

Meridional Overturning Circulation ◽

Subtropical Gyre ◽

Global Ocean ◽

Labrador Sea ◽

Deep Water Formation ◽

Water Formation ◽

Labrador Sea Water

AbstractDeep water formation in the northern North Atlantic has been of long-standing interest because the resultant water masses, along with those that flow over the Greenland–Scotland Ridge, constitute the lower limb of the Atlantic meridional overturning circulation (AMOC), which carries these cold, deep waters southward to the subtropical region and beyond. It has long been assumed that an increase in deep water formation would result in a larger southward export of newly formed deep water masses. However, recent observations of Lagrangian floats have raised questions about this linkage. Motivated by these observations, the relationship between convective activity in the Labrador Sea and the export of newly formed Labrador Sea Water (LSW), the shallowest component of the deep AMOC, to the subtropics is explored. This study uses simulated Lagrangian pathways of synthetic floats produced with output from a global ocean–sea ice model. It is shown that substantial recirculation of newly formed LSW in the subpolar gyre leads to a relatively small fraction of this water exported to the subtropical gyre: 40 years after release, only 46% of the floats are able to reach the subtropics. Furthermore, waters produced from any one particular convection event are not collectively and contemporaneously exported to the subtropical gyre, such that the waters that are exported to the subtropical gyre have a wide distribution in age.

Download Full-text

An Outsized Role for the Labrador Sea in the Multidecadal Variability of the Atlantic Overturning Circulation

10.31223/x5zp68 ◽

2021 ◽

Author(s):

Stephen Yeager ◽

Fred Castruccio ◽

Ping Chang ◽

Gokhan Danabasoglu ◽

Elizabeth Maroon ◽

...

Keyword(s):

North Atlantic ◽

Climate Models ◽

Sea Water ◽

Meridional Overturning Circulation ◽

Climate Simulation ◽

Labrador Sea ◽

Overturning Circulation ◽

Buoyancy Forcing ◽

Realistic Representation ◽

Meridional Overturning

Climate models are essential tools for investigating intrinsic North Atlantic variability related to variations in the Atlantic meridional overturning circulation (AMOC), but recent observations have called into question the fidelity of models that emphasize the importance ofLabrador Sea processes. A multi-century pre-industrial climate simulation that resolves ocean mesoscale eddies has a realistic representation of key observed subpolar Atlantic phenomena,including the dominance of density-space overturning in the eastern subpolar gyre, and thus provides uniquely credible context for interpreting short observational records. Despite weak mean surface diapycnal transformation in the Labrador Sea, multidecadal AMOC variability can be traced to anomalous production of dense Labrador Sea Water with local buoyancy forcing in the interior Labrador Sea playing a significant driving role.

Download Full-text

A sea change in our view of overturning in the subpolar North Atlantic

Science ◽

10.1126/science.aau6592 ◽

2019 ◽

Vol 363 (6426) ◽

pp. 516-521 ◽

Cited By ~ 94

Author(s):

M. S. Lozier ◽

F. Li ◽

S. Bacon ◽

F. Bahr ◽

A. S. Bower ◽

...

Keyword(s):

North Atlantic ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Climate Change Projections ◽

Overturning Circulation ◽

Intergovernmental Panel ◽

Freshwater Transport ◽

Deep Waters ◽

Meridional Overturning ◽

Prevailing View

To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

Download Full-text

Arctic–North Atlantic Interactions and Multidecadal Variability of the Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli3462.1 ◽

2005 ◽

Vol 18 (19) ◽

pp. 4013-4031 ◽

Cited By ~ 190

Author(s):

Johann H. Jungclaus ◽

Helmuth Haak ◽

Mojib Latif ◽

Uwe Mikolajewicz

Keyword(s):

North Atlantic ◽

Deep Water ◽

Ocean Model ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Deep Water Formation ◽

Overturning Circulation ◽

Multidecadal Variability ◽

Water Formation ◽

Meridional Overturning

Abstract Analyses of a 500-yr control integration with the non-flux-adjusted coupled atmosphere–sea ice–ocean model ECHAM5/Max-Planck-Institute Ocean Model (MPI-OM) show pronounced multidecadal fluctuations of the Atlantic overturning circulation and the associated meridional heat transport. The period of the oscillations is about 70–80 yr. The low-frequency variability of the meridional overturning circulation (MOC) contributes substantially to sea surface temperature and sea ice fluctuations in the North Atlantic. The strength of the overturning circulation is related to the convective activity in the deep-water formation regions, most notably the Labrador Sea, and the time-varying control on the freshwater export from the Arctic to the convection sites modulates the overturning circulation. The variability is sustained by an interplay between the storage and release of freshwater from the central Arctic and circulation changes in the Nordic Seas that are caused by variations in the Atlantic heat and salt transport. The relatively high resolution in the deep-water formation region and the Arctic Ocean suggests that a better representation of convective and frontal processes not only leads to an improvement in the mean state but also introduces new mechanisms determining multidecadal variability in large-scale ocean circulation.

Download Full-text

Decadal changes in the Atlantic Meridional Overturning Circulation in high-resolution simulations of the subpolar North Atlantic

10.5194/egusphere-egu21-7789 ◽

2021 ◽

Author(s):

Claus W. Böning ◽

Arne Biastoch ◽

Klaus Getzlaff ◽

Patrick Wagner ◽

Siren Rühs ◽

...

Keyword(s):

High Resolution ◽

North Atlantic ◽

Deep Convection ◽

Alternative Interpretation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Labrador Sea ◽

Overturning Circulation ◽

First Results ◽

Meridional Overturning

<p>A series of global ocean - sea ice model simulations is used to investigate the spatial structure and temporal variability of the sinking branch of the meridional overturning circulation (AMOC) in the subpolar North Atlantic. The experiments include hindcast simulations of the last six decades based on the high-resolution (1/20&#176;) VIKING20X-model forced by the CORE and JRA55-do reanalysis products, supplemented by sensitivity studies with a 1/4&#176;-configuration (ORCA025) aimed at elucidating the roles of variations in the wind stress and buoyancy fluxes. The experiments exhibit different multi-decadal trends in the AMOC, reflecting the well-known sensitivity of ocean-only models to subtle details in the configuration of the subarctic freshwater forcing. All experiments, however, concur in that the dense, southward branch of the overturning is mainly fed by &#8220;sinking&#8221; (in density space) in the Irminger and Iceland Basins, in accordance with the first results of the OSNAP observational program. Remarkably, the contribution of the Labrador Sea has remained small throughout the whole simulation period, even during the phase of extremely strong convection in the early 1990s: i.e., the rate of deep water exported from the subpolar North Atlantic by the DWBC off Newfoundland never differed by more than O(1 Sv) from the DWBC entering the Labrador Sea at Cape Farewell. The model solutions indicate a particular concentration of the sinking along the deep boundary currents south of the Denmark Straits and south of Iceland, pointing to a prime importance for the AMOC of the outflows from the Nordic Seas and their subsequent enhancement by the entrainment of intermediate waters. Since these include the water masses formed by deep convection in the Labrador and southern Irminger Seas, our study offers an alternative interpretation of the dynamical role of decadal changes in Labrador Sea convection intensity in terms of a remote effect on the deep transports established in the outflow regimes.</p>

Download Full-text

Antarctic Sea Ice Control on the Depth of North Atlantic Deep Water

Journal of Climate ◽

10.1175/jcli-d-18-0519.1 ◽

2019 ◽

Vol 32 (9) ◽

pp. 2537-2551 ◽

Cited By ~ 6

Author(s):

Louis-Philippe Nadeau ◽

Raffaele Ferrari ◽

Malte F. Jansen

Keyword(s):

Sea Ice ◽

Deep Water ◽

Formation Rate ◽

Deep Ocean ◽

Meridional Overturning Circulation ◽

Ice Formation ◽

Overturning Circulation ◽

Antarctic Sea Ice ◽

Meridional Overturning

Abstract Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

Download Full-text

The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0629.1 ◽

2018 ◽

Vol 31 (13) ◽

pp. 5165-5188 ◽

Cited By ~ 6

Author(s):

He Wang ◽

Sonya Legg ◽

Robert Hallberg

Keyword(s):

North Atlantic ◽

Climate Models ◽

Deep Convection ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Labrador Sea ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning

This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate.

Download Full-text

Impact of Labrador Sea Convection on the North Atlantic Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/jpo3178.1 ◽

2007 ◽

Vol 37 (9) ◽

pp. 2207-2227 ◽

Cited By ~ 99

Author(s):

Robert S. Pickart ◽

Michael A. Spall

Keyword(s):

North Atlantic ◽

World Ocean ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning ◽

The North Atlantic ◽

Density Space

Abstract The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.

Download Full-text

How Does Labrador Sea Water Enter the Deep Western Boundary Current?

Journal of Physical Oceanography ◽

10.1175/2007jpo3807.1 ◽

2008 ◽

Vol 38 (5) ◽

pp. 968-983 ◽

Cited By ~ 18

Author(s):

Jaime B. Palter ◽

M. Susan Lozier ◽

Kara L. Lavender

Keyword(s):

Heat Flux ◽

Water Mass ◽

Sea Water ◽

Western Boundary Current ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Western Boundary ◽

Boundary Current ◽

Deep Western Boundary Current ◽

Labrador Sea Water

Abstract Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.

Download Full-text