scholarly journals Labrador Sea subsurface density as a precursor of multidecadal variability in the North Atlantic: a multi-model study

2021 ◽  
Vol 12 (2) ◽  
pp. 419-438
Author(s):  
Pablo Ortega ◽  
Jon I. Robson ◽  
Matthew Menary ◽  
Rowan T. Sutton ◽  
Adam Blaker ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Climate Model ◽  
Decadal Variability ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Western Boundary ◽  
Multidecadal Variability ◽  
Boundary Density

Abstract. The subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large-scale ocean circulation changes. This article analyses the interrelationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the subpolar gyre (SPG) circulation, and the upper-ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked to the strength of the subpolar AMOC and gyre circulation, and it is also linked to the subtropical AMOC, although the strength of this relationship is model-dependent and affected by the inclusion of the Ekman component. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including the mean density stratification in the Labrador Sea, the strength and depth of the AMOC, and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link to the subtropical AMOC might be more realistic.

scholarly journals Labrador Sea sub-surface density as a precursor of multi-decadal variability in the North Atlantic: a multi-model study

2020 ◽  
Author(s):  
Pablo Ortega ◽  
Jon I. Robson ◽  
Matthew Menary ◽  
Rowan T. Sutton ◽  
Adam Blaker ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Climate Model ◽  
Decadal Variability ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Western Boundary ◽  
Overturning Circulation ◽  
Boundary Density

Abstract. The Subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large scale ocean circulation changes. This article analyses the inter-relationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the Subpolar Gyre (SPG) circulation, and the upper ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked with the strength of subpolar AMOC and gyre circulation, and is also linked with the subtropical AMOC, although the strength of this relationship is model dependent. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including: the mean density stratification in the Labrador Sea; the strength and depth of the AMOC; and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link with the subtropical AMOC may be more realistic. This would imply that RAPID AMOC measurements might not be adequate to represent decadal to multidecadal changes in the subpolar overturning circulation.

scholarly journals Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model

2015 ◽  
Vol 28 (19) ◽  
pp. 7659-7677 ◽  
Author(s):  
Claude Frankignoul ◽  
Guillaume Gastineau ◽  
Young-Oh Kwon
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Atmospheric Response ◽  
Subpolar Gyre ◽  
Climate System Model ◽  
The North ◽  
The North Atlantic

Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.

scholarly journals Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation

2010 ◽  
Vol 6 (5) ◽  
pp. 1811-1852 ◽  
Author(s):  
A. Bozbiyik ◽  
M. Steinacher ◽  
F. Joos ◽  
T. F. Stocker
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Carbon Stocks ◽  
Meridional Overturning Circulation ◽  
Climate Conditions ◽  
Tropical Regions ◽  
The North ◽  
Northern Latitudes

Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes that are well documented in the literature but there is a clear distinction between northern and southern perturbations. Changes in the physical variables affect, in return, the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large re-organizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results.

Co-variability of salinity and temperature changes in the North Atlantic

2021 ◽  
Author(s):  
Levke Caesar ◽  
Gerard McCarthy
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Surface Fluxes ◽  
Meridional Overturning Circulation ◽  
Geophysical Research ◽  
Overturning Circulation ◽  
The North ◽  
Cold Anomaly ◽  
The North Atlantic

<p>While there is increasing paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened over the last one to two hundred years (Caesar et al., 2018; Thornalley et al., 2018), this is not confirmed by climate model simulations. Instead, the new simulations from the 6th Coupled Model Intercomparison Project (CMIP6) show a slight strengthening of the multimodel mean AMOC from 1850 until about 1985 (Menary et al., 2020), attributed to anthropogenic aerosol forcing. Arguing for a recent weakening of the AMOC, some studies attribute the emergence of the North Atlantic warming hole as a sign of the reduced meridional heat transport associated with a weaker AMOC (e.g. Caesar et al., 2018), yet this cold anomaly has also been interpreted as being aerosol-forced (Booth et al., 2012) and therefore not necessarily a sign of a weakening AMOC but rather a possible driver of a strengthening of the AMOC.</p><p>Looking beyond temperature, a fresh anomaly has recently emerged in the subpolar North Atlantic (Holliday et al., 2020). While a strengthening AMOC has been linked with an increase in salinity in the subpolar gyre region (Menary et al., 2013), an AMOC weakening would, due to the salt-advection feedback, likely lead to a reduction in salinity in the North Atlantic region. To shed some light on the question of whether the cold anomaly is internally (AMOC) or externally (aerosol-forced) driven we consider the co-variability of salinity and temperature in the North Atlantic in respect of changes in surface fluxes or alternate drivers.</p><p> </p><p>References</p><p>Booth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N., 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393): 228–232.</p><p>Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. and Saba, V., 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700): 191-196.</p><p>Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., Hátún, H., Johns, W., Josey, S.A., Larsen, K.M.H., Mulet, S., Oltmanns, M., Reverdin, G., Rossby, T., Thierry, V., Valdimarsson, H. and Yashayaev, I., 2020. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1): 585.</p><p>Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., Müller, W.A., Matei, D. and Lee, S.-K., 2013. Mechanisms of aerosol-forced AMOC variability in a state of the art climate model. Journal of Geophysical Research: Oceans, 118(4): 2087-2096.</p><p>Menary, M.B., Robson, J., Allan, R.P., Booth, B.B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L. and Zhang, R., 2020. Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophysical Research Letters, 47(14): e2020GL088166.</p><p>Thornalley, D.J.R., Oppo, D.W., Ortega, P., Robson, J.I., Brierley, C.M., Davis, R., Hall, I.R., Moffa-Sanchez, P., Rose, N.L., Spooner, P.T., Yashayaev, I. and Keigwin, L.D., 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature, 556(7700): 227-230.</p>

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>

A centennial-scale Arctic - North Atlantic recharge oscillator in a coupled climate model

2021 ◽  
Author(s):  
Robin Waldman ◽  
Christophe Cassou ◽  
Aurore Voldoire
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Climate Model ◽  
Natural Variability ◽  
Meridional Overturning Circulation ◽  
The Arctic ◽  
Western Boundary ◽  
Coupled Climate Model ◽  
Overturning Circulation ◽  
Salinity Variability

<p>In global climate models, low-frequency natural variability related to the Atlantic Ocean overturning circulation is a common behaviour. Such intrinsic climate variability is a potential source of decadal climate predictability. However, over longer term scenario simulations, this natural variability becomes a major source of uncertainty. In this study, we document a large and sustained centennial variability in the 3500-year pre-industrial control run of the CNRM-CM6 coupled climate model which is driven by the North Atlantic ocean, and more specifically its meridional overturning circulation (AMOC). We propose a new AMOC dynamical decomposition highlighting the dominant role of mid-depth density anomalies at the western boundary as the driver of this centennial variability. We relate such density variability to deep convection and overflows in the western subpolar gyre, themselves controlled by and intense salinity variability of the upper layers. Finally, we show that such salinity variability is the result of periodic freshwater recharge and descharge events from the Arctic Ocean, themselves triggered by stochastic atmospheric forcing.</p>

scholarly journals Investigating the impact of Lake Agassiz drainage routes on the 8.2 ka cold event with a climate model

2009 ◽  
Vol 5 (3) ◽  
pp. 471-480 ◽  
Author(s):  
Y.-X. Li ◽  
H. Renssen ◽  
A. P. Wiersma ◽  
T. E. Törnqvist
Keyword(s):  
Atlantic Ocean ◽  
North Atlantic ◽  
Climate Model ◽  
North Atlantic Ocean ◽  
Sensitivity Study ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Cold Event ◽  
The North ◽  
The Impact

Abstract. The 8.2 ka event is the most prominent abrupt climate change in the Holocene and is often believed to result from catastrophic drainage of proglacial lakes Agassiz and Ojibway (LAO) that routed through the Hudson Bay and the Labrador Sea into the North Atlantic Ocean, and perturbed Atlantic meridional overturning circulation (MOC). One key assumption of this triggering mechanism is that the LAO freshwater drainage was dispersed over the Labrador Sea. Recent data, however, show no evidence of lowered δ18O values, indicative of low salinity, from the open Labrador Sea around 8.2 ka. Instead, negative δ18O anomalies are found close to the east coast of North America, extending as far south as Cape Hatteras, North Carolina, suggesting that the freshwater drainage may have been confined to a long stretch of continental shelf before fully mixing with North Atlantic Ocean water. Here we conduct a sensitivity study that examines the effects of a southerly drainage route on the 8.2 ka event with the ECBilt-CLIO-VECODE model. Hosing experiments of four routing scenarios, where freshwater was introduced to the Labrador Sea in the northerly route and to three different locations along the southerly route, were performed to investigate the routing effects on model responses. The modeling results show that a southerly drainage route is possible but generally yields reduced climatic consequences in comparison to those of a northerly route. This finding implies that more freshwater would be required for a southerly route than for a northerly route to produce the same climate anomaly. The implicated large amount of LAO drainage for a southerly routing scenario is in line with a recent geophysical modelling study of gravitational effects on sea-level change associated with the 8.2 ka event, which suggests that the volume of drainage might be larger than previously estimated.

Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation

2008 ◽  
Vol 38 (9) ◽  
pp. 1913-1930 ◽  
Author(s):  
Armin Köhl ◽  
Detlef Stammer
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Special Focus ◽  
Western Boundary ◽  
Consistent Estimate ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.

scholarly journals Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation

2011 ◽  
Vol 7 (1) ◽  
pp. 319-338 ◽  
Author(s):  
A. Bozbiyik ◽  
M. Steinacher ◽  
F. Joos ◽  
T. F. Stocker ◽  
L. Menviel
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Carbon Stocks ◽  
Meridional Overturning Circulation ◽  
Climate Conditions ◽  
Tropical Regions ◽  
Proxy Records ◽  
The North

Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes qualitatively in agreement with results documented in the literature, but there is a clear distinction between northern and southern perturbations. Changes in the physical variables, in turn, affect the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 concentration by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large reorganizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results. The proxy records, in general, show good agreement with the model's response to a North Atlantic freshwater perturbation.

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