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

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.

Download Full-text

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

Earth System Dynamics ◽

10.5194/esd-12-419-2021 ◽

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.

Download Full-text

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

10.5194/egusphere-egu21-5324 ◽

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

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.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.&#160;ReferencesBooth, 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&#8211;232.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.Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-L&#243;pez, C., H&#225;t&#250;n, 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.Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., M&#252;ller, 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.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.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.

Download Full-text

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

10.5194/egusphere-egu21-5888 ◽

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

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.

Download Full-text

Pending recovery in the strength of the meridional overturning circulation at 26° N

10.5194/os-2019-134 ◽

2020 ◽

Author(s):

Ben I. Moat ◽

David A. Smeed ◽

Eleanor Frajka-Williams ◽

Damien G. Desbruyères ◽

Claudie Beaulieu ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Ocean Reanalysis ◽

Buoyancy Forcing ◽

Modelling Studies ◽

Meridional Overturning ◽

Lower Magnitude

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26° N has now been continuously measured by the RAPID array over the period Apr 2004–Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy-forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45° N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26° N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26° N is higher than its previous low. Extending the record at 26° N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45° N by 0–2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

Download Full-text

The Origins of Late-Twentieth-Century Variations in the Large-Scale North Atlantic Circulation

Journal of Climate ◽

10.1175/jcli-d-13-00125.1 ◽

2014 ◽

Vol 27 (9) ◽

pp. 3222-3247 ◽

Cited By ~ 69

Author(s):

Stephen Yeager ◽

Gokhan Danabasoglu

Keyword(s):

Twentieth Century ◽

North Atlantic ◽

Decadal Variability ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Overturning Circulation ◽

The North ◽

Late Twentieth ◽

The North Atlantic ◽

Late Twentieth Century

Abstract Surface forcing perturbation experiments are examined to identify the key forcing elements associated with late-twentieth-century interannual-to-decadal Atlantic circulation variability as simulated in an ocean–sea ice hindcast configuration of the Community Earth System Model, version 1 (CESM1). Buoyancy forcing accounts for most of the decadal variability in both the Atlantic meridional overturning circulation (AMOC) and the subpolar gyre circulation, and the key drivers of these basin-scale circulation changes are found to be the turbulent buoyancy fluxes: evaporation as well as the latent and sensible heat fluxes. These three fluxes account for almost all of the decadal AMOC variability in the North Atlantic, even when applied only over the Labrador Sea region. Year-to-year changes in surface momentum forcing explain most of the interannual AMOC variability at all latitudes as well as most of the decadal variability south of the equator. The observed strengthening of Southern Ocean westerly winds accounts for much of the simulated AMOC variability between 30°S and the equator but very little of the recent AMOC change in the North Atlantic. Ultimately, the strengthening of the North Atlantic overturning circulation between the 1970s and 1990s, which contributed to a pronounced SST increase at subpolar latitudes, is explained almost entirely by trends in the atmospheric surface state over the Labrador Sea.

Download Full-text

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

Journal of Climate ◽

10.1175/jcli-d-15-0007.1 ◽

2015 ◽

Vol 28 (19) ◽

pp. 7659-7677 ◽

Cited By ~ 8

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.

Download Full-text

Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models

10.5194/egusphere-egu2020-2981 ◽

2020 ◽

Author(s):

Susan Lozier ◽

Matthew Menary ◽

Laura Jackson

Keyword(s):

North Atlantic ◽

Climate Models ◽

Climate Model ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Overturning Circulation ◽

Important Indicator ◽

The North ◽

North Atlantic Subpolar Gyre ◽

Meridional Overturning

The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.

Download Full-text

Pending recovery in the strength of the meridional overturning circulation at 26°N

10.5194/egusphere-egu2020-5785 ◽

2020 ◽

Author(s):

Ben Moat ◽

David Smeed ◽

Eleanor Frajka-Williams ◽

Damien Desbruyeres ◽

Claudie Beaulieu ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Large Scale ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Ocean Reanalysis ◽

Buoyancy Forcing ◽

Modelling Studies ◽

Meridional Overturning ◽

Lower Magnitude

<div> <div> <div> The strength of the Atlantic meridional overturning circulation (AMOC) at 26&#176;N has now been continuously measured by the RAPID array over the period April 2004 - Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008-2010. In 2013-2015, a period of strong buoyancy- forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3-9 years. At 45&#176;N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26&#176;N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26&#176;N is higher than its previous low. Extending the record at 26&#176;N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45&#176;N by 0-2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering. </div> </div> </div>

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

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

10.5194/egusphere-egu21-1164 ◽

2021 ◽

Author(s):

Tomas Jonathan ◽

Mike Bell ◽

Helen Johnson ◽

David Marshall

Keyword(s):

Global Climate ◽

Dominant Role ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Relative Role ◽

Overturning Circulation ◽

Near Surface ◽

Boundary Density ◽

Meridional Overturning

The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting&#160; potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model&#160; control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales.&#160;The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.

Download Full-text