scholarly journals Fingerprints for Early Detection of Changes in the AMOC

2020 ◽  
Vol 33 (16) ◽  
pp. 7027-7044 ◽  
Author(s):  
L. C. Jackson ◽  
R. A. Wood
Keyword(s):  
Early Detection ◽  
Large Scale ◽  
Climate Model ◽  
Internal Variability ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
The Past ◽  
Vertical Density ◽  
Meridional Overturning ◽  
Additional Value

AbstractDifferent strategies have been proposed in previous studies for monitoring the Atlantic meridional overturning circulation (AMOC). As well as arrays to directly monitor the AMOC strength, various fingerprints have been suggested to represent an aspect of the AMOC based on properties such as temperature and density. The additional value of fingerprints potentially includes the ability to detect a change earlier than a change in the AMOC itself, the ability to extend a time series back into the past, and the ability to detect crossing a threshold. In this study we select metrics that have been proposed as fingerprints in previous studies and evaluate their ability to detect AMOC changes in a number of scenarios (internal variability, weakening from increased greenhouse gases, weakening from hosing and hysteresis) in the eddy-permitting coupled climate model HadGEM3-GC2. We find that the metrics that perform best are the temperature metrics based on large-scale differences, the large-scale meridional density gradient, and the vertical density difference in the Labrador Sea. The best metric for monitoring the AMOC depends somewhat on the processes driving the change. Hence the best strategy would be to consider multiple fingerprints to provide early detection of all likely AMOC changes.

scholarly journals Increased risk of near term global warming due to a recent AMOC weakening

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Rémy Bonnet ◽  
Didier Swingedouw ◽  
Guillaume Gastineau ◽  
Olivier Boucher ◽  
Julie Deshayes ◽  
...  
Keyword(s):  
Global Warming ◽  
Low Frequency ◽  
Internal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
The Past ◽  
Increased Risk ◽  
Meridional Overturning ◽  
Near Term ◽  
New Generation

AbstractSome of the new generation CMIP6 models are characterised by a strong temperature increase in response to increasing greenhouse gases concentration1. At first glance, these models seem less consistent with the temperature warming observed over the last decades. Here, we investigate this issue through the prism of low-frequency internal variability by comparing with observations an ensemble of 32 historical simulations performed with the IPSL-CM6A-LR model, characterized by a rather large climate sensitivity. We show that members with the smallest rates of global warming over the past 6-7 decades are also those with a large internally-driven weakening of the Atlantic Meridional Overturning Circulation (AMOC). This subset of members also matches several AMOC observational fingerprints, which are in line with such a weakening. This suggests that internal variability from the Atlantic Ocean may have dampened the magnitude of global warming over the historical era. Taking into account this AMOC weakening over the past decades means that it will be harder to avoid crossing the 2 °C warming threshold.

scholarly journals Pacific Shallow Meridional Overturning Circulation in a Warming Climate

2011 ◽  
Vol 24 (24) ◽  
pp. 6424-6439 ◽  
Author(s):  
Daiwei Wang ◽  
Mark A. Cane
Keyword(s):  
Surface Layer ◽  
Large Scale ◽  
Climate Model ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
First Century ◽  
Western Boundary ◽  
Warming Climate ◽  
Meridional Overturning ◽  
Twenty First Century

Abstract By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface-layer transport tend to be weakening (strengthening) in the Northern (Southern) Hemisphere as a response to large-scale surface wind changes over the tropical Pacific. The STC pycnocline transport convergence into the equatorial Pacific Ocean from higher latitudes shows a robust weakening in the twenty-first century. This weakening is mainly through interior pathways consistent with the relaxation of the zonal pycnocline tilt, whereas the transport change through western boundary pathways is small and not consistent across models. It is found that the change of the western boundary pycnocline transport is strongly affected by the shoaling of the pycnocline base. In addition, there is a robust weakening of the Indonesian Throughflow (ITF) transport in a warming climate. In the multimodel ensemble mean, the response to greenhouse warming of the upper-ocean mass balance associated with the STCs is such that the weakening of the equatorward pycnocline transport convergence is balanced by a weakening of the poleward surface-layer transport divergence and the ITF transport of similar amounts.

scholarly journals A stable Atlantic Meridional Overturning Circulation in a changing North Atlantic Ocean since the 1990s

2020 ◽  
Vol 6 (48) ◽  
pp. eabc7836
Author(s):  
Yao Fu ◽  
Feili Li ◽  
Johannes Karstensen ◽  
Chunzai Wang
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
North Atlantic Ocean ◽  
Global Climate ◽  
Global Climate Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The Past ◽  
Meridional Overturning

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important to global climate. Model simulations suggest that the AMOC may have been weakening over decades. However, existing array-based AMOC observations are not long enough to capture multidecadal changes. Here, we use repeated hydrographic sections in the subtropical and subpolar North Atlantic, combined with an inverse model constrained using satellite altimetry, to jointly analyze AMOC and hydrographic changes over the past three decades. We show that the AMOC state in the past decade is not distinctly different from that in the 1990s in the North Atlantic, with a remarkably stable partition of the subpolar overturning occurring prominently in the eastern basins rather than in the Labrador Sea. In contrast, profound hydrographic and oxygen changes, particularly in the subpolar North Atlantic, are observed over the same period, suggesting a much higher decoupling between the AMOC and ocean interior property fields than previously thought.

scholarly journals Distinct Modes of Internal Variability in the Global Meridional Overturning Circulation Associated with the Southern Hemisphere Westerly Winds

2012 ◽  
Vol 42 (5) ◽  
pp. 785-801 ◽  
Author(s):  
Wei Wei ◽  
Gerrit Lohmann ◽  
Mihai Dima
Keyword(s):  
Surface Temperature ◽  
Large Scale ◽  
Internal Variability ◽  
Meridional Overturning Circulation ◽  
Sea Surface ◽  
Earth System ◽  
Overturning Circulation ◽  
Westerly Winds ◽  
Meridional Overturning ◽  
Southern Hemisphere Westerly Winds

Abstract The internal variability of the global meridional overturning circulation (GMOC) in long-term integration of the earth system model Community Earth System Models (COSMOS) is examined in this study. Two distinct modes of the GMOC, which are closely linked to the Southern Hemisphere westerly winds (SWW) anomalies, are found to vary on multidecadal and centennial time scales. The dominant mode is characterized by Southern Ocean dynamics: strengthening and poleward shift of the SWW associated with a positive phase of the southern annular mode yield Ekman-induced northward mass transport, including a zonally asymmetric response in the Southern Ocean sea surface temperature and a cooling in the tropical Pacific Ocean due to large-scale upwelling. The second mode projects mainly onto the Atlantic meridional overturning circulation (AMOC). It is driven by a combination of SWW variation and buoyancy forcing. Based on the relationship between the two modes together with the wind perturbation experiments, the authors emphasize that the full AMOC response to the SWW change takes several centuries in their model. The sea surface temperature in Northern Hemisphere high latitudes is significantly affected in this mode, showing a large-scale warming. Their results from a mid-Holocene experiment imply that both modes are independent from the climate background conditions in the Holocene. Finally, the authors argue that the natural modes of GMOC are important to understand trends in ocean circulation, with consequences for heat and carbon budgets for past, present, and future climate.

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 On the Path of the Gulf Stream and the Atlantic Meridional Overturning Circulation

2010 ◽  
Vol 23 (11) ◽  
pp. 3146-3154 ◽  
Author(s):  
Terrence M. Joyce ◽  
Rong Zhang
Keyword(s):  
Gulf Stream ◽  
Climate Model ◽  
Numerical Models ◽  
Internal Variability ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
The Relationship ◽  
Ocean Observations

Abstract The Atlantic meridional overturning circulation (AMOC) simulated in various ocean-only and coupled atmosphere–ocean numerical models often varies in time because of either forced or internal variability. The path of the Gulf Stream (GS) is one diagnostic variable that seems to be sensitive to the amplitude of the AMOC, yet previous modeling studies show a diametrically opposed relationship between the two variables. In this note this issue is revisited, bringing together ocean observations and comparisons with the GFDL Climate Model version 2.1 (CM2.1), both of which suggest a more southerly (northerly) GS path when the AMOC is relatively strong (weak). Also shown are some examples of possible diagnostics to compare various models and observations on the relationship between shifts in GS path and changes in AMOC strength in future studies.

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

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

<p>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.</p>

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

Science ◽  
2019 ◽  
Vol 363 (6426) ◽  
pp. 516-521 ◽  
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.

Subpolar Gyre – AMOC – Atmosphere Interactions on Multidecadal Timescales in a Version of the Kiel Climate Model

2021 ◽  
Author(s):  
Jing Sun ◽  
Mojib Latif ◽  
Wonsun Park
Keyword(s):  
North Atlantic ◽  
Climate Model ◽  
Deep Convection ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Surface Salinity ◽  
The North ◽  
Ocean Coupling ◽  
Meridional Overturning ◽  
The North Atlantic

<p>There is a controversy about the nature of multidecadal climate variability in the North Atlantic (NA) region, concerning the roles of ocean circulation and atmosphere-ocean coupling. Here we describe NA multidecadal variability from a version of the Kiel Climate Model, in which both subpolar gyre (SPG)-Atlantic Meridional Overturning Circulation (AMOC) and atmosphere-ocean coupling are essential. The oceanic barotropic streamfuntions, meridional overturning streamfunctions, and sea level pressure are jointly analyzed to derive the leading mode of Atlantic variability. This mode accounting for about 23.7 % of the total combined variance is oscillatory with an irregular periodicity of 25-50 years and an e-folding time of about a decade. SPG and AMOC mutually influence each other and together provide the delayed negative feedback necessary for maintaining the oscillation. An anomalously strong SPG, for example, drives higher surface salinity and density in the NA’s sinking region. In response, oceanic deep convection and AMOC intensify, which, with a time delay of about a decade, reduces SPG strength by enhancing upper-ocean heat content. The weaker gyre circulation leads to lower surface salinity and density in the sinking region, which eventually reduces deep convection and AMOC strength. There is a positive ocean-atmosphere feedback between the sea surface temperature and low-level atmospheric circulation over the Southern Greenland area, with related wind stress changes reinforcing SPG changes, thereby maintaining the (damped) multidecadal oscillation against dissipation. Stochastic surface heat-flux forcing associated with the North Atlantic Oscillation drives the eigenmode.</p>

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