Larger Role for Shallow Intermediate Waters in Ocean Circulation

The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of poleward heat transport and an important part of the global climate system. How it responded to past changes inforcing, such as experienced during Quaternary interglacials, is an intriguing and open question. Previous modelling studies suggest an enhanced AMOC in the mid-Holocene compared to the pre-industrial period. In previous simulations from the Palaeoclimate Modelling Intercomparison Project (PMIP), this arose from feedbacks between sea ice and AMOC changes, which also depended on resolution. Here I present aninitial analysis of the recently available PMIP4 simulations. This shows the overall strength of the AMOC does not markedly change between the mid-Holocene and piControl experiments (at least looking at the maximum of the mean meridional mass overturning streamfunction below 500m at 30oN and 50oN). This is not inconsistent with the proxy reconstructions using sortable silt and Pa/Th for the mid-Holocene. Here we analyse changes in the spatial structure of the meridional overturning circulation, along with their fingerprints on the surface temperature (computed through regression). We then estimate the percentage of the simulated surface temperature changes between the mid-Holocene and pre-industrial period that can be explained by AMOC. Furthermore, the analysis for the changes in the AMOC spatial structure has been extended to see if the same patterns of change hold for the last interglacial. The simulations will be compared to existing proxy reconstructions, as well as new palaeoceanographic reconstructions.

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Observed Evidence of a Stable Atlantic Meridional Overturning Circulation since the 1990s

10.5194/egusphere-egu2020-8758 ◽

2020 ◽

Author(s):

Yao Fu ◽

Feili Li ◽

Johannes Karstensen ◽

N. Penny Holliday ◽

Chunzai Wang

Keyword(s):

North Atlantic ◽

Global Climate ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Distribution Model ◽

Overturning Circulation ◽

State Estimate ◽

The Past ◽

Stable Partition ◽

Meridional Overturning

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important in the global climate system due to its role in the meridional heat and freshwater distribution. Model simulations and constructed AMOC indices suggest that the AMOC may have been weakening for decades. However, direct AMOC observations, introduced in 2004 in the subtropics (the RAPID program) and in 2014 in the subpolar North Atlantic (the OSNAP program), are not sufficiently long to capture changes dating back to previous periods. Here we use repeated hydrographic sections in the subtropical and subpolar North Atlantic through the early 1990s to the mid-2010s, combined with a box inverse model that is constrained using satellite altimetry, to analyze hydrographic changes and the AMOC. In combination with a state-of-the-art ocean state estimate, GECCO2, we show that despite dramatic hydrographic changes in the subtropical and subpolar North Atlantic over the past two and half decades, the AMOC has not significantly weakened over the same period. Our hydrography-based estimates also illustrate a remarkably stable partition of the subpolar overturning between the Labrador basin and the eastern subpolar basins on decadal timescales since the 1990s.

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Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

Journal of Climate ◽

10.1175/jcli-d-12-00478.1 ◽

2013 ◽

Vol 26 (18) ◽

pp. 7167-7186 ◽

Cited By ~ 45

Author(s):

Carl Wunsch ◽

Patrick Heimbach

Keyword(s):

Ocean Circulation ◽

Extreme Values ◽

Statistical Significance ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Western Boundary ◽

Boundary Current ◽

Gaussian Stochastic Process ◽

Overturning Circulation ◽

Meridional Overturning

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

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Latest Pliocene Northern Hemisphere glaciation amplified by intensified Atlantic meridional overturning circulation

Communications Earth & Environment ◽

10.1038/s43247-020-00023-4 ◽

2020 ◽

Vol 1 (1) ◽

Author(s):

Tatsuya Hayashi ◽

Toshiro Yamanaka ◽

Yuki Hikasa ◽

Masahiko Sato ◽

Yoshihiro Kuwahara ◽

...

Keyword(s):

Northern Hemisphere ◽

North Atlantic ◽

Deep Water ◽

Global Climate ◽

Atlantic Meridional Overturning Circulation ◽

North Atlantic Deep Water ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Early Stages ◽

Meridional Overturning

Abstract The global climate has been dominated by glacial–interglacial variations since the intensification of Northern Hemisphere glaciation 2.7 million years ago. Although the Atlantic meridional overturning circulation has exerted strong influence on recent climatic changes, there is controversy over its influence on Northern Hemisphere glaciation because its deep limb, North Atlantic Deep Water, was thought to have weakened. Here we show that Northern Hemisphere glaciation was amplified by the intensified Atlantic meridional overturning circulation, based on multi-proxy records from the subpolar North Atlantic. We found that the Iceland–Scotland Overflow Water, contributing North Atlantic Deep Water, significantly increased after 2.7 million years ago and was actively maintained even in early stages of individual glacials, in contrast with late stages when it drastically decreased because of iceberg melting. Probably, the active Nordic Seas overturning during the early stages of glacials facilitated the efficient growth of ice sheets and amplified glacial oscillations.

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Sensitivity of the Atlantic Meridional Overturning Circulation to Model Resolution in CMIP6 HighResMIP Simulations

10.5194/egusphere-egu2020-18216 ◽

2020 ◽

Author(s):

Dorotea Iovino ◽

Malcolm J. Roberts ◽

Laura C. Jackson ◽

Christopher D. Roberts ◽

Virna Meccia ◽

...

Keyword(s):

Heat Transport ◽

Ocean Circulation ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

The Arctic ◽

Climate Modelling ◽

Florida Current ◽

Model Resolution ◽

Overturning Circulation ◽

Meridional Overturning

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the three-dimensional ocean circulation that transports warm and salty water northward, and exports cold and dense water from the Arctic southward.The simulated AMOC in Coupled Model Intercomparison Project models (both coupled and ocean-only) has been studied extensively. However, correctly simulating the AMOC with these models remains a challenge for the climate modelling community. One model aspect that can affect the AMOC representation is the model&#160;resolution (i.e. grid spacing).Here, we examine key aspects of the North Atlantic Ocean circulation using a multi-model, multi-resolution ensemble based on the CMIP6 HighResMIP coupled experiments.&#160;The AMOC and associated heat transport tend to become stronger as&#160;model resolution increases, particularly when the ocean resolution changes from non-eddying to eddy-present and eddy-rich. However, the circulation remains too&#160;shallow compared to observations for most models, and this, together with temperature biases, cause the northward&#160;heat transport to be too low for a given overturning&#160;strength.In the period 2015-2050, the overturning circulation tends to decline more rapidly in the higher&#160;resolution models by more than 20% compared to the control state, which is related to both themean state and to the subpolar gyre contribution to deep water formation. The main part of the&#160;decline comes from the Florida Current component of the circulation.

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Nonlinear Effects of Wind on Atlantic Ocean Circulation

Eos ◽

10.1029/2022eo220013 ◽

2022 ◽

Vol 103 ◽

Author(s):

Jack Lee

Keyword(s):

Atlantic Ocean ◽

Wind Stress ◽

Ocean Circulation ◽

Nonlinear Effects ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning

Simulations reveal the influence of reduced and enhanced wind stress on the Atlantic Meridional Overturning Circulation.

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

Science Advances ◽

10.1126/sciadv.abc7836 ◽

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.

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Theoretical Models Advance Knowledge of Ocean Circulation

Eos ◽

10.1029/2019eo132047 ◽

2019 ◽

Vol 100 ◽

Author(s):

Sarah Stanley

Keyword(s):

Ocean Circulation ◽

Theoretical Models ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Future Research ◽

Overturning Circulation ◽

Meridional Overturning ◽

Advance Knowledge

A review of recent advancements highlights key insights into the Atlantic Meridional Overturning Circulation and what might be in store for future research.

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Adjustment of the global climate to an abrupt slowdown of the Atlantic meridional overturning circulation

Ocean Circulation: Mechanisms and Impacts—Past and Future Changes of Meridional Overturning - Geophysical Monograph Series ◽

10.1029/173gm19 ◽

2007 ◽

pp. 295-313 ◽

Cited By ~ 28

Author(s):

Wei Cheng ◽

Cecilia M. Bitz ◽

John C. H. Chiang

Keyword(s):

Global Climate ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning

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An Indicator of the Multiple Equilibria Regime of the Atlantic Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/2009jpo4215.1 ◽

2010 ◽

Vol 40 (3) ◽

pp. 551-567 ◽

Cited By ~ 74

Author(s):

Selma E. Huisman ◽

Matthijs den Toom ◽

Henk A. Dijkstra ◽

Sybren Drijfhout

Keyword(s):

Ocean Circulation ◽

Circulation Model ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Multiple Equilibrium ◽

Global Ocean ◽

Overturning Circulation ◽

Freshwater Transport ◽

Freshwater Budget ◽

Meridional Overturning

Abstract Recent model results have suggested that there may be a scalar indicator Σ monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity Σ is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of Σ is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to Σ. The sign of Σ precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.

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