Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Johnson, H. L.; Cessi, Paola; Marshall, David P.; Schloesser, Fabian; Spall, Michael A.

doi:10.1029/2019jc015330

Cited by 89 publications

(89 citation statements)

References 190 publications

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“…5). These waves appear in the Hovmöller diagram to be slower than predicted due to the decaying amplitude of Kelvin waves, which radiate energy into the interior via westward propagating Rossby waves (Johnson and Marshall 2002). The southward Kelvin wave arrives at the southern tip of Africa (''D'' in Fig.…”

Section: A Transient Response To a Step Change In Nadw Formationmentioning

confidence: 94%

“…The reduced gravity model represents the upper northward branch of the AMOC (Fig. 1) and is similar to models used in previous studies (e.g., Johnson and Marshall 2004;Allison et al 2011;Jones and Cessi 2016). However, this 1.5-layer reduced gravity model omits a number of key components of the global ocean overturning circulation, including a realistic surface forcing in the North Atlantic, which modifies the NADW formation rate, and a realistic surface schematic of the global ocean overturning circulation, adapted from Talley (2013).…”

Section: Model Description and Methodsmentioning

confidence: 99%

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Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

Sun

Thompson

Eisenman

2020

Journal of Physical Oceanography

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Climate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.

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Section: A Transient Response To a Step Change In Nadw Formationmentioning

confidence: 94%

Section: Model Description and Methodsmentioning

confidence: 99%

Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

Sun

Thompson

Eisenman

2020

Journal of Physical Oceanography

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“…However, whether the current generation of low‐resolution climate models adequately simulates the sensitivity of the AMOC to a warming climate is still a matter of debate (e.g., Caesar et al, ; Rahmstorf et al, ) with some authors suggesting that the sensitivity in nature could be higher than previously thought (Heuzé, ; Reintges et al, ). For more exhaustive overviews about the AMOC and mechanisms driving its strength and variability we refer the reader to comprehensive reviews such as Lumpkin and Speer (2007), Kuhlbrodt et al (), Buckley and Marshall (), Zhang et al (), Johnson et al (2019) and Weijer et al ().…”

Section: Introductionmentioning

confidence: 99%

The Atlantic Meridional Overturning Circulation in High‐Resolution Models

et al. 2020

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The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC. Key Points:• Observations and high-resolution models have changed view on the AMOC pathways • High-resolution models suggest the presence of previously unknown high-frequency AMOC variability • High-resolution models allow to estimate the intrinsic/chaotic component of the AMOCCorrespondence to:

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“…Most of the models considered in this study have relatively coarse resolution, including non-eddying oceans (≥ 1°x1°), which means that they might be missing some key dynamics for the AMOC (Johnson et al, 2019) that could be important to represent realistic linkages. The current analysis, however, includes two models at eddy-permitting resolution (HadGEM3-…”

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confidence: 99%

“…differences (Hirschi et al, 2020;Johnson et al, 2019). A recent analysis based on HadGEM3-GC3.1 (a later version of HadGEM3-GC2) configured at different horizontal resolutions has shown that long-standing model biases, including in the…”

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confidence: 99%

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

Ortega¹,

Robson²,

Menary³

et al. 2020

Preprint

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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.

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Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

Cited by 89 publications

References 190 publications

Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

The Atlantic Meridional Overturning Circulation in High‐Resolution Models

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

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