Dynamical Attribution of Recent Variability in Atlantic Overturning

Pillar, Helen; Heimbach, Patrick; Johnson, H. L.; Marshall, David P.

doi:10.1175/jcli-d-15-0727.1

Cited by 58 publications

(110 citation statements)

References 69 publications

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“…The computations use the tangent linear and adjoint versions of the NEMO ocean model 20 in its ORCA2 configuration (2 • ×2 • horizontal resolution and 31 vertical levels). Although this configuration is relatively coarse, the general approach represents state-of-the-art in adjoint ocean modeling 16,18,19 . In addition, this resolution allows filtering out baroclinic instability that would contaminate the solutions otherwise.…”

Section: Discussionmentioning

confidence: 99%

Arctic sea-ice decline weakens the Atlantic Meridional Overturning Circulation

2017

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The ongoing decline of Arctic sea ice 1, 2 exposes the ocean to anomalous surface heat and freshwater fluxes, resulting in positive buoyancy anomalies that can affect ocean circulation.In this study, we use an optimal flux perturbation framework and comprehensive climate model simulations to estimate the sensitivity of the Atlantic meridional overturning circulation (AMOC) to such buoyancy forcing over the Arctic and globally, and more generally to sea ice decline. It is found that on decadal timescales flux anomalies over the subpolar North Atlantic have the largest impact on the AMOC, while on multi-decadal timescales (longer than 20 years), flux anomalies in the Arctic become more important. These positive buoyancy anomalies spread to the North Atlantic, weakening the AMOC and its poleward heat transport. Therefore, the Arctic sea ice decline may explain the suggested slow-down of the AMOC and the "Warming Hole" 3, 4 persisting in the subpolar North Atlantic. 1 Observations of climate change in the Arctic ocean and the North AtlanticGlobal climate change is now affecting various components of the Earth's climate system. In particular, the extent of Arctic sea ice has been declining over the past several decades 1, 2 , with an annual-mean areal reduction of ∼20% since 1980 ( Fig. 1) and even stronger decrease in September (∼30%). At the same time, the Atlantic Meridional Overturning Circulation (AMOC), a crucial component of oceanic circulation monitored over the past decade by the RAPID array 5 at 26.5 • N, is arguably slowing down 6 at a rate as high as 0.4 Sv yr −1 (Fig. 2a). Although the attribution of this recent AMOC slow-down remains an open question 7, 8 in view of oceanic natural decadal variability 9 , indirect evidence based on the proxies of AMOC strength 4 supports the hypothesis that the AMOC is gradually weakening (Fig. 2b) as part of ongoing climate change. Complementary to these present-day observations, numerical experiments using state-of-the-art climate models under future CO 2 emission scenarios consistently predict a gradual AMOC slow-down during this century 10 .The long-term AMOC decline has been conjectured to cause the so-called Warming Hole persisting in the subpolar North Atlantic 3, 4 especially pronounced between 50 and 60 • N ( Fig. 2c and S1). Indeed, the weakening of oceanic poleward heat transport associated with this decline is arguably the most likely explanation for why this region is warming at a slower rate than the rest of the globe (or possibly even cooling down). This relative cooling moderates local impacts of anthropogenically forced climate change over the ocean 11 .Nevertheless, beyond general conceptual understanding of this mechanism 12 , there exists 2 no agreement on the exact causes of the AMOC slow-down and the Warming Hole, nor their attribution to a particular external forcing. The main goal of the present study is to investigate whether these phenomena could be driven by the ongoing Arctic climate change. AMOC sensitivity to Arctic surface heat and freshw...

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Section: Discussionmentioning

confidence: 99%

Arctic sea-ice decline weakens the Atlantic Meridional Overturning Circulation

2017

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“…This quantification is most efficiently accomplished using an adjoint modeling approach that provides the linear sensitivity of the AMOC at a single latitude to changes in surface forcing over the globe, for all forcing lead times (Pillar et al 2016). Sensitivity glider sampled a strong anticyclonic eddy between the M3 and M4 moorings (Fig.…”

Section: Hydrography Across the Irminger And Labrador Seasmentioning

confidence: 99%

“…In particular, a break point in coherence may occur at the subpolar-subtropical gyre boundary in the North Atlantic (Bingham et al 2007;Baehr et al 2009). Furthermore, a recent modeling study has suggested that the low-frequency variability of the RAPID-MOCHA appears to be an integrated response to buoyancy forcing over the subpolar gyre (Pillar et al 2016). Thus, a measure of the overturning in the subpolar basin contemporaneous with a measure of the buoyancy forcing in that basin likely offers the best possibility of understanding the mechanisms that underpin AMOC variability.…”

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confidence: 99%

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

Lozier

Bacon

Bower

et al. 2017

Bulletin of the American Meteorological Society

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190

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For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017.

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“…Given limited resources, the ASTE adjoint sensitivity tool (e.g., Heimbach et al, 2011;Pillar et al, 2016) can be used to identify optimal sites for float deployments in data-deficient regions such as the eastern Arctic as follows. First, a target quantity of interest (QoI), such as the mean salinity of Box B within a depth range covering the AW layer (145-250 m, green box in Figure 2c), is defined.…”

Section: The Need For Additional Alps Data For Observing-network Designmentioning

confidence: 99%