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...

Variations in the strength of the Atlantic meridional overturning circulation (AMOC) are a major potential source of decadal and longer climate variability in the Atlantic. This study analyzes continuous integrations of tangent linear and adjoint versions of an ocean general circulation model [Océan Parallélisé (OPA)] and rigorously shows the existence of a weakly damped oscillatory eigenmode of the AMOC centered in the North Atlantic Ocean and controlled solely by linearized ocean dynamics. In this particular GCM, the mode period is roughly 24 years, its e-folding decay time scale is 40 years, and it is the least-damped oscillatory mode in the system. Its mechanism is related to the westward propagation of large-scale temperature anomalies in the northern Atlantic in the latitudinal band between 308 and 608N. The westward propagation results from a competition among mean eastward zonal advection, equivalent anomalous westward advection caused by the mean meridional temperature gradient, and westward propagation typical of long baroclinic Rossby waves. The zonal structure of temperature anomalies alternates between a dipole (corresponding to an anomalous AMOC) and anomalies of one sign (yielding no changes in the AMOC). Further, it is shown that the system is nonnormal, which implies that the structure of the least-damped eigenmode of the tangent linear model is different from that of the adjoint model. The ''adjoint'' mode describes the sensitivity of the system (i.e., it gives the most efficient patterns for exciting the leading eigenmode). An idealized model is formulated to highlight the role of the background meridional temperature gradient in the North Atlantic for the mode mechanism and the system nonnormality.

International audienceUnderstanding the preferential timescales of variability in the North Atlantic, usually associated with the Atlantic meridional overturning circulation (AMOC), is essential for the prospects for decadal prediction. However, the wide variety of mechanisms proposed from the analysis of climate simulations, potentially dependent on the models themselves, has stimulated the debate of which processes take place in reality. One mechanism receiving increasing attention, identified both in idealized models and observations, is a westward propagation of subsurface buoyancy anomalies that impact the AMOC through a basin-scale intensification of the zonal density gradient, enhancing the northward transport via thermal wind balance. In this study, we revisit a control simulation from the Institut Pierre-Simon Laplace Coupled Model 5A (IPSL-CM5A), characterized by a strong AMOC periodicity at 20 years, previously explained by an upper ocean–atmosphere–sea ice coupled mode driving convection activity south of Iceland. Our study shows that this mechanism interacts constructively with the basin-wide propagation in the subsurface. This constructive feedback may explain why bi-decadal variability is so intense in this coupled model as compared to others

We explore the mechanisms by which Arctic sea ice decline affects the Atlantic meridional overturning circulation (AMOC) in a suite of numerical experiments perturbing the Arctic sea ice radiative budget within a fully coupled climate model. The imposed perturbations act to increase the amount of heat available to melt ice, leading to a rapid Arctic sea ice retreat within 5 years after the perturbations are activated. In response, the AMOC gradually weakens over the next ~100 years. The AMOC changes can be explained by the accumulation in the Arctic and subsequent downstream propagation to the North Atlantic of buoyancy anomalies controlled by temperature and salinity. Initially, during the first decade or so, the Arctic sea ice loss results in anomalous positive heat and salinity fluxes in the subpolar North Atlantic, inducing positive temperature and salinity anomalies over the regions of oceanic deep convection. At first, these anomalies largely compensate one another, leading to a minimal change in upper ocean density and deep convection in the North Atlantic. Over the following years, however, more anomalous warm water accumulates in the Arctic and spreads to the North Atlantic. At the same time, freshwater that accumulates from seasonal sea ice melting over most of the upper Arctic Ocean also spreads southward, reaching as far as south of Iceland. These warm and fresh anomalies reduce upper ocean density and suppress oceanic deep convection. The thermal and haline contributions to these buoyancy anomalies, and therefore to the AMOC slowdown during this period, are found to have similar magnitudes. We also find that the related changes in horizontal wind-driven circulation could potentially push freshwater away from the deep convection areas and hence strengthen the AMOC, but this effect is overwhelmed by mean advection.