Arctic sea-ice decline weakens the Atlantic Meridional Overturning Circulation

Abstract

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...

Asymmetric Cooling of the Atlantic and Pacific Arctic During the Past Two Millennia: A Dual Observation‐Modeling Study

“…However, the volcanic forcing is also generally hemispherically symmetric in the high latitudes. However, as other studies have shown, this feedback loop could be sensitive to the mean distribution of sea ice (Sevellec et al, 2017;Zhong et al, 2010) and affected by air-sea interactions (Suo et al, 2017;Zhu et al, 2015). Previous model studies have shown that sea ice expansion in response to the sequence of decadal-spaced volcanic eruptions during the late thirteenth century suppressed deep water formation in the northern North Atlantic, resulting in a weakened AMOC (Slawinska & Robock, 2018;Zhong et al, 2010).…”

Stability of the Atlantic Meridional Overturning Circulation: A Review and Synthesis

“…Another positive feedback is the subsequent increase in sea ice cover (Drijfhout, ; Liu et al, ; Vellinga et al, ). The problem of the two‐way interaction between the AMOC and Arctic sea ice and its effect on the AMOC stability is an area of ongoing research (Liu & Fedorov, ; Liu et al, ; Sévellec et al, ; Sun et al, ).…”

“…We believe that this review is timely for several reasons. First of all, climate change continues at a relentless pace and strongly affects the freshwater budget of the Arctic and subpolar North Atlantic, for instance, through impacts on sea ice cover (e.g., Sévellec et al, ; Stroeve et al, ), the Greenland Ice Sheet mass balance (e.g., Enderlin et al, ), and enhanced river discharge in the Arctic watershed (e.g., Peterson et al, ). A rigorous assessment of the resilience of the AMOC to these changes is critical, given that the AMOC is thought to be most sensitive to perturbations in the freshwater budget of the subpolar North Atlantic, and such perturbations have been implicated in slowdowns, or even shutdowns, of the AMOC in the past (Lynch‐Stieglitz, ).…”

Projected Freshening of the Arctic Ocean in the 21st Century

“…Upper layer freshwater in the Arctic Ocean maintains strong near‐surface stratification, which supports Arctic sea ice formation (Aagaard & Coachman, ). Changes in freshwater export from the Arctic could also potentially influence both deep water formation in the North Atlantic and the Atlantic Meridional Overturning Circulation (Aagaard et al, ; Aagaard and Carmack, ; Haak et al, ; Haine et al, ; Holland et al, ; Jahn & Holland, ; Jungclaus et al, ; Nummelin et al, ; Sévellec et al, ; Thornalley et al, ).…”