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1. Arctic freshwater impact on the Atlantic Meridional Overturning Circulation: status and prospects

   https://doi.org/10.1098/rsta.2022.0185  

   Haine, Thomas W. ; Siddiqui, Ali H. ; Jiang, Wenrui ( December 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   Arguably, the most conspicuous evidence for anthropogenic climate change lies in the Arctic Ocean. For example, the summer-time Arctic sea ice extent has declined over the last 40 years and the Arctic Ocean freshwater storage has increased over the last 30 years. Coupled climate models project that this extra freshwater will pass Greenland to enter the sub-polar North Atlantic Ocean (SPNA) in the coming decades. Coupled climate models also project that the Atlantic Meridional Overturning Circulation (AMOC) will weaken in the twenty-first century, associated with SPNA buoyancy increases. Yet, it remains unclear when the Arctic anthropogenic freshening signal will be detected in the SPNA, or what form the signal will take. Therefore, this article reviews and synthesizes the state of knowledge on Arctic Ocean and SPNA salinity variations and their causes. This article focuses on the export processes in data-constrained ocean circulation model hindcasts. One challenge is to quantify and understand the relative importance of different competing processes. This article also discusses the prospects to detect the emergence of Arctic anthropogenic freshening and the likely impacts on the AMOC. For this issue, the challenge is to distinguish anthropogenic signals from natural variability.

   This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’.

    

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2. The Mechanisms of the Atlantic Meridional Overturning Circulation Slowdown Induced by Arctic Sea Ice Decline

   https://doi.org/10.1175/JCLI-D-18-0231.1  

   Liu, Wei ; Fedorov, Alexey ; Sévellec, Florian ( February 2019 , Journal of Climate)

   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.

    

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3. Impact of industrial versus biomass burning aerosols on the Atlantic Meridional Overturning Circulation

   https://doi.org/10.1038/s41612-024-00602-8  

   Allen, Robert J. ; Vega, Claire ; Yao, Eva ; Liu, Wei ( March 2024 , npj Climate and Atmospheric Science)

   The ocean’s major circulation system, the Atlantic Meridional Overturning Circulation (AMOC), is slowing down. Such weakening is consistent with warming associated with increasing greenhouse gases, as well as with recent decreases in industrial aerosol pollution. The impact of biomass burning aerosols on the AMOC, however, remains unexplored. Here, we use the Community Earth System Model version 1 Large Ensemble to quantify the impact of both aerosol types on the AMOC. Despite relatively small changes in North Atlantic biomass burning aerosols, significant AMOC evolution occurs, including weakening from 1920 to ~1970 followed by AMOC strengthening. These changes are largely out of phase relative to the corresponding AMOC evolution under industrial aerosols. AMOC responses are initiated by thermal changes in sea surface density flux due to altered shortwave radiation. An additional dynamical mechanism involving the North Atlantic sea-level pressure gradient is important under biomass-burning aerosols. AMOC-induced ocean salinity flux convergence acts as a positive feedback. Our results show that biomass-burning aerosols reinforce early 20th-century AMOC weakening associated with greenhouse gases and also partially mute industrial aerosol impacts on the AMOC. Recent increases in wildfires suggest biomass-burning aerosols may be an important driver of future AMOC variability.

    

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4. The abyssal origins of North Atlantic decadal predictability

   https://doi.org/10.1007/s00382-020-05382-4  

   Yeager, Stephen ( July 2020 , Climate Dynamics)

   The fundamental mechanisms that explain high subpolar North Atlantic (SPNA) decadal predictability within a particular modeling framework are described. The focus is on the Community Earth System Model (CESM), run in both a historical forced-ocean configuration as well as in a fully coupled configuration initialized from the former. The initialized prediction experiments comprise the CESM Decadal Prediction Large Ensemble (CESM-DPLE)—a 40-member set of retrospective hindcasts documented in Yeager et al. (Bull Am Meteorol Soc 99:1867–1886. https://doi.org/10.1175/bams-d-17-0098.1, 2018). Heat budget analysis confirms the driving role of advective heat convergence in skillful prediction of SPNA upper ocean heat content out to decadal lead times. The key ocean dynamics are topographically-coupled overturning/gyre fluctuations that are geographically centered over the mid-Atlantic ridge (MAR). Long-lasting predictive skill for ocean heat transport can be related to predictable barotropic gyre and sigma-coordinate AMOC circulations, but depth-coordinate AMOC is far less predictable except in the deepest layers. The foundation of ocean memory (and circulation predictive skill) in CESM-DPLE is Labrador Sea Water thickness, which propagates predictably through interior pathways towards the MAR where large anomalies accumulate and persist. Abyssal thickness anomalies drive predictable decadal changes in the gyre circulation, including changes in sea level gradient and near surface flow, that account for the high predictability of SPNA upper ocean heat content. 

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5. Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

   https://doi.org/10.1007/s00382-022-06386-y  

   Couldrey, Matthew P. ; Gregory, Jonathan M. ; Dong, Xiao ; Garuba, Oluwayemi ; Haak, Helmuth ; Hu, Aixue ; Hurlin, William J. ; Jin, Jiangbo ; Jungclaus, Johann ; Köhl, Armin ; et al ( August 2022 , Climate Dynamics)

   The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content ($$\Delta$$$\Delta$OHC), and the spatial pattern of ocean dynamic sea level ($$\Delta \zeta$$$\Delta \zeta$). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling$$\hbox {CO}_{{2}}$$${\text{CO}}_2$. Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of$$\Delta$$$\Delta$OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects$$\Delta \zeta$$$\Delta \zeta$in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean$$\Delta \zeta$$$\Delta \zeta$and$$\Delta$$$\Delta$OHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.

    

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