Understanding AMOC stability: the North Atlantic Hosing Model Intercomparison Project

<p>The Atlantic Meridional Overturning Circulation (AMOC) is thought to exist in multiple states of equilibria. In the present climate, the AMOC is believed to be in a relatively strong state, bringing warm waters into the North Atlantic and contributing to mild winters over Europe. However, proxy data show evidence of abrupt declines in the strength of the AMOC, often associated with the initiation of ice ages. The abrupt shifts in the strength of the AMOC are usually referred to as ‘tipping points’. Presently, state-of-the-art climate models are unable to spontaneously reproduce tipping points in the AMOC, preventing an accurate study of the climate impacts of an abrupt AMOC shutdown. Contextually, although it is deemed unlikely that the AMOC will collapse in response to climate change, it is expected to further slow down into the 21<sup>st</sup> century. The impacts of this weakening, relative to those of global warming, are poorly understood, especially on daily timescales.</p><p>            To address this question, we run water hosing experiments with the EC-Earth3 earth system model to investigate the impacts of an AMOC abrupt weakening on the winter climate variability focusing on the North Atlantic and Europe. We confirm results from previous studies showing a large decrease in temperature, precipitation, and an increase in the jet stream over Europe. However, we further investigate the moisture budget and the impacts on daily weather regimes and blocking. In contrast to previous hypotheses, we find that the reduction in precipitation over Europe is due to changes in the storm tracks rather than thermodynamic effects. Further, we find a significant increase in the frequency and persistence of NAO+ days. Finally, we show precipitation and temperature extremes that are expected in response to the AMOC weakening.</p><p>            Our results show the climate impacts on weather events that can be expected from an AMOC weakening alone, and are relevant to understanding the relative roles of greenhouse gas forcing and AMOC weakening on the European climate in simulations of future climate change.</p>

Climate change simulations project the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in the 21st century, though the rate and extent of the decline vary across models. In the Euro-Atlantic region, where the AMOC strongly influences sea surface temperature (SST) variability, climate projections also show significant uncertainty in the evolution of large-scale atmospheric circulation. We hypothesise that the decline of the AMOC and the uncertainty in Euro-Atlantic atmospheric circulation are interconnected. To test this, we analyse three coordinated experiments from the CMIP6 archive (SSP2-4.5, SSP5-8.5 and abrupt-4xCO2) adopting a storyline approach. In particular, we compare groups of models projecting a larger AMOC decline to groups of models projecting a smaller AMOC decline. Our results indicate that a stronger AMOC weakening is associated with an intensification of the North Atlantic storm track and jet stream, as well as an increased frequency of the NAO+ weather regime. We link these atmospheric changes to the influence of a reduced warming of the subpolar North Atlantic, known as the North Atlantic Warming Hole (NAWH), associated with the AMOC weakening. To further support our findings, we conduct an additional experiment using the EC-Earth3 model, comparing an abrupt-4xCO2 experiment with one where the AMOC is fixed at preindustrial levels. This experiment corroborates the CMIP6 analysis and validates our hypothesis. We conclude that the AMOC plays a critical role in driving atmospheric changes over the Euro-Atlantic region. Improved monitoring and better constraints on future AMOC decline would help reduce uncertainty in predictions of atmospheric circulation and climate impacts over Europe.

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial mechanism of poleward heat transport in the ocean and climate system. It modulates the redistribution of heat and carbon in the northern hemisphere. The state of AMOC in recent decades has revealed a slowdown compared to the industrial era. Its state is linked to a number of physical factors, including sea level. Along the eastern seaboard of North America, on long timescales, the imprint of the AMOC is projected onto sea level patterns. The relationship between AMOC weakening and sea level is not clearly understood. This study investigates the state of the AMOC in recent decades and its link to the regional sea level using CMIP6 and RAPID datasets.One of the most critical questions in ocean science is whether climate models and observations of the state of the AMOC in recent decades are consistent. If these datasets show significant differences, it could lead to a bias in our projected long-term climate knowledge. This study shows the potential of sea level data to inform the evolution of the AMOC to constrain and improve future projections.

Northward ocean heat transport (OHT) plays a key role in Arctic climate variability and change. Unforced climate model simulations suggest that at decadal and longer timescales, strengthened Atlantic meridional overturning circulation (AMOC) is correlated with increased OHT into the Arctic. Yet greenhouse gas forced simulations predict increased Arctic OHT while AMOC weakens. Here we partition OHT changes into contributions from dynamic circulation changes and thermodynamic temperature advection, as well as meridional overturning and gyre changes. We find that under decadal‐scale internal variability, strengthened AMOC converges heat in the subpolar gyre; anomalous heat is advected into the Arctic by both time mean circulations and strengthened gyre circulations. Under greenhouse gas forcing, weakened AMOC reduces subpolar gyre heat convergence; yet Arctic OHT increases as mean overturning and strengthened gyre circulations advect warmed surface waters. Thus, caution should be exercised when inferring Arctic OHT from AMOC, as the relationship between OHT and AMOC changes depends on whether they are internally generated or externally forced.

The amount of shortwave radiation absorbed by atmospheric water vapor is highly model dependent. This study examines how differences in the atmospheric water vapor shortwave radiation absorption affect the CO2-induced climate response pattern. We control the atmospheric water vapor shortwave radiation absorption in Community Earth System Model 1.2.2 (CESM1-CAM4-POP2) by altering the water vapor shortwave absorptivity parameter k by 60% to 120% of the default value. The pre-industrial control simulations with different k values are integrated for 150 years and additional 150 years are integrated after abruptly quadrupling CO2 concentrations. Regardless of the k value, the Atlantic meridional overturning circulation (AMOC) weakens in response to the quadrupling of CO2. However, the simulation with a higher k value exhibits a faster AMOC recovery approximately 30 years after the quadrupled CO2, with the lowest k simulation exhibiting a persistent AMOC weakening with no sign of recovery for the entire 300-year integration period. The faster AMOC restoration with a larger k value is attributed to the climatologically colder and saltier subpolar North Atlantic sea surface condition arising from the larger Arctic sea ice fraction due to colder temperature associated with stronger atmospheric shortwave absorption. The colder and more saline subpolar North Atlantic sea surface facilitates a more rapid destratification of surface density, establishing a favorable condition for the AMOC restoration. The faster restoration of the AMOC with the higher k value leads to a larger inter-hemispheric energy asymmetry followed by a more northward ITCZ shift as well as a stronger equilibrium climate sensitivity. This study demonstrates the complex interaction among different elements within the Earth system, encompassing radiation, sea ice, AMOC, and large-scale atmospheric circulation, suggesting a way to reduce uncertainties in future climate projections by improving the parameterization of shortwave radiation absorption by atmospheric water vapor.

We examine North Atlantic climate variability using an ensemble of ocean reanalysis datasets to study the Atlantic Meridional Overturning Circulation (AMOC) from 1979 to 2018. The dataset intercomparison shows good agreement for the latest period (1995–2018) for AMOC dynamics, characterized by a weaker overturning circulation after 1995 and a more intense one during 1979–1995, with varying intensity across the various datasets. The correlation between leading empirical orthogonal functions suggests that the AMOC weakening has connections with cooler (warmer) sea surface temperature (SST) and lower (higher) ocean heat content in the subpolar (subtropical) gyre in the North Atlantic. Barotropic stream function and Gulf Stream index reveal a shrinking subpolar gyre and an expanding subtropical gyre during the strong-AMOC period and vice versa, consistently with Labrador Sea deep convection reduction. We also observed an east–west salt redistribution between the two periods. Additional analyses show that the AMOC variability is related to the North Atlantic Oscillation phase change around 1995. One of the datasets included in the comparison shows an overestimation of AMOC variability, notwithstanding the model SST bias reduction via ERA-Interim flux adjustments: further studies with a set of numerical experiments will help explain this behavior.

During the penultimate deglaciation, which largely coincided with Heinrich Stadial 11 (HS-11, ~136-129 ka), meltwater pulses cooled the North Atlantic and weakened the Atlantic Meridional Overturning Circulation (AMOC), driving a southward shift of the Intertropical Convergence Zone (ITCZ) and arid conditions in northern South America. Although deglacial effects persisted for several millennia into the subsequent Last Interglacial period (LIG, ~129-115 ka), the response of the ITCZ to this transitional climate state remains poorly constrained. Here, we present paleoenvironmental records from a marine sediment core north of the Orinoco River delta, where runoff-sensitive proxies track northern South American rainfall and Atlantic ITCZ migration, and benthic δ¹³C records indicate AMOC strength. Our records show a gradual increase in precipitation during the early LIG, indicating a progressive northward migration of the ITCZ. Notably, the onset of peak wet conditions at 126.5±1 ka coincides with stabilized benthic δ¹³C values, consistent with the re-establishment of a fully developed interglacial AMOC. This temporal alignment suggests that the lingering effects of the penultimate deglaciation, such as gradual cessation of freshwater influence, subpolar North Atlantic SST warming and AMOC recovery, played an important role in shaping tropical hydroclimate during the first 4 millennia of the LIG, and should be incorporated in climate models.

The Atlantic meridional overturning circulation (AMOC) plays an important role in regulating global and regional climates, especially the European climate, as it affects northward heat transport. Climate model simulations project a decline in the strength of AMOC under future climate change, although high uncertainties exist across models. The potential slowdown of AMOC could cause large and rapid changes in the climate and is therefore regarded as a ‘low probability-high impact’ event. However, how a weakening AMOC affects temperature and precipitation extremes remains poorly understood. Here we use a state-of-the-art global climate model, a new version of the Alfred Wegener Institute Climate Model (AWI-CM3), to quantify these impacts on Europe. We have performed freshwater hosing experiments to weaken the strength of AMOC and to investigate its sensitivity to different freshwater forcing regions. Our results reveal that the largest decline in AMOC generally appears when additional freshwater is put directly at regions of deepwater formation, especially around the Irminger Sea. As atmospheric responses, widespread cooling and less precipitation are found in the Northern Hemisphere mid-latitudes. We further identify the changes in precipitation and temperature extremes on different timescales. Droughts and cold days are very pronounced under AMOC attenuations, and we argue that they may have a stronger impact than the system's mean responses. Our results provide implications for understanding European weather and climate response to a weakening of AMOC in the past, present, and future.

All climate models project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) strength in response to greenhouse gas forcing. However, the climate impacts of the AMOC decline in relation to other drivers of climate change, cannot be assessed from existing Coupled Model Intercomparison Project (CMIP) simulations. To address this issue, we conduct idealized experiments using the EC-Earth3 climate model. We compare an abrupt 4xCO2 simulation with an identical one, except we artificially fix the AMOC strength at preindustrial levels. With this design, we can formally attribute differences in climate change impacts between these two experiments to the AMOC decline. In addition, we quantify the state-dependence of AMOC impacts by comparing the aforementioned experiments with a preindustrial simulation in which we artificially reduce the AMOC strength. Our findings demonstrate that AMOC decline impacts are state-dependent, thus understanding AMOC impacts on future climate change requires targeted model experiments.

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in regulating the global climate. The AMOC change in response to global warming has important environmental and, potentially, societal impacts but remains an issue with large uncertainty. Here we use a series of coupled climate model experiments to reveal the overlooked role of Atlantic subtropical salinification, a robust consequence of an intensified hydrological cycle, in inhibiting AMOC weakening under global warming. Without subtropical salinification, the AMOC weakening more than doubles in response to a doubling of CO2, primarily driven by a reduced zonal salinity gradient that weakens the geostrophic component of AMOC through the thermal wind relation. This larger AMOC weakening reduces surface warming in the Northern Hemisphere by as much as 1–3 K at northern high latitudes when subtropical salinification is inhibited.

During deglaciation disintegration of large-scale continental ice sheets represents a continuous threat to reduce the strength of the Atlantic meridional overturning circulation (AMOC) via meltwater perturbations to the northern high latitudes. Nevertheless, an abrupt AMOC recovery is detected half-way through the last deglaciation and  a growing number of studies using Earth System Models (ESMs) of varying complexity have shown that atmospheric CO2 concentrations and ice sheet volume can influence the operational mode of the AMOC, eventually including the coexistence of multiple states and associated threshold behavior for intermediate climate states between full glacial (e.g. Last Glacial Maximum, LGM) and full interglacial (e.g. pre-industrial, PD)  conditions. In this study we present results from coordinated sensitivity experiments conducted as part of the German climate modeling initiative (PalMod), using three complex ESMs (AWI-ESM, CESM and MPI-ESM). Besides differences in the impact of CO2 and ice volume changes, we also investigate how variations in these boundary conditions control the AMOC sensitivity to deglacial meltwater injections in the North Atlantic. We find that the AMOC strength responds to ice sheet and/or CO2 changes in all models, with partly opposing effects.  A similar AMOC strength for PD and LGM conditions is detected in AWI-ESM and MPI-ESM, while CESM shows a weaker LGM AMOC. This weaker LGM state is also characterized by a relatively pronounced AMOC sensitivity to freshwater perturbations. Our inter-comparison experiments suggest that this specific behavior in CESM can be detected for atmospheric concentrations between LGM and intermediate levels of ~220 ppm. This further corroborates in particular the impact of CO2 changes to modulate the trajectory of deglacial climate changes by an alteration of the AMOC susceptibility to meltwater injections as recently suggested (Sun et al., Glob. Planet. Change, 2021; Barker & Knorr, Nat. Commun., 2021).   

Changes in the Atlantic Meridional Overturning Circulation (AMOC) impact the redistribution of heat across the climate system and can therefore influence surface temperatures over land. A large AMOC weakening, frequently analysed through idealised model simulations (e.g., freshwater hosing experiments), would lead to a strong cooling over the Northern Hemisphere. This cooling is most pronounced for winter months, suggesting a potential influence on cold extreme events; and for Europe, this influence has been hinted at. However, whether a more realistic interannual variability in the AMOC, rather than an idealised long-term weakening, also influences European mean temperatures and cold extremes is thus far unknown.To unravel this issue, we use the historical simulations of the 50-member MPI-ESM1.2-LR large ensemble, whose size is particularly suitable for analysing extremes. In these simulations, we categorise European temperatures based on their preceding interannual AMOC strengths. For yearly mean temperatures in a pre-industrial climate, we find that the distribution of temperatures following weak interannual AMOC strengths is significantly shifted towards colder values compared to years preceded by strong interannual AMOC strengths. Among all seasons, this shift is largest in winter; and spatially it is accentuated for northern latitudes. When considering present-day climate, the same shift still occurs, although less pronounced and strongest now for Eastern Europe. For daily extreme cold temperatures, the distribution of events is again colder following years of prevalent weak AMOC strengths; and this difference also becomes less clear and moves south-eastward in present-day climate. We complete the analysis by looking at the potential chain of physical atmospheric mechanisms that explains not only the connection between AMOC strengths and European extreme cold temperatures but also the evolution of this connection in the recent past.

The ocean absorbs most of the excess heat induced by anthropogenic greenhouse gas emissions. Among the different processes, a key role has been attributed to the Atlantic Meridional Overturning Circulation (AMOC), both for heat uptake and redistribution among basins, yet a comprehensive view remains elusive. In this study, we investigate how a weakening of the AMOC would influence ocean heat storage in a warming climate by comparing two idealized experiments with two different AMOC evolutions from the global climate model EC‐Earth3, after abrupt carbon dioxide quadrupling. We find that AMOC weakening influences both vertical and inter‐basin heat redistribution. For a stronger AMOC decline, more heat is stored in the Indo‐Pacific Ocean and in the intermediate ocean layers above 700 m, while less heat accumulates in the deeper ocean layers, especially in the Atlantic. Globally, we find a 3% increase in cumulated ocean heat uptake at the end of the simulations.

Abstract. We investigate the impact of the strength of the Atlantic Meridional Overturning Circulation (AMOC) at 26∘ N on the prediction of North Atlantic sea surface temperature anomalies (SSTAs) a season ahead. We test the dependence of sea surface temperate (SST) predictive skill in initialised hindcasts on the phase of the AMOC at 26∘N, invoking a seesaw mechanism driven by AMOC fluctuations, with positive SSTAs north of 26∘ N and negative SSTAs south of 26∘ N after a strong AMOC and vice versa. We use initialised simulations with the MPI-ESM-MR (where MR is mixed resolution) seasonal prediction system. First, we use an assimilation experiment between 1979–2014 to confirm that the AMOC leads a SSTA dipole pattern in the tropical and subtropical North Atlantic, with the strongest AMOC fingerprints after 2–4 months. Going beyond previous studies, we find that the AMOC fingerprint has a seasonal dependence and is sensitive to the length of the observational window used, i.e. stronger over the last decade than for the entire time series back to 1979. We then use a set of ensemble hindcast simulations with 30 members, starting each February, May, August and November between 1982 and 2014. We compare the changes in skill between composites based on the AMOC phase a month prior to each start date to simulations without considering the AMOC phase and find subtle influence of the AMOC mechanism on seasonal SST prediction skill. We find higher subtropical SST hindcast skill at a 2–4-month lead time for June–July–August (JJA) SSTA composites based on the AMOC phase at May start dates than for the full time period. In other regions and seasons, we find a negligible impact of the AMOC seesaw mechanism on seasonal SST predictions due to atmospheric influence, calling for caution when considering such a mechanism. Our method shows that, for May start dates following strong AMOC phases, summer SST hindcast skill over the subtropics increases significantly compared to that of weak AMOC phases. This suggests that in the assessment of SST skill for a season ahead an eye should be kept on the initial AMOC state.

The Atlantic meridional overturning circulation (AMOC) is a vital component of the global ocean circulation and the heat engine of the climate system. Through the use of a coupled general circulation model, this study examines the role of synoptic systems on the AMOC and presents evidence that internally generated high-frequency, synoptic-scale weather variability in the atmosphere could play a significant role in maintaining the overall strength and variability of the AMOC, thereby affecting climate variability and change. Results of a novel coupling technique show that the strength and variability of the AMOC are greatly reduced once the synoptic weather variability is suppressed in the coupled model. The strength and variability of the AMOC are closely linked to deep convection events at high latitudes, which could be strongly affected by the weather variability. Our results imply that synoptic weather systems are important in driving the AMOC and its variability. Thus, interactions between atmospheric weather variability and AMOC may be an important feedback mechanism of the global climate system and need to be taken into consideration in future climate change studies.