Atlantic Meridional Overturning Circulation Strength Research Articles

The Atlantic Meridional Overturning Circulation and the Cabbeling Effect

AbstractNorth Atlantic meridional density gradients have been identified as a main driver of the Atlantic meridional overturning circulation (AMOC). Due to the cabbeling effect, these density gradients are increasingly dominated by temperature gradients in a warming ocean, and a direct link exists between North Atlantic mean temperature and AMOC strength. This paper quantifies the impact of this mechanism in the Stommel and Gnanadesikan models. Owing to different feedback mechanisms being included, a 1°C warming of North Atlantic mean ocean temperature strengthens the AMOC by 3% in the Gnanadesikan model and 8% in the Stommel model. In the Gnanadesikan model that increase is equivalent to a 4% strengthening of Southern Hemisphere winds and can compensate for a 14% increase in the hydrological cycle. Furthermore, mean temperature strongly controls a freshwater forcing threshold for the strong AMOC state, suggesting that the cabbeling effect needs to be considered to explain past and future AMOC variability.

Pliocene Model Intercomparison Project (PlioMIP2) simulations using the Model for Interdisciplinary Research on Climate (MIROC4m)

Abstract. The second phase of the Pliocene Model Intercomparison Project (PlioMIP2) has attracted many climate modelling groups in its continuing efforts to better understand the climate of the mid-Piacenzian warm period (mPWP) when atmospheric CO2 was last closest to present-day levels. Like the first phase, PlioMIP1, it is an internationally coordinated initiative that allows for a systematic comparison of various models in a similar manner to the Paleoclimate Modelling Intercomparison Project (PMIP). Model intercomparison and model–data comparison now focus specifically on the interglacial at marine isotope stage KM5c (3.205 Ma), and experimental design is not only based on new boundary conditions but includes various sensitivity experiments. In this study, we present results from long-term model integrations using the MIROC4m (Model for Interdisciplinary Research on Climate) atmosphere–ocean coupled general circulation model, developed at the institutes CCSR, NIES and FRCGC in Japan. The core experiment, with CO2 levels set to 400 ppm, shows a warming of 3.1 ∘C compared to the pre-industrial period, with two-thirds of the warming being attributed to the increase in CO2. Although this level of warming is less than that in the equivalent PlioMIP1 experiment, there is slightly better agreement with proxy sea surface temperature (SST) data at PRISM3 (PRISM – Pliocene Research Interpretation and Synoptic Mapping) locations, especially in the northern North Atlantic where there were large model–data discrepancies in PlioMIP1. Similar spatial changes in precipitation and sea ice are seen and the Arctic remains ice-free in the summer in the core experiments of both phases. Comparisons with both the proxy SST data and proxy surface air temperature data from paleobotanical sites indicate a weaker polar amplification in model results. Unlike PlioMIP1, the Atlantic Meridional Overturning Circulation (AMOC) is now stronger than that of the pre-industrial period, even though increasing CO2 tends to weaken it. This stronger AMOC is a consequence of a closed Bering Strait in the PlioMIP2 paleogeography. Also, when present-day boundary conditions are replaced by those of the Pliocene, the dependency of the AMOC strength on CO2 is significantly weakened. Sensitivity tests show that lower values of CO2 give a global SST which is overall more consistent with the PRISM3 SST field presented in PlioMIP1, while SSTs at many of the PRISM4 sites are still too high to be reconciled with any of the model results. On the other hand, tropical Pacific SST in the core experiment agrees well with more recent proxy data, which suggested that PRISM3 SST there was overestimated. Future availability of climate reconstructions from proxy data will continue to help evaluate model results. The inclusion of dynamical vegetation and the effects of all possible extreme orbital configurations outside KM5c should be considered in future experiments using MIROC4m for the mPWP.

Dynamics of Spontaneous (Multi) Centennial‐Scale Variations of the Atlantic Meridional Overturning Circulation Strength During the Last Interglacial

AbstractRecent reconstructions of bottom water δ13C during the last interglacial (LIG) suggest short‐lived variability in the Atlantic meridional overturning circulation (AMOC). Spontaneous (multi) centennial‐scale variability of the AMOC simulated in the Earth system model of intermediate complexity iLOVECLIM are investigated for that period. The model simulates abrupt AMOC transitions occurring at 300 years frequency and correspond to a switch of the AMOC vigor between a strong (∼17 Sv) and a weak (∼11 Sv) state. The onset of these abrupt transitions is associated with changes in orbital forcings resulting in the decline of summer insolation in the high latitudes of the North Atlantic and affecting the sea ice cover in two key deep convection regions: (1) the northern Nordic Seas (NNS) and (2) the northwest North Atlantic (NWNA). Northward inflow of Atlantic surface water increases the convection depth in (1) and strengthens the Greenland Iceland Norway (GIN) Seas overturning circulation. Subsequent ocean‐atmosphere interactions involving sea ice, ocean heat release, anomalies of evaporation‐precipitation, and wind stress over the Nordic Seas lead also to an increase in deep convection in (2), followed by increase in the AMOC strength.

Pending recovery in the strength of the meridional overturning circulation at 26° N

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26∘ N has now been continuously measured by the RAPID array over the period April 2004–September 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic multidecadal variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy forcing by the atmosphere drove intense water-mass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45∘ N, observations suggest that the AMOC may already be increasing. Examining 26∘ N, we find that the AMOC is no longer weakening, though the recent transport is not above the long-term mean. Extending the record backwards in time at 26∘ N with ocean reanalysis from GloSea5, the transport fluctuations at 26∘ N are consistent with a 0- to 2-year lag from those at 45∘ N, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering nor how the AMOC at 26∘ N responds to intense buoyancy forcing.

Fingerprints for Early Detection of Changes in the AMOC

AbstractDifferent strategies have been proposed in previous studies for monitoring the Atlantic meridional overturning circulation (AMOC). As well as arrays to directly monitor the AMOC strength, various fingerprints have been suggested to represent an aspect of the AMOC based on properties such as temperature and density. The additional value of fingerprints potentially includes the ability to detect a change earlier than a change in the AMOC itself, the ability to extend a time series back into the past, and the ability to detect crossing a threshold. In this study we select metrics that have been proposed as fingerprints in previous studies and evaluate their ability to detect AMOC changes in a number of scenarios (internal variability, weakening from increased greenhouse gases, weakening from hosing and hysteresis) in the eddy-permitting coupled climate model HadGEM3-GC2. We find that the metrics that perform best are the temperature metrics based on large-scale differences, the large-scale meridional density gradient, and the vertical density difference in the Labrador Sea. The best metric for monitoring the AMOC depends somewhat on the processes driving the change. Hence the best strategy would be to consider multiple fingerprints to provide early detection of all likely AMOC changes.

Stability of the Atlantic overturning circulation under intermediate (MIS3) and full glacial (LGM) conditions and its relationship with Dansgaard-Oeschger climate variability

Paleoclimatic records reveal that millennial-scale climate variability during the Pleistocene was most pronounced during intermediate glacial conditions, like Marine Isotope Stage 3 (MIS3), rather than during interglacial and fully glaciated climates, like the Last Glacial Maximum (LGM). The rapid transitions between cold stadials and warm interstadials recorded in Greenland ice cores during MIS3, referred to as Dansgaard–Oeschger (D-O) events, have been correlated with millennial-scale climate variations worldwide. Although the origin of D-O events is a matter of controversy, striking evidence shows that variations in the strength of the Atlantic meridional overturning circulation (AMOC) were involved. Therefore, understanding the stability properties of the ocean circulation under different background climate conditions is key to understanding D-O millennial-scale climate variability. In the present study, the stability of the AMOC to northern high-latitude freshwater perturbations under MIS3 and LGM boundary conditions is investigated by using the coupled climate model CCSM3. Stability diagrams constructed from a large set of equilibrium experiments reveal a nonlinear dependence of AMOC strength on freshwater forcing under both MIS3 and LGM conditions. The MIS3 baseline state is close to an AMOC stability threshold, which makes the MIS3 climate unstable with respect to minor perturbations. A similar threshold behavior in AMOC stability is observed under LGM conditions; however, larger freshwater perturbations are necessary to pass the threshold and weaken the AMOC. The threshold’s displacement relative to the MIS3 background climate is attributable to differences in the atmospheric hydrologic cycle and North Atlantic sea ice transport. Different atmospheric moisture transports are attributable to thermodynamic and dynamic processes related to differences in greenhouse gas forcing and ice-sheet height between MIS3 and the LGM. We conclude that the higher stability of the AMOC during the LGM is a physically plausible explanation for millennial-scale D-O-type climate variability being suppressed under full glacial conditions, whereas minor perturbations in freshwater fluxes could have triggered D-O climate shifts during MIS3.

Impact of ocean resolution and mean state on the rate of AMOC weakening

We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO_2 at different horizontal resolutions in a state-of-the-art climate model and in a small ensemble of models with differing resolutions. There is a strong influence of the ocean mean state on the AMOC weakening: models with a more saline western subpolar gyre have a greater formation of deep water there. This makes the AMOC more susceptible to weakening from an increase in CO_2 since weakening ocean heat transports weaken the contrast between ocean and atmospheric temperatures and hence weaken the buoyancy loss. In models with a greater proportion of deep water formation further north (in the Greenland-Iceland-Norwegian basin), deep-water formation can be maintained by shifting further north to where there is a greater ocean-atmosphere temperature contrast. We show that ocean horizontal resolution can have an impact on the mean state, and hence AMOC weakening. In the models examined, those with higher resolutions tend to have a more westerly location of the North Atlantic Current and stronger subpolar gyre. This likely leads to a greater impact of the warm, saline subtropical Atlantic waters on the western subpolar gyre resulting in greater dense water formation there. Although there is some improvement of the higher resolution models over the lower resolution models in terms of the mean state, both still have biases and it is not clear which biases are the most important for influencing the AMOC strength and response to increasing CO_2.

CMIP6 Models Predict Significant 21st Century Decline of the Atlantic Meridional Overturning Circulation

AbstractWe explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multimodel ensemble. Comparison with RAPID and SAMBA observations suggests that the ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850–2014) are generally neutral, but many models exhibit an AMOC peak around the 1980s. Ensemble mean AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34–45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%.

Role of AMOC in Transient Climate Response to Greenhouse Gas Forcing in Two Coupled Models

AbstractAs the greenhouse gas concentrations increase, a warmer climate is expected. However, numerous internal climate processes can modulate the primary radiative warming response of the climate system to rising greenhouse gas forcing. Here the particular internal climate process that we focus on is the Atlantic meridional overturning circulation (AMOC), an important global-scale feature of ocean circulation that serves to transport heat and other scalars, and we address the question of how the mean strength of AMOC can modulate the transient climate response. While the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting a faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise the stronger AMOC in CESM2 by permitting a slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial time scale.

Beyond Forcing Scenarios: Predicting Climate Change through Response Operators in a Coupled General Circulation Model

Global Climate Models are key tools for predicting the future response of the climate system to a variety of natural and anthropogenic forcings. Here we show how to use statistical mechanics to construct operators able to flexibly predict climate change. We perform our study using a fully coupled model - MPI-ESM v.1.2 - and for the first time we prove the effectiveness of response theory in predicting future climate response to CO2 increase on a vast range of temporal scales, from inter-annual to centennial, and for very diverse climatic variables. We investigate within a unified perspective the transient climate response and the equilibrium climate sensitivity, and assess the role of fast and slow processes. The prediction of the ocean heat uptake highlights the very slow relaxation to a newly established steady state. The change in the Atlantic Meridional Overturning Circulation (AMOC) and of the Antarctic Circumpolar Current (ACC) is accurately predicted. The AMOC strength is initially reduced and then undergoes a slow and partial recovery. The ACC strength initially increases due to changes in the wind stress, then undergoes a slowdown, followed by a recovery leading to a overshoot with respect to the initial value. Finally, we are able to predict accurately the temperature change in the North Atlantic.

Assessing the potential capability of reconstructing glacial Atlantic water masses and AMOC using multiple proxies in CESM

Reconstructing the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) is essential for understanding glacial-interglacial climate change and the carbon cycle. However, despite many previous studies, uncertainties remain regarding the glacial water mass distributions in the Atlantic and the AMOC intensity. Here we use an isotope enabled ocean model with multiple geotracers (δ13C, εNd, 231Pa/230Th, δ18O and Δ14C) and idealized water tracers to study the potential constraints on LGM ocean circulation from multiple proxies. Our model suggests that the glacial Atlantic water mass distribution can be accurately constrained by the air-sea gas exchange signature of water masses (δ13CAS), but εNd might overestimate the North Atlantic Deep Water (NADW) percentage in the deep Atlantic probably because of the boundary source of Nd. A sensitivity experiment with an AMOC of similar geometry but much weaker strength suggests that the correct AMOC geometry is more important than the AMOC strength for simulating the observed glacial δ13C and εNd distributions. The kinematic tracer 231Pa/230Th is sensitive to AMOC intensity, but the interpretation might be complicated by the AMOC geometry and AABW transport changes during the LGM. δ18O in the benthic foraminifera (δ18Oc) from the Florida Straits provides a consistent measure of the upper ocean boundary current in the model, which potentially provides an unambiguous method to reconstruct glacial AMOC intensity. Finally, we propose that the moderate difference between AMOC intensity at LGM and PD, if any, is caused by the competition of the responses to CO2 forcing and continental ice sheet forcing.

Atlantic Meridional Overturning Circulation and δ13C Variability During the Last Interglacial

AbstractThe Atlantic Meridional Overturning Circulation (AMOC) is thought to be relatively vigorous and stable during Interglacial periods on multimillennial (equilibrium) timescales. However, recent proxy (δ13C benthic) reconstructions suggest that higher frequency variability in deep water circulation may have been common during some interglacial periods, including the Last Interglacial (LIG, 130–115 ka). The origin of these isotope variations and their implications for past AMOC remain poorly understood. Using iLOVECLIM, an Earth system model of intermediate complexity (EMIC) allowing the computation of and direct comparison to proxy reconstructions, we perform a transient experiment of the LIG (125–115 ka) forced only by boundary conditions of greenhouse gases and orbital forcings. The model simulates large centennial‐scale variations in the interior of the North Atlantic similar in timescale and amplitude to changes resolved by high‐resolution reconstructions from the LIG. In the model, these variations are caused by changes in the relative influence of North Atlantic Deep Water (NADW) and southern source water (SSW) and are closely linked to large (∼50%) changes in AMOC strength provoked by saline input and associated deep convection areas south of Greenland. We identify regions within the subpolar North Atlantic with different sensitivity and response to changes in preformed of NADW and to changes in NADW versus SSW influence, which is useful for proxy record interpretation. This could explain the relatively large δ13C gradient (∼0.4%0) observed at ∼3 km water depth in the subpolar North Atlantic at the inception of the last glacial.

Forcing of western tropical South Atlantic sea surface temperature across three glacial-interglacial cycles

Abstract The western tropical Atlantic (WTA) supplies warm and saline waters to the upper-limb of the Atlantic Meridional Overturning Circulation (AMOC) and may store excess heat and salinity during periods of AMOC slowdown. Since previous sea surface temperature (SST) reconstructions from the WTA typically focus on the Last Glacial Maximum and the last deglaciation, additional long-term records spanning multiple glacial-interglacial transitions are needed in order to elucidate the drivers of long-term WTA SST variability. We performed Mg/Ca analyses on the surface-dwelling planktic foraminifera Globigerinoides ruber (pink) from a sediment core raised from the southern WTA to reconstruct SST changes over the past 322 kyr. We evaluate the relative importance of atmospheric pCO2, AMOC strength and trade-wind intensity in driving the thermal evolution of the WTA across three glacial-interglacial cycles. Our results indicate a lack of pronounced glacial-interglacial variability in the SST record, prompting us to exclude atmospheric pCO2 as a direct driver of SST variations in the southern WTA. Similarly, we conclude that variations in AMOC strength also likely did not have a strong influence on long-term WTA SST, based on the low and relatively stable interhemispheric SST gradient over the past 322 kyr. Our results reveal high-amplitude variability in zonal SST gradients within the (sub)tropical South Atlantic and similarities between the long-term patterns of the intrahemispheric meridional SST gradient and our SST record. Based on these findings, we propose that changes in the intrahemispheric meridional SST gradient modulate southeast trade wind intensity, which in turn drives variations in zonal SST gradients and southern WTA SSTs.

South Asian monsoon response to weakening of Atlantic meridional overturning circulation in a warming climate

Observational records and climate model projections reveal a considerable decline in the Atlantic Meridional Overturning Circulation (AMOC). Changes in the AMOC can have a significant impact on the global climate. Sustained warming due to increased greenhouse gas emissions is projected to weaken the AMOC, which in turn can lead to changes in the location of Inter-tropical convergence zone (ITCZ), oceanic and atmospheric large-scale circulation, tropical precipitation and regional monsoons. Using proxy records, observations and CMIP6 simulations of IITM Earth System Model (IITM-ESM), we investigate the changes in the AMOC and associated changes in the large-scale circulation and precipitation patterns over the South Asian monsoon region. Transient CO2 simulation and additional model sensitivity experiments with realistic surface heat and freshwater perturbation anomalies under the experimental protocol of Flux Anomaly Forcing Model Intercomparison Project (FAFMIP) performed with IITM-ESM reveal a decline in the strength of AMOC. The weakening of AMOC is associated with enhanced heat and freshwater forcing in the North Atlantic resulting in the reduction of northward oceanic heat transport and an enhanced northward atmospheric heat transport. Changes in AMOC lead to weakening of large-scale north–south temperature gradient and regional land-sea thermal gradient, which in turn weaken the regional Hadley circulation and, monsoon circulation over the South Asian region. Both the FAFMIP and transient CO2 experiments reveal consistent results of weakening South Asian Monsoon circulation with a decline of AMOC, while precipitation exhibits contrasting responses as precipitation changes are dominated by the thermodynamic response. The suite of observational and numerical analysis provides a mechanistic hypothesis for the weakening of South Asian monsoon circulation concomitant with a weakening of AMOC in a warming climate.

The Mean State and Variability of the North Atlantic Circulation: A Perspective From Ocean Reanalyses

AbstractThe observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal time scale changes across the North Atlantic including in heat content, heat transport, and the circulation. However, there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However, the ability of the reanalyses to represent the dynamics must also be assessed. We use an ensemble of global ocean reanalyses to examine the time mean state and interannual‐decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular, we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50°N from the mid‐1990s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5°N since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5°N since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved.

Did North Atlantic cooling and freshening from 3.65–3.5 Ma precondition Northern Hemisphere ice sheet growth?

The North Atlantic Current (NAC) as part of the Atlantic Meridional Overturning Circulation (AMOC) is the major supplier of heat into the northern North Atlantic. Pliocene changes of AMOC strength were speculated to either have amplified or diminished the Northern Hemisphere Glaciation (NHG) 2.7 million years ago (Ma). However, from the North Atlantic, little evidence is known about AMOC changes at around 3.6 Ma. At this time the intensification of NHG started and culminated in the first major glacial M2 event at 3.3 Ma. To elaborate the climatic effects of variations in the NAC during this early stage of NHG, we here present millennial-scale resolved records from Deep Sea Drilling (DSDP) Site 610A in the northern North Atlantic. Our data of planktic foraminiferal Mg/Ca-based sea surface temperatures (SSTMg/Ca) and ice volume corrected salinity approximations (δ18OIVC-seawater) span the critical time period 4–3.3 Ma. From 3.65 to 3.5 Ma, we observe a distinct ~3.5 °C cooling and ~0.7‰ freshening of the sea surface, which we interpret to reflect a weakened NAC. At the same time Arctic sea ice grew and benthic δ13C in the South Atlantic suggest a weakened AMOC. We conclude that the weakened NAC in response to a sluggish AMOC fostered sea ice formation in the Arctic Ocean and high-latitude North Atlantic, which might have preconditioned the climate for subsequent continental glaciations.

The Relationship Between U.S. East Coast Sea Level and the Atlantic Meridional Overturning Circulation: A Review.

Scientific and societal interest in the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and U.S. East Coast sea level has intensified over the past decade, largely due to (1) projected, and potentially ongoing, enhancement of sea level rise associated with AMOC weakening and (2) the potential for observations of U.S. East Coast sea level to inform reconstructions of North Atlantic circulation and climate. These implications have inspired a wealth of model‐ and observation‐based analyses. Here, we review this research, finding consistent support in numerical models for an antiphase relationship between AMOC strength and dynamic sea level. However, simulations exhibit substantial along‐coast and intermodel differences in the amplitude of AMOC‐associated dynamic sea level variability. Observational analyses focusing on shorter (generally less than decadal) timescales show robust relationships between some components of the North Atlantic large‐scale circulation and coastal sea level variability, but the causal relationships between different observational metrics, AMOC, and sea level are often unclear. We highlight the importance of existing and future research seeking to understand relationships between AMOC and its component currents, the role of ageostrophic processes near the coast, and the interplay of local and remote forcing. Such research will help reconcile the results of different numerical simulations with each other and with observations, inform the physical origins of covariability, and reveal the sensitivity of scaling relationships to forcing, timescale, and model representation. This information will, in turn, provide a more complete characterization of uncertainty in relevant relationships, leading to more robust reconstructions and projections.

The Stability of the AMOC During Heinrich Events Is Not Dependent on the AMOC Strength in an Intermediate Complexity Earth System Model Ensemble

AbstractPrevious studies that used Earth system models of intermediate complexity showed that stronger background winds drove a more vigorous and stable Atlantic Meridional Overturning Circulation (AMOC), while those with weaker winds had a more sluggish and unstable AMOC. In other studies, ensembles under vertical mixing uncertainty showed the opposite effect, where the simulations with a stronger AMOC were more unstable. To tackle this conundrum, we produce a model ensemble featuring uncertainties related to wind forcing and vertical mixing to understand the role of feedbacks on the AMOC stability. We show that the stability of the AMOC is not influenced by vertical mixing and the AMOC strength, and rather, it is determined by the strength of the Northern Hemisphere winds. Paleoproxies indicate an AMOC shutdown during the last Heinrich Stadial. Our comparisons to sea surface temperature proxies show a better fit with the simulations under a stable AMOC, which corresponds to a forced off‐state. The sign of the AMOC‐driven freshwater transport in the South Atlantic, which is regarded as an index for its stability, is shown not to be an absolute measure, although its evolution agrees with the salt advection feedback.

Investigating the feedbacks between CO2, vegetation and the AMOC in a coupled climate model

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system, however its sensitivity to the terrestrial biosphere has been largely overlooked. Here the HadCM3 coupled climate model is run for millennial timescales to investigate the feedbacks between vegetation and the AMOC at increasing CO2. The impact of agricultural conversion (termed land-use change; LUC) and the role of the simulated ‘background’ vegetation (termed land cover change; LCC) are investigated. LUC cools climate in regions of high crop fraction due to increased albedo. LCC is shown to evolve at higher CO2, with a northward migration of the tree line in the Northern Hemisphere and dieback of the Amazon. This generally acts to enhance the impact of climate change primarily due to albedo changes. Density in the Greenland-Iceland-Norwegian (GIN) Seas is crucial in driving the AMOC. Increasing CO2 decreases regional sea surface density, reducing convection and weakening the AMOC. The inclusion of LCC is shown to be responsible for a significant proportion of this weakening; reflecting the amplification effect it has on climate change. This acts to decrease the surface density in the GIN Seas. At elevated CO2 (1400 ppm) the inclusion of dynamic vegetation is shown to drive a reduction in AMOC strength from 6 to 20%. Despite the cooling effect of LUC, the impact on the AMOC is shown to be small reflecting minimal impact it has on GIN Sea density. These results indicate the importance of including dynamic vegetation in future AMOC studies using HadCM3, but LUC may be insignificant. In the context of other climate models however, the importance of vegetation is likely to be overshadowed by other systemic model biases.

Sortable silt records of intermediate-depth circulation and sedimentation in the Southwest Labrador Sea since the Last Glacial Maximum

The Labrador Sea is a vital region for the Atlantic Meridional Overturning Circulation (AMOC), where overflow waters from the Nordic Seas mix with locally produced Labrador Sea Water (LSW), before exiting to the interior of the Atlantic Ocean. The dynamical sedimentary proxy of mean sortable silt size (SS¯) can give information on past changes in deep water circulation speed and the strength of AMOC. We have produced SS¯ records from two core sites at depths between 1500 and 2000 m on the continental slope east of Newfoundland, to reconstruct changes in intermediate depth water circulation speed, including Glacial North Atlantic Intermediate Water and Labrador Sea Water over the past 22,000 years. Increases in SS¯ appear to coincide with much of the deglaciation as well as the mid-late Holocene. End-member modeling suggests that ice-rafted debris (IRD) is an important factor in interpreting SS¯ during the deglaciation. We find that a robust increase in SS¯ is likely unrelated to IRD during the past 5 ka, and probably reflects increased flow at intermediate depths due to local production of LSW strengthening as Nordic Seas overflows weakened at this depth. Our results highlight both the complications of producing SS¯ records in IRD-rich, slope environments and the promise that this proxy nevertheless has for reconstructing dynamical changes in deep ocean currents.