Atlantic Meridional Overturning Circulation Research Articles

Impacts and State‐Dependence of AMOC Weakening in a Warming Climate

AbstractAll 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 alone cannot be isolated from other drivers of climate change using existing Coupled Model Intercomparison Project simulations. To address this issue, we conduct idealized experiments using the EC‐Earth3 climate model. We compare an abrupt 4×CO2 simulation with the same experiment, 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.

A diminished North Atlantic nutrient stream during Younger Dryas climate reversal

The high rate of biological productivity in the North Atlantic is stimulated by the advective supply of nutrients into the region via the Gulf Stream (nutrient stream). It has been proposed that the projected future decline in the Atlantic Meridional Overturning Circulation (AMOC) will cause a reduction in nutrient supply and resulting productivity. In this work, we examine how the nutrient stream changed over the Younger Dryas climate reversal that marked the transition out of the last ice age. Gulf Stream nutrient content decreased, and oxygen content increased at the Florida Straits during this time of weakened AMOC. The decreased nutrient stream was accompanied by a reduction in biological productivity at higher latitudes in the North Atlantic, which supports the link postulated in theoretical and modeling studies.

Fast and Slow Responses of Atlantic Meridional Overturning Circulation to Antarctic Meltwater Forcing

AbstractAntarctic meltwater discharge has been largely emphasized for its potential role in climate change mitigation, not only by reducing global warming, but also by stabilizing the Atlantic Meridional Overturning Circulation (AMOC). Despite the tremendous impact of the AMOC on the climate system, its temporal evolution in response to the meltwater remains poorly understood. Here, we investigate the meltwater impacts on the AMOC based on the GFDL CM2.1 experiments and discover its fast weakening and slow strengthening to the Antarctic meltwater discharge. Cold ocean surface caused by meltwater spread throughout the globe and eventually strengthened the AMOC. However, in the early stages, the tropical temperature response could stimulate the Rossby wave teleconnection, modulating atmospheric circulation in the North Atlantic, and weakening convection and even the AMOC. This counterintuitive evolution implies a potential destabilizing effect of Antarctic meltwater, underscoring the importance of the atmospheric dynamics in the interaction between the two poles.

Atlantic Meridional Overturning Circulation Influence on the Annual Mean Intertropical Convergence Zone Location in the Miocene

AbstractThe Intertropical Convergence Zone (ITCZ) has an annual mean location north of the equator today. The factors determining this location and the evolution to its modern state are actively debated. Here we investigate how the Atlantic Meridional Overturning Circulation (AMOC) influences the ITCZ during the early‐to‐middle Miocene. By conducting a sensitivity study with an open Canadian Arctic Archipelago gateway, we show that North Atlantic Deep‐Water formation strengthens the AMOC, in alignment with Miocene North Atlantic ventilation proxies. A vigorous AMOC increases northward Atlantic Ocean heat transport and cross‐equatorial atmospheric energy transport shifts southwards to compensate, pushing the ITCZ northwards. Our study supports AMOC development as a strong contributor to the ITCZ's northern location today. Existing proxy‐based interpretations of ITCZ history are too sparse to strongly confirm these results. We predict a strong in‐phase relationship between AMOC strength and ITCZ's northward location, which should be testable in high resolution paleoclimate records.

North Atlantic Ocean–Originated Multicentennial Oscillation of the AMOC: A Coupled Model Study

Abstract Using a CESM1 control simulation, we conduct a follow-up study to advance our earlier theoretical research on the multicentennial oscillation (MCO) of the Atlantic meridional overturning circulation (AMOC). The modeled AMOC MCO primarily arises from internal oceanic processes in the North Atlantic, potentially representing a North Atlantic Ocean–originated mode of AMOC multicentennial variability (MCV) in reality. Specifically, this AMOC MCO is mainly driven by salinity variation in the subpolar upper North Atlantic, which dominates local density variation. Salinity anomaly in the subpolar upper ocean is enhanced by the well-known positive salinity advection feedback that is realized through anomalous advection in the subtropical to subpolar upper ocean. Meanwhile, mean advection moves salinity anomaly in the subtropical intermediate ocean northward, weakening the subpolar upper salinity anomaly and leading to its phase change. The salinity anomalies have a clear three-dimensional life cycle around the North Atlantic. The mechanism and time scale of the modeled AMOC MCO are consistent with our earlier theoretical studies. In the theoretical model, artificially deactivating either the anomalous or mean advection in the AMOC upper branch prevents it from exhibiting AMOC MCO, underscoring the indispensability of both the anomalous and mean advections in this North Atlantic Ocean–originated AMOC MCO. In our coupled model simulation, the South Atlantic and Southern Oceans do not exhibit variabilities synchronous with the AMOC MCO; the Arctic Ocean’s contribution to the subpolar upper salinity anomaly is much weaker than the North Atlantic. Hence, this North Atlantic Ocean–originated AMOC MCO is distinct from the previously proposed Southern Ocean–originated and Arctic Ocean–originated AMOC MCOs.

The North Atlantic Subpolar Ocean Dynamics during the Past 21,000 Years

Numerous studies have suggested that the North Atlantic subpolar gyre (SPG), Atlantic Meridional Overturning Circulation (AMOC), and Arctic sea ice impact the polar and global climate. Here, we use a fully linked atmosphere-ocean-sea ice Earth system model to investigate the North Atlantic subpolar ocean dynamics over the last 21 thousand years before the present (ka). We found that the SPG strength, net ocean surface heat flux, and mixed layer depth in the North Atlantic deep convection sites declined during the Heinrich 1 (H1; ∼19–17 ka) and Younger Dryas (YD; ∼12.9–11.3 ka) cold events. Consequently, the deep convection and AMOC strength declined, reducing the northward meridional heat transport and causing the expansion of Atlantic sea ice coverage. We also found that the North Atlantic subpolar net ocean surface heat flux varied coherently with AMOC strength throughout the past 21 ka. Subsequently, we observed a sea ice-capping mechanism wherein an increase (decrease) in Atlantic sea ice coverage during H1/YD (Bølling-Allerød (BA; 17–14.35 ka)) reduces (increases) net ocean surface heat flux and deep convection, thereby influencing the AMOC strength. Meanwhile, the SPG and AMOC strengths have been in-phase throughout the past 21 ka, except during the abrupt termination and input of freshwater flux during the BA and Meltwater Pulse 1A (∼14.4–13.9 ka) events, respectively. In conclusion, our study suggests that a sudden shift in freshwater discharge into the subpolar North Atlantic may disturb the polar ocean dynamics.

Global, Arctic, and Antarctic sea ice volume predictions using score-driven threshold climate models

The literature on sea ice predictions uses a variety of general circulation models (GCMs), which suggest diverse predictions of the date of ice-free or almost ice-free oceans, and focus mainly on the Arctic. GCMs are not sensitive enough to tipping points in the Atlantic meridional overturning circulation (AMOC), and they underestimate the sensitivity of Arctic sea ice to carbon emissions. We contribute to the literature in two ways: First, we use a novel time series model, named the score-driven threshold climate (SDTC) model, and we report global, Arctic, and Antarctic sea ice predictions. For the SDTC model, the estimations are computationally less demanding than those of the GCMs. Second, we combine long-run 1,000-year frequency climate data from 798,000 to 1,000 years ago, and short-run annual data from year 850 to year 2014. We present the evolution of long-run and short-run climate data with descriptive statistics. We estimate the SDTC model using annual data from 850 to 2014 for Arctic and Antarctic sea ice volume Icet and Antarctic land surface temperature Tempt. We use the atmospheric CO2,t concentration as a clustering variable to define periods of climate change. Hence, CO2,t concentration drives the regime-switching dynamics of parameters in the SDTC model. We report in-sample interval forecasts of global, Arctic, and Antarctic sea ice from 1980 to 2014. Observed global and Arctic sea ice volumes are below the forecasted interval from 2003. The observed Antarctic sea ice volume is below the forecasted interval from 2011. We report out-of-sample interval forecasts of sea ice from 2015 to 2314. The out-of-sample forecasts, μ[μ±2σ], indicate that if the current trend of climate change continues, then Arctic sea ice will disappear around 2058[2049,2068], and global and Antarctic sea ice will disappear around 2174[2123,2270].

Impact of volcanic eruptions on CMIP6 decadal predictions: a multi-model analysis

Abstract. In recent decades, three major volcanic eruptions of different intensity have occurred (Mount Agung in 1963, El Chichón in 1982 and Mount Pinatubo in 1991), with reported climate impacts on seasonal to decadal timescales that could have been potentially predicted with accurate and timely estimates of the associated stratospheric aerosol loads. The Decadal Climate Prediction Project component C (DCPP-C) includes a protocol to investigate the impact of volcanic aerosols on the climate experienced during the years that followed those eruptions through the use of decadal predictions. The interest of conducting this exercise with climate predictions is that, thanks to the initialisation, they start from the observed climate conditions at the time of the eruptions, which helps to disentangle the climatic changes due to the initial conditions and internal variability from the volcanic forcing. The protocol consists of repeating the retrospective predictions that are initialised just before the last three major volcanic eruptions but without the inclusion of their volcanic forcing, which are then compared with the baseline predictions to disentangle the simulated volcanic effects upon climate. We present the results from six Coupled Model Intercomparison Project Phase 6 (CMIP6) decadal prediction systems. These systems show strong agreement in predicting the well-known post-volcanic radiative effects following the three eruptions, which induce a long-lasting cooling in the ocean. Furthermore, the multi-model multi-eruption composite is consistent with previous work reporting an acceleration of the Northern Hemisphere polar vortex and the development of El Niño conditions the first year after the eruption, followed by a strengthening of the Atlantic Meridional Overturning Circulation the subsequent years. Our analysis reveals that all these dynamical responses are both model- and eruption-dependent. A novel aspect of this study is that we also assess whether the volcanic forcing improves the realism of the predictions. Comparing the predicted surface temperature anomalies in the two sets of hindcasts (with and without volcanic forcing) with observations we show that, overall, including the volcanic forcing results in better predictions. The volcanic forcing is found to be particularly relevant for reproducing the observed sea surface temperature (SST) variability in the North Atlantic Ocean following the 1991 eruption of Pinatubo.

The Irminger Gyre as a Key Driver of the Subpolar North Atlantic Overturning

AbstractThe lower limb of the Atlantic meridional overturning circulation (AMOC) is the equatorward flow of dense waters formed through the cooling and freshening of the poleward‐flowing upper limb. In the subpolar North Atlantic (SPNA), upper limb variability is primarily set by the North Atlantic Current, whereas lower limb variability is less well understood. Using observations from a SPNA mooring array, we show that variability of the AMOC's lower limb is connected to poleward flow in the interior Irminger Sea. We identify this poleward flow as the northward branch of the Irminger Gyre (IG), accounting for 55% of the AMOC's lower limb variability. Over 2014–2018, wind stress curl fluctuations over the Labrador and Irminger Seas drive this IG and AMOC variability. On longer (>annual) timescales, however, an increasing trend in the thickness of intermediate water, from 2014 to 2020, within the Irminger Sea coincides with a decreasing trend in IG strength.

Weakening of the Atlantic Meridional Overturning Circulation abyssal limb in the North Atlantic

Weakening of the Atlantic Meridional Overturning Circulation abyssal limb in the North Atlantic

Middle to late Holocene climate change in the monsoon-dominated southeastern Tibetan Plateau and its relationship with human activity

Aeolian deposits on the southeastern Tibetan Plateau (SETP) can provide evidence of how prehistoric humans were influenced by climatic and environmental changes in high-altitude regions. We present the results of multi-proxy analyses of a middle–late Holocene aeolian section in the SETP, together with the probability density of the ages of various cultural sites within and around the study area. Quartz optically stimulated luminescence dating provides a reliable chronology for this section, and analyses of geochemical elements, color and grain size are used to reconstruct the pattern of climate change during the middle–late Holocene. The results suggest a trend of gradually decreasing moisture from the middle Holocene (∼6 ka) onwards, followed by a slight increase at ∼2 ka, which is supported by TraCE-21 ka simulation results. These changes were closely related to variation in the Indian summer monsoon (ISM) caused by decreasing Northern Hemisphere summer insolation and an increase in the Atlantic Meridional Overturning Circulation. The variations in dust activity are inversely related with moisture and there was a significant decrease in dust activity after ∼2 ka. The drought conditions during the 4.2 ka event recorded within the section appear to be a continuation of the trend of middle–late Holocene climatic deterioration, which is superimposed on a weakening trend of the ISM. The permanent human occupation of the study area after ∼2 ka corresponded to an interval of mild and wet climate when dust activity was relatively weak. An observed correlation between the intensity of human activity and dust storms suggests that the enhancement of human activities played a significant role in increasing the occurrence of dust storms. Overall, our results provide new insights into human-environmental interactions in the SETP.

Global-scale future climate projections from ACCESS model contributions to CMIP6

This paper describes projected climate evolution and outcomes simulated by the Australian Community Climate and Earth System Simulator (ACCESS) to varying future scenarios, including of socio-ecological and technological development, and land-use and land-cover change. Contributions to the Coupled Model Intercomparison Project Phase 6 (CMIP6) from the climate model version, ACCESS-CM2, and the fully coupled Earth System Model version, ACCESS-ESM1.5, are presented for the near-future (2020–2050), 21st Century (2000–2100) and longer-term (2100–2300). Scenario differentiation in the near future is aided by high-density sampling in large-ensemble ACCESS-ESM1.5, more clearly illustrating projected 2020–2050 global changes in temperature, precipitation and aerosol optical depth. Over the 21st Century, the heightened equilibrium climate sensitivity of ACCESS-CM2 relative to ACCESS-ESM1.5 results in persistently greater surface air temperature increases and larger amplified polar warming, leading to more rapid sea ice decline. Although weakening of the Atlantic meridional overturning circulation (AMOC) occurs in both models, 21st Century recovery under aggressive-mitigation and overshoot scenarios only occurs in ACCESS-ESM1.5; AMOC weakening continues under all scenarios in ACCESS-CM2 through to 2100. Longer-term climate response from simulations extending to 2300 depict opposing hemispheric responses of polar surface air temperatures and sea ice in both models under scenarios based on aggressive mitigation action, leading to a resurgence of surface ocean warming and Antarctic sea ice decline. Under a future scenario where development is driven by continued fossil fuel use, both AMOC and Antarctic Bottom Water Formation continue to weaken across 2200–2300 in both models, reaching such low levels in ACCESS-CM2 that these pivotal components of global meridional overturning circulation could be considered essentially to have ceased.

Persistent climate model biases in the Atlantic Ocean's freshwater transport

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is considered to be one of the most dangerous climate tipping elements. The salt–advection feedback plays an important role in AMOC tipping behaviour, and its strength is strongly connected to the freshwater transport carried by the AMOC at 34° S, below indicated by FovS. Available observations have indicated that FovS has a negative sign for the present-day AMOC. However, most climate models of the Coupled Model Intercomparison Project (CMIP, phase 3 and phase 5) have an incorrect FovS sign. Here, we analyse a high-resolution and a low-resolution version of the Community Earth System Model (CESM) to identify the origin of these FovS biases. Both CESM versions are initialised from an observed ocean state, and FovS biases quickly develop under fixed pre-industrial forcing conditions. The most important model bias is a too fresh Atlantic Surface Water, which arises from deficiencies in the surface freshwater flux over the Indian Ocean. The second largest bias is a too saline North Atlantic Deep Water and arises through deficiencies in the freshwater flux over the Atlantic Subpolar Gyre region. Climate change scenarios branched from the pre-industrial simulations have an incorrect FovS upon initialisation. Most CMIP phase 6 models have similar biases to those in the CESM. Due to the biases, the value of FovS is not in agreement with available observations, and the strength of the salt advection feedback is underestimated. Values of FovS are projected to decrease under climate change, and their response is also dependent on the various model biases. To better project future AMOC behaviour, an urgent effort is needed to reduce biases in the atmospheric components of current climate models.

Modulation of regional carbon uptake by AMOC and alkalinity changes in the subpolar North Atlantic under a warming climate

IntroductionThe slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and associated consequences on ocean carbon uptake could have large implications for the Earth’s climate system and its global carbon cycle.MethodsThis study analyzes ten Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and reveals that the regional carbon uptake in the subpolar North Atlantic under a high CO2 emission scenario moderately correlates with the decline in AMOC at 40°N. AMOC transports warm and salty subtropical waters to the subpolar regions.ResultsModels with stronger AMOC slowdown generally exhibit weaker surface warming and larger decline of surface salinity and alkalinity. We consider two plausible mechanisms linking the AMOC slowdown to the decline of regional CO2 uptake: the reduction in surface alkalinity and diminished subduction. The decline of surface salinity and alkalinity reduces the ocean’s capacity to buffer acids leading to a reduced CO2 uptake. This important contribution is unique to the North Atlantic. Diminished convective mixing and subduction of surface water can further decrease the downward transport of anthropogenic carbon, as also shown in previous research. The centennial trends of pCO2 are decomposed into four components driven by temperature, salinity, alkalinity and dissolved inorganic carbon, revealing that alkalinity and dissolved inorganic carbon are both significant contributors. The alkalinity-driven pCO2 essentially follows surface salinity, establishing the linkage between AMOC slowdown and alkalinity decline.DiscussionOur results indicate that alkalinity changes are important for the interplay between AMOC and the regional carbon sequestration ability across the late 20th and the entirety of the 21st century in the subpolar North Atlantic.

Collapse and slow recovery of the Atlantic Meridional Overturning Circulation (AMOC) under abrupt greenhouse gas forcing

Collapse and slow recovery of the Atlantic Meridional Overturning Circulation (AMOC) under abrupt greenhouse gas forcing

A multi-model assessment of the early last deglaciation (PMIP4 LDv1): a meltwater perspective

Abstract. The last deglaciation (∼20–11 ka BP) is a period of a major, long-term climate transition from a glacial to interglacial state that features multiple centennial- to decadal-scale abrupt climate variations whose root cause is still not fully understood. To better understand this time period, the Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an internationally coordinated endeavour in simulating the last deglaciation whilst encompassing a broad range of models. Here, we present a multi-model intercomparison of 17 transient simulations of the early part of the last deglaciation (∼20–15 ka BP) from nine different climate models spanning a range of model complexities and uncertain boundary conditions and forcings. The numerous simulations available provide the opportunity to better understand the chain of events and mechanisms of climate changes between 20 and 15 ka BP and our collective ability to simulate them. We conclude that the amount of freshwater forcing and whether it follows the ice sheet reconstruction or induces an inferred Atlantic meridional overturning circulation (AMOC) history, heavily impacts the deglacial climate evolution for each simulation rather than differences in the model physics. The course of the deglaciation is consistent between simulations except when the freshwater forcing is above 0.1 Sv – at least 70 % of the simulations agree that there is warming by 15 ka BP in most places excluding the location of meltwater input. For simulations with freshwater forcings that exceed 0.1 Sv from 18 ka BP, warming is delayed in the North Atlantic and surface air temperature correlations with AMOC strength are much higher. However, we find that the state of the AMOC coming out of the Last Glacial Maximum (LGM) also plays a key role in the AMOC sensitivity to model forcings. In addition, we show that the response of each model to the chosen meltwater scenario depends largely on the sensitivity of the model to the freshwater forcing and other aspects of the experimental design (e.g. CO2 forcing or ice sheet reconstruction). The results provide insight into the ability of our models to simulate the first part of the deglaciation and how choices between uncertain boundary conditions and forcings, with a focus on freshwater fluxes, can impact model outputs. We can use these findings as helpful insight in the design of future simulations of this time period.

Tropical Atlantic variability in EC-EARTH: impact of the radiative forcing

AbstractUnderstanding the impact of radiative forcing on climate variability and change in the Tropical Atlantic is crucial for different socio-economic sectors, given their substantial impacts in both local and remote regions. To properly evaluate the effect of a changing climate on the variability, the use of standard transient historical and scenario simulations requires very large ensembles. A computationally cheaper alternative implemented in this study consists of performing two 250-year-long atmosphere-ocean coupled simulations with EC-EARTH 3.3 (CMIP6 version) with fixed radiative forcing at the years 2000 and 2050, representative of present and future climate conditions, respectively. The changes in the leading modes of Tropical Atlantic variability (TAV), including the Atlantic Niño/Niña and the Subtropical North Atlantic pattern, have been assessed in three target seasons: spring (MAM), summer (JJ) and early winter (ND). While the change in sea surface temperature (SST) climatology shows homogeneous warming, the difference between future and present SST variability exhibits a distinct behaviour consistent along the seasonal cycle, with a decrease in the equatorial region and an increase at subtropical latitudes. This study explores the processes associated with the suppressed/enhanced TAV, with a particular focus on the less-explored early winter season. In agreement with previous studies, the Atlantic Meridional Overturning Circulation (AMOC) shows a weakening in strength, but the results also show an increase in variability. The AMOC-related deepening of the equatorial thermocline and the flattening linked to weakened trade winds are consistent with the suppressed SST variability of the Atlantic Niño/Niña. On the other hand, the enhanced SST variability at subtropical latitudes is mainly associated with an increase in turbulent heat flux variability, with a minor contribution of the mixed layer depth variability. Variability in turbulent heat flux is influenced primarily by latent heat flux, connected to changes in precipitation variability.

Hydrothermal and eco-environmental evolution on the southeastern Chinese Loess Plateau since the last deglaciation: evidence from terrestrial mollusk records

The ongoing global warming is altering hydrothermal patterns and eco-environments. However, it is unclear how they would respond under a natural warming scenario. Terrestrial mollusks in loess-paleosol sequences could serve as natural biological archives to explore this issue. In this study, terrestrial mollusk records from two loess-paleosol profiles on the southeastern Chinese Loess Plateau (CLP) were analyzed. Our results show that the cold-aridiphilous species decreased and the overall biodiversity increased gradually during the last deglaciation (∼19.0–11.5 ka BP), which was accompanied with the shift from cold-dry to warm-humid climate. Moreover, the mollusk communities reacted to Heinrich event 1 (H1) cooling. In the early-middle Holocene (∼11.5–5.5 ka BP), mollusk communities were dominated by the thermo-humidiphilous species, characterized by stable community composition, increased species richness and evenness with warm-humid climate. Thereafter, there was a noticeable decrease in species diversity and increase in the cold-aridiphilous species in the mid-late Holocene (∼5.5–0 ka BP), associated with a gradual shift towards a cold-dry climate. On the whole, it is noteworthy that the count of thermo-humidiphilous species increased remarkably at ∼16 ka BP, which is synchronous with a significant increase in the Northern Hemisphere summer insolation. Interestingly, this change of terrestrial mollusks preceded the formation of the Holocene paleosol at ∼8 ka as indicated by the pronounced increase in magnetic susceptibility and decrease of mean grain size. This suggests that the substantial East Asian summer monsoon (EASM) precipitation lagged the onset of deglacial warming on the southeastern CLP. Furthermore, this delay may be attributed to the negative feedback of North Atlantic meltwater, which caused a decline in the intensity of Atlantic Meridional Overturning Circulation (AMOC) and inhibited the rapid enhancement of EASM precipitation during the last deglaciation. Altogether, this provides crucial geological background evidence for exploring the potential regional hydrothermal and biodiversity responses to global warming in the near future.

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

Abstract. Atmospheric carbon dioxide concentration (pCO2) has increased by approximately 80 ppm from the Last Glacial Maximum (LGM) to the early Holocene. The change in this atmospheric greenhouse gas is recognized as a climate system response to gradual change in insolation. Previous modeling studies suggested that the deglacial increase in atmospheric pCO2 is primarily attributed to the release of CO2 from the ocean. Additionally, it has been suggested that abrupt change in the Atlantic meridional overturning circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, understanding remains limited regarding oceanic circulation changes and the factors responsible for changes in chemical tracers in the ocean during the last deglaciation and their impact on atmospheric pCO2. In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes similar to those observed in reconstructions. We investigate the reliability of simulated changes in the ocean carbon cycle by comparing the simulated carbon isotope ratios with sediment core data, and we examine potential biases and overlooked or underestimated processes in the model. Qualitatively, the modeled changes in atmospheric pCO2 are consistent with ice core records. For example, during Heinrich Stadial 1 (HS1), atmospheric pCO2 increases by 10.2 ppm, followed by a reduction of 7.0 ppm during the Bølling–Allerød (BA) period and then by an increase of 6.8 ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 during these events compared to values derived from ice core data. Radiocarbon and stable isotope signatures (Δ14C and δ13C) indicate that the model underestimates both the activated deep-ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean and the active ventilation in the North Pacific Intermediate Water (NPIW) during HS1. The relatively small changes in simulated atmospheric pCO2 during HS1 might be attributable to these underestimations of ocean circulation variation. The changes in Δ14C associated with strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with values derived from sediment core records. However, although the data indicate continuous increase in δ13C in the deep ocean throughout the YD period, the model shows the opposite trend. It suggests that the model either simulates excessive weakening of the AMOC during the YD period or has limited representation of geochemical processes, including marine ecosystem response and terrestrial carbon storage. Decomposing the factors behind the changes in ocean pCO2 reveals that variations in temperature and alkalinity have the greatest impact on change in atmospheric pCO2. Compensation for the effects of temperature and alkalinity suggests that the AMOC changes and the associated bipolar climate changes contribute to the decrease in atmospheric pCO2 during the BA and the increase in atmospheric pCO2 during the YD period.

Optimizing the key parameter to accelerate the recovery of AMOC under a rapid increase of greenhouse gas forcing

Atlantic Meridional Overturning Circulation (AMOC) plays a central role in long-term climate variations through its heat and freshwater transports, which can collapse under a rapid increase of greenhouse gas forcing in climate models. Previous studies have suggested that the deviation of model parameters is one of the major factors in inducing inaccurate AMOC simulations. In this work, with a low-resolution earth system model, the authors try to explore whether a reasonable adjustment of the key model parameter can help to re-establish the AMOC after its collapse. Through a new optimization strategy, the extra freshwater flux (FWF) parameter is determined to be the dominant one affecting the AMOC's variability. The traditional ensemble optimal interpolation (EnOI) data assimilation and new machine learning methods are adopted to optimize the FWF parameter in an abrupt 4×CO2 forcing experiment to improve the adaptability of model parameters and accelerate the recovery of AMOC. The results show that, under an abrupt 4×CO2 forcing in millennial simulations, the AMOC will first collapse and then re-establish by the default FWF parameter slowly. However, during the parameter adjustment process, the saltier and colder sea water over the North Atlantic region are the dominant factors in usefully improving the adaptability of the FWF parameter and accelerating the recovery of AMOC, according to their physical relationship with FWF on the interdecadal timescale.