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Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

Regional hydrological sensitivity (i.e., precipitation change per degree local surface warming) contributes substantially to the uncertainty in future precipitation projections over tropical oceans. Here, we investigate the sensitivity of relative precipitation (P*, precipitation divided by the basin average precipitation) to local sea surface temperature (SST) change by dissecting it into three components, namely the sensitivity of P* to relative SST (SSTrel, SST minus the tropical mean SST) changes, the sensitivity of P* to surface convergence changes, and the sensitivity of surface convergence to SST gradient changes. We show that the relationships between P* and SSTrel, and between P*, surface convergence, and SST gradients are largely constant during climate change. This allows us to constrain regional hydrological sensitivity based on present‐day SST‐precipitation relationships. The sensitivity of surface convergence to SST gradient changes is a main source of uncertainty in regional hydrological sensitivity and is likely underestimated in GCMs. Plain Language Summary: Understanding how precipitation will change over tropical oceans is important because these changes influence the atmospheric circulation, which in turn affects the global climate and weather patterns. Climate models disagree on their projections of precipitation changes over tropical oceans in part due to a lack of understanding on how precipitation should respond to a given amount of local surface warming. We find that the sensitivity of precipitation to future changes in local sea surface temperature (which is commonly referred to as regional hydrological sensitivity) largely depends on the present‐day relationship between precipitation and local sea surface temperature, as well as that between precipitation and the spatial gradient in sea surface temperature, and both relationships are observable and thus can serve as constraints. We find that inter‐model differences in regional hydrological sensitivity result primarily from differences in the response of surface winds to sea surface temperature gradient changes. Key Points: Regional hydrological sensitivity is an important source of uncertainty in rainfall projections over tropical oceansRegional hydrological sensitivity can be constrained by components of rainfall‐temperature relationship that stay constant during warmingUncertainty in regional hydrological sensitivity originates largely from surface convergence sensitivity to temperature gradient changes [ABSTRACT FROM AUTHOR]

Persistent precipitation deficits are among the most impactful consequences of global warming. Here we focus on changes in the annual number of dry days (NDD) and in the annual maximum length of dry spells due to a quadrupling of atmospheric CO2. We use atmosphere‐only simulations to decompose the projected changes into additive contributions. A fast adjustment leads to a global increase in NDD despite notable regional exceptions (e.g., South Asia and Sahel). The effect of the uniform component of the surface ocean warming is model‐dependent but shapes the regional distribution of the NDD response in each model. Finally, the ocean warming pattern also contributes to large uncertainties, likely through contrasting changes in large‐scale circulation. Our results thus highlight the complexity of the NDD response, with policy‐relevant practical implications for mitigation and adaptation strategies. Plain Language Summary: Global warming is expected to intensify the global water cycle, including the intensity and frequency of precipitation extremes. Yet, the response of the dry side of the daily precipitation distribution to increased atmospheric CO2 has received so far less attention. Here we show that this response remains highly model‐dependent across the latest generation of global climate models. Furthermore, we use atmosphere‐only simulations to isolate different drivers of the precipitation response. A fast radiative and vegetation adjustment to increased CO2 leads to an overall increase in the mean annual number of dry days (NDD) despite some regional exceptions. The effect of the ocean surface warming is highly model‐dependent, only partly due to the diversity of the simulated patterns of sea surface temperature anomalies. The regional response of NDD and of the maximum length of dry spells thus does not simply scale with global warming across the different models. Key Points: AGCM simulations are used to split the dry day response to increased CO2 into fast adjustment versus slower uniform and patterned SST effectsNone of the three components consistently dominates the global land response of dry days across the selected CMIP6 modelsThe uniform SST warming dominates the regional anomalies, which however do not scale with global warming across the multi‐model ensemble [ABSTRACT FROM AUTHOR]

Previous studies documented possible connections between low frequency climate modes in the Northern Hemisphere ocean basins. We use observed sea surface temperatures and 270 large ensemble climate model simulations, which allows for improved methods of separating external and internal variability, such as removing the ensemble mean from each simulation. Detrending methods for observations have also improved since some of these previous studies were conducted. We also devise a modified statistical test using bootstrapping that is tuned specifically to this analysis. With these tools, we reexamine relationships among these modes. While previous studies have argued for the existence of an inter‐basin link, our results suggest that any internal connections between these modes are indistinguishable from random noise. Further, we show that external forcing affects each region in similar ways. This suggests that anthropogenic warming can cause an indirect link between the two basins, confounding the interpretation of a potential relationship. Plain Language Summary: We reexamine possible connections between climate patterns in the North Atlantic and North Pacific oceans. Improved climate model simulations and appropriate statistical methods allow us to build on previous research of these linkages. In contrast to previous studies, no natural connections are detected. However, global warming is shown to affect each region in similar ways, suggesting that climate change could cause an indirect link between the two basins. Key Points: A multi model large ensemble archive and modified statistical assumptions provide a framework for studying climatic relationshipsAn analysis of relationships between leading Northern Hemisphere climate modes does not show any internal connectionsExternal forcing such as global warming is shown to be a possible confounding factor in climate relationships [ABSTRACT FROM AUTHOR]

The Interdecadal Pacific Oscillation (IPO) features a basin‐scale horseshoe‐like sea surface temperature (SST) anomaly pattern. Its cold‐phase shift around 1999, implicated as a driver for the early‐2000s global warming slowdown, has been potentially linked to the Atlantic warming during the positive‐phase Atlantic Multidecadal Variability (AMV). However, the key mechanism for the trans‐basin Atlantic‐Pacific teleconnection remains debatable. Here, we show that an AMV‐SST cooling can initiate a pan‐Pacific response. The North Atlantic cooling induces westerly wind anomalies over the central‐western equatorial Pacific as Kelvin‐wave responses and easterly wind anomalies over the far‐eastern equatorial Pacific as Rossby‐wave responses. Additionally, anomalous lows are generated over the extratropical North and South Pacific through the midlatitude Rossby wave propagation. The tropical and midlatitude teleconnections act together to develop into a warm‐phase IPO‐like pattern through the wind‐induced latent heat. Our results suggest a potential of advancing the predictability of IPO through a skillful simulation of AMV. Plain Language Summary: The Interdecadal Pacific Oscillation (IPO) is the most prominent mode of decadal climate variability over the whole Pacific basin, characterized by a horseshoe‐shaped sea surface temperature (SST) anomaly pattern. Its Atlantic counterpart, the Atlantic Multidecadal Variability (AMV) is characterized by a basin‐wide SST warming or cooling pattern across the North Atlantic. In recent decades, the Atlantic warming strongly tied to the positive phase of AMV has significantly strengthened the tropical Pacific cooling, contributing to the recent global warming hiatus. Yet, the root cause of the trans‐basin Atlantic‐Pacific teleconnection on decadal timescales remains unclear. In this study, using the idealized pacemaker simulations with an imposed AMV‐SST anomaly pattern in a fully coupled model, we rigorously investigate how an AMV‐SST cooling impacts the Pacific‐basin decadal‐scale climate change. We found that an AMV cooling initiates a tropical Gill‐type response and midlatitude Rossby wave trains. The tropical and midlatitude teleconnections work together to form a warm‐phase IPO‐like SST anomaly pattern. Our results highlight that realistic simulation of the AMV may improve the predictability of the IPO state and predictions of global warming rates. Key Points: The Atlantic Multidecadal Variability cooling can drive an Interdecadal Pacific Oscillation‐like warm‐phase patternThe North Atlantic cooling excites a tropical Gill‐type response and midlatitude Rossby wave train responsesBoth the tropical and midlatitude dynamical pathways are central to the decadal‐scale trans‐basin Atlantic‐Pacific connection [ABSTRACT FROM AUTHOR]

Recent evidence shows that wind‐driven ocean currents, like the western boundary currents, are strongly affected by global warming. However, due to insufficient observations both on temporal and spatial scales, the impact of climate change on large‐scale ocean gyres is still not clear. Here, based on satellite observations of sea surface height and sea surface temperature, we find a consistent poleward shift of the major ocean gyres. Due to strong natural variability, most of the observed ocean gyre shifts are not statistically significant, implying that natural variations may contribute to the observed trends. However, climate model simulations forced with increasing greenhouse gases suggest that the observed shift is most likely to be a response of global warming. The displacement of ocean gyres, which is coupled with the poleward shift of extratropical atmospheric circulation, has broad impacts on ocean heat transport, regional sea level rise, and coastal ocean circulation. Plain Language Summary: Ocean circulation plays a vital role in regulating the weather and climate and supporting marine life. Therefore, it is important to understand whether and how it responds to global warming. However, the available observations are currently sparse and short in duration, making it difficult to track the dynamic changes of large‐scale ocean circulation. Here, we show for the first time, independent satellite observational evidence demonstrating that the large‐scale ocean gyres are moving poleward during the past four decades. Further analysis based on climate models and various other data sets reveal that the poleward shifting of the ocean gyre circulation is most likely to be a consequence of global warming, which so far has not been well recognized by the public and the scientific community. Key Points: Satellite observations show a consistent poleward shift of the major ocean gyres during the past four decadesDue to strong natural variability, most of the observed ocean gyre shifts are not statistically significantClimate model simulations suggest that the observed shift is most likely to be a response to global warming [ABSTRACT FROM AUTHOR]

Key Points: The uplift event (~10.5–8 Ma) revealed by glycerol dialkyl glycerol tetraethers temperature records from the Xining Basin is synchronous with regional dryingThe cooling amplitude over land is less than that over the ocean during the CO2‐dominated Late Miocene cooling event (~7–5.4 Ma)Tectonic forcing, rather than pCO2, dominated regional continental climate patterns and ecosystem transitions during the Late Miocene During the Late Miocene, the climate patterns and ecosystems of continental land masses experienced crucial transitions, but whether the principal driver was regional tectonic forcing or a decline in CO2 concentrations remains debated. Here we present a terrestrial paleotemperature record spanning ~12.7–5.2 Ma based on tetraether lipids extracted from the northeastern Tibetan Plateau. Our results reveal a sharp cooling (~8 °C) during ~10.5–8 Ma, asynchronous with minor fluctuations in global sea temperatures, suggesting a rapid tectonic uplift of ~1 km in extent. This event appears consistent with the simultaneous aridification and transitions of ecosystems experienced in adjacent regions. Moreover, the amplitude of the cooling over land is less than that which occurred over the ocean during the CO2‐dominated Late Miocene cooling event (~7–5.4 Ma). We therefore concluded that tectonic forcing, rather than a decline in CO2 levels, most likely dominated continental climate patterns and ecosystem transitions during the Late Miocene. Plain Language Summary: Animals and plants in many regions had to survive a gradual drying and cooling environment during the Late Miocene epoch (ca. 12–5 Ma). Exploring the drivers and mechanisms behind Late Miocene climate change is helpful when considering current global warming trends. Although the exact reason for these Late Miocene changes remains unclear, two suspects have been identified, that is, a decline in CO2 levels and/or regional tectonic activity. In this study we used microbial cell membrane lipid fossils to reconstruct the quantitative paleotemperatures and paleoaltitudes of the Xining Basin, on the northeastern Tibetan Plateau, during the Late Miocene. Our results revealed a surficial uplift event (~10.5–8 Ma) delineated by the identification of a rapid cooling process during a period that experienced constant sea surface temperatures, leading to an increasingly dry climate and ecosystem changes over large areas. Moreover, the amplitude of this cooling over land appears to have been less than that observed over the ocean during the CO2‐dominated Late Miocene cooling event (~7–5.4 Ma). Therefore, we would posit that regional tectonic activities played a more important role than CO2 levels in the changes to terrestrial climate patterns and ecosystems during the Late Miocene. [ABSTRACT FROM AUTHOR]

Arabian Sea (AS) warming has been significantly accelerated since the 1990s, in particular in the spring season. Here we link the AS warming changes to the Atlantic multidecadal oscillation (AMO). A set of Atlantic pacemaker experiments with a slab mixed‐layer ocean model successfully reproduces the AS spring multidecadal variability and its connection with the AMO. An atmospheric teleconnection from the Atlantic to the AS in the preceding winter and associated thermodynamic air‐sea feedback is found to be important. The teleconnection can be reestablished by the atmospheric model when the AMO sea surface temperature (SST) and its trans‐basin footprint over the western Pacific are prescribed simultaneously. The western Pacific SST warming associated with the AMO positive phase induces a Gill‐type Rossby wave over the AS, showing anomalously low pressures and converging southerlies that weaken winter northerlies. Thus, the wind‐evaporation‐SST feedback results in and maintains the AS warm SST anomalies to the subsequent spring. Plain Language Summary: The rapid warming of the Arabian Sea (AS) since the 1990s not only has significant impacts on the monsoon climate change and extreme weather events (flood, heat wave, and cyclone) in Arabian Peninsula and Indian subcontinent but also poses increasingly severe risks of damage to the coastal and marine ecosystems. However, the cause of this recent acceleration of AS warming remains unclear. Here using observations and atmosphere‐ocean coupled models, we link the observed AS warming changes to the Atlantic multidecadal oscillation (AMO) and show that more than 70% of AS multidecadal variability can be explained by the AMO. This Atlantic‐AS teleconnection involves atmosphere‐ocean interactions across multiple ocean basins, with a contribution from the western Pacific (WP). The SST footprint of the AMO over the WP acts as a relay for the effect of the AMO on the AS by exciting an atmospheric teleconnection and subsequently thermodynamic feedbacks over the AS. The concurrent cold‐to‐warm phase shift of the AMO and its WP SST footprint since the 1990s contribute constructively to the rapid warming of the AS. Our results highlight an unexpected multiple‐basin interaction at decadal timescales, which plays a key role in the attribution of historical regional SST warming. Key Points: The observed Arabian Sea warming changes (slowdown and acceleration) are linked to the Atlantic multidecadal oscillation for the first timeThe teleconnection involves air‐sea interactions across multiple ocean basins, with a contribution from western Pacific that acts as a relayThe decadal phase shift of Atlantic‐Western Pacific trans‐basin variability since the 1990s contributes to the rapid warming of Arabian Sea [ABSTRACT FROM AUTHOR]

Arctic moisture intrusions have played an important role in warming the Arctic over the past few decades. A prior study found that Coupled Model Intercomparison Project Phase 5 (CMIP5) models exhibit large regional biases in the moisture flux across 70°N. It is shown here that the systematic misrepresentation of the moisture flux is related to the models' overprediction of zonal wavenumber k = 2 contribution and underprediction of k = 1 contribution to the flux. Models with a warmer tropical upper troposphere and El‐Niño‐like tropical surface temperature tend to simulate stronger k = 2 flux, while k = 1 flux is uncorrelated with tropical upper tropospheric temperature and is associated with La‐Niña‐like surface temperature. The models also overpredict the transient eddy moisture flux while underpredicting the stationary eddy flux. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show trends in moisture flux that is consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. Plain Language Summary: The Arctic is the region where climate change has been most rapid. A growing body of work indicates that moisture intrusions into Arctic have played an important role in warming the Arctic over the past decades. Coupled Model Intercomparison Project Phase 5 (CMIP5) models have served as a critical tool for projecting future climate changes. Therefore, it is imperative to evaluate whether the physical processes governing moisture intrusions are accurately represented by the models. It is shown here that there is a systematic misrepresentation of the moisture flux into the Arctic related to the models' biased representation of tropical temperatures. Moreover, future projections in Representative Concentration Pathway 8.5 (RCP8.5) simulations show moisture flux trends that are consistent with biases in historical simulations, suggesting that these CMIP5 projections reflect the same error(s) that cause the model biases. It is common practice to regard averages across climate model as being the true response to climate forcing. The findings here question this widespread assumption and underscore the need to pay close attention to model biases and their causes. Key Points: Climate models overpredict (underpredict) zonal‐wavenumber k = 2 (k = 1) contribution to moisture flux across Arctic boundaryThe models with warmer tropical upper troposphere tend to simulate stronger k = 2 flux, while no such relation is present for k = 1 fluxClimate models overpredict transient eddy contribution to the flux at the expense of stationary eddy contribution [ABSTRACT FROM AUTHOR]

Recent studies have revealed a global, poleward migration trend of tropical cyclones (TCs) in terms of annual‐averaged latitude of lifetime maximum intensity under the global warming. The TCs in the western North Pacific (WNP) make the largest contribution to this trend. One existing hypothesis is that there is a linkage between poleward migration of TCs and anthropogenic forcing. Here we introduce a new measurement, the lifetime‐averaged latitude weighted by TC destructive potential, to detect the meridional migration of the WNP TCs, which is more reliable and meaningful as it considers not only past data uncertainty but also public concerns. Our results show that the trend of WNP TC migration reversed during the recent warming slowing‐down period (after ~1999). Different from the existing hypothesis, it is the change of regional pattern of sea surface temperature that contributes greatly to the meridional migration of WNP TCs. Plain Language Summary: Tropical cyclones (TCs) account for the majority of natural catastrophic losses in the developed world and are the second leading cause (next to floods) of death and injury among natural disasters affecting developing countries. The challenges for climate detection and attribution research with regard to TCs are to determine whether an observed change in TC activity exceeds the natural variability and to attribute significant changes to anthropogenic forcing (e.g., greenhouse gases or aerosols). Recent studies have indicated that the TCs over the western North Pacific (WNP) systematically migrated poleward, which is plausibly linked to the expansion of the tropics associated with anthropogenic forcing. However, utilizing our new measurement, we find that the poleward migration of WNP TCs reversed direction during the recent warming slowing‐down period (after ~1999). We thus propose a new mechanism associated with the regional pattern of sea surface temperature change, which is characterized by a relatively warm or cold sea surface temperature pool in the region (20–40°N, 140–160°E). Compared with the previous theory based on anthropogenic forcing and well‐known dominant modes of variability, the new mechanism provides a more convincing explanation for the migration of WNP TCs since 1980, especially for the recent warming slowing‐down period. Key Points: The trend of tropical cyclone migration over the western North Pacific reversed during the recent warming slowing‐down period (after ~1999)It is the change of regional sea surface temperature that contributed greatly to the meridional migration trend of tropical cyclonesThe effect of regional sea surface temperature change came from its impact on storm potential intensity and large‐scale circulation [ABSTRACT FROM AUTHOR]

Abstract: On interannual to decadal time scales, the climate mode with many of the strongest societal impacts is the El Niño–Southern Oscillation (ENSO). However, quantifying ENSO's changes in a warming climate remains a formidable challenge, due to both the noise arising from internal variability and the complexity of air‐sea feedbacks in the tropical Pacific Ocean. In this work, we use large (≥30‐member) ensembles of climate simulations to show that anthropogenic climate change can produce systematic increases in ENSO teleconnection strength over many land regions, driving increased interannual variability in regional temperature extremes and wildfire frequency. As the spatial character of this intensification exhibits strong land‐ocean contrasts, a causal role for land‐atmosphere feedbacks is suggested. The identified increase in variance occurs in multiple model ensembles, independent of changes in sea surface temperature variance. This suggests that in addition to changes in the overall likelihoods of heat and wildfire extremes, the variability in these events may also be a robust feature of future climate. [ABSTRACT FROM AUTHOR]

A global ¼° ocean/sea‐ice 50‐member ensemble simulation is analyzed to disentangle the imprints of the atmospheric forcing and the chaotic ocean variability on regional sea level trends over the satellite altimetry period. We find that the chaotic ocean variability may mask atmospherically forced regional sea level trends over 38% of the global ocean area from 1993 to 2015, and over 47% of this area from 2005 to 2015. These regions are located in the western boundary currents, in the Southern Ocean and in the subtropical gyres. While these results do not question the anthropogenic origin of global mean sea level rise, they give new insights into the intrinsically oceanic versus atmospheric forcing of regional sea level trends and provide new constraints on the measurement time required to attribute regional sea level trends to the atmospheric forcing or to climate change. Plain Language Summary: As a direct consequence of anthropogenic influences, global mean sea level rises in response to ocean warming and land ice melting. Since the early 1990s, satellite altimetry has revealed large regional contrasts in sea level trends, controlled by temperature and salinity changes, oceanic processes and atmospheric forcing. Using an ensemble of forced eddying ocean simulations, we show that regional sea level trends over the altimetric period are only partly determined by the atmospheric evolution (both natural and anthropogenic): nonlinear ocean processes produce additional sea level trends that are inherently random, which can compete in certain regions with the externally forced trends. These results do not question the existence of global and regional sea level trends, but suggest that sea level trends may not be unambiguously attributed to external causes in certain regions. Key Points: Atmospheric drivers force regional sea level trends over most of the global ocean area since 1993Chaotic ocean variability may mask these atmospherically forced regional trends over a substantial fraction of the global ocean areaIn the latter regions, altimeter‐derived regional sea level trends may not be representative of anthropogenic or atmospheric causes [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is a key component of the global climate system. Recent studies suggested a twentieth‐century weakening of the AMOC of unprecedented amplitude (~15%) over the last millennium. Here we present a record of δ18O in benthic foraminifera from sediment cores retrieved from the Laurentian Channel and demonstrate that the δ18O trend is linked to the strength of the AMOC. In this 100‐year record, the AMOC signal decreased steadily to reach its minimum value in the late 1970s, where the weakest AMOC signal then remains constant until 2000. We also present a longer δ18O record of 1,500 years and highlight the uniqueness of the last century δ18O trend. Moreover, the Little Ice Age period is characterized by statistically heavier δ18O, suggesting a relatively weak AMOC. Implications for understanding the mechanisms driving the intensity of AMOC under global warming and high‐latitude freshwater input are discussed. Plain Language Summary: Oceanic circulation in the North Atlantic transports huge amounts of water, heat, salt, carbon, and nutrients around the globe. As such, changes in the strength of oceanic currents can yield profound changes in both North American and European climate, in addition to affecting the African and Indian summer monsoon rainfall. In this study, we used geochemical evidence to highlight a slowdown in the North Atlantic Ocean circulation over the last century. This change appears to be unique over the last 1,500 years and could be related to global warming and freshwater input from ice sheet melt. Based on our data, we also suggest that the period often called "The Little Ice Age" was characterized by a slowdown, of less amplitude than the modern weakening, in the North Atlantic Ocean circulation. Thus, our results contribute to ongoing investigations of the state of the circulation in the North Atlantic by providing a robust reconstruction of its variability over the last 1,500 years. Key Points: The oxygen isotope in the Laurentian Channel can trace variations in the subsurface western North Atlantic circulationThe twentieth century is characterized by the weakest Atlantic meridional overturning circulation of the last 1,500 yearsThe Little Ice Age is also characterized by relatively higher oxygen isotope, which can be linked to a weaker AMOC [ABSTRACT FROM AUTHOR]