The Atlantic Multidecadal Variability in Observations and in a Large Historical Multimodel Ensemble: Forced and Internal Variability

<p>Vegetation in the northern high latitudes shows a characteristic pattern of persistent changes as documented by multi-decadal satellite observations. The prevailing explanation that these mainly increasing trends (greening) are a consequence of external CO<sub>2</sub> forcing, i.e., due to the ubiquitous effect of CO2-induced fertilization and/or warming of temperature-limited ecosystems, however does not explain why some areas also show decreasing trends of vegetation cover (browning). We propose here to consider the dominant mode of multi-decadal internal climate variability in the north Atlantic region, the Atlantic Multidecadal Variability (AMV), as the missing link in the explanation of greening and browning trend patterns in the northern high latitudes. Such a link would also imply potential for decadal predictions of ecosystem changes in the northern high latitudes.</p><p>An analysis of observational and reanalysis data sets for the period 1979-2019 shows that locations characterized by greening trends largely coincide with warming summer temperature and increasing precipitation. Wherever either cooling or decreasing precipitation occurs, browning trends are observed over this period. These precipitation and temperature patterns are significantly correlated with a North Atlantic sea surface temperature index that represents the AMV signal, indicating its role in modulating greening/browning trend patterns in the northern high latitudes.</p><p>Using two large ensembles of coupled Earth system model simulations (100 members of MPI-ESM-LR Grand Ensemble and 32 members of the IPSL-CM6A-LR Large Ensemble), we separate the greening/browning pattern caused by external CO<sub>2</sub> forcing from that caused by internal climate variability associated with the AMV. These sets of model simulations enable a clean separation of the externally forced signal from internal variability. While the greening and browning patterns in the simulations do not agree with observations in terms of magnitude and location, we find consistent internally generated greening/browning patterns in both models caused by changes in temperature and precipitation linked to the AMV signal. These greening/browning trend patterns are of the same magnitude as those caused by the external forcing alone. Our work therefore shows that internally-generated changes of vegetation in the northern lands, driven by AMV, are potentially as large as those caused by external CO<sub>2</sub> forcing. We thus argue that the observed pattern of greening/browning in the northern high latitudes could originate from the combined effect of rising CO<sub>2</sub> as well as the AMV.</p>

Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951–2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO–AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS.

In this study, the impacts of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) on the western Pacific warm pool (WPWP) were investigated. Our results show that the WPWP is linked with the PDO and the AMO at multiple time scales. On the seasonal time scales, the WPWP and the PDO/AMO reinforce each other, while at decadal time scales the forcing roles of the PDO and the AMO dominate. Notably, a positive PDO tends to enlarge the WPWP at both seasonal and decadal time scales, while a positive AMO tends to reduce the WPWP at decadal time scales. Furthermore, the decadal variability of the WPWP can be well predicted based on the PDO and AMO.

Internal climate variability is a major source of uncertainty in decadal predictions of summer rainfall in the Sahel, where it can amplify or attenuate the impact of anthropogenically forced changes. Yet, its complexity and inherent randomness make it difficult to interpret, communicate, and integrate into decision-relevant information. Here we introduce a novel framework to construct physically plausible storylines of internal variability (IVSs), based on a large set of multi-model large ensembles, which provides a robust sampling of internal variability and model uncertainty. Selecting Pacific Decadal Variability (PDV) and Atlantic Multidecadal Variability (AMV) as remote drivers of Sahelian climate, we identify coherent trajectories of internal variability that are physically interpretable.We find that the proposed IVSs capture - and separate - a range of plausible near-term, decadal futures shaped by Atlantic and Pacific variability patterns, shedding light on the dynamics that modulate decadal rainfall regimes. In particular we identify a high-impact storyline of a positive AMV in-phase with a negative PDV leads to wet anomalies which roughly double the forced response over the Western Sahel, while the contrasting storyline largely offsets the effect of climate change in this region. By combining physical understanding with robust statistical methods and multi-model large ensembles, the proposed method seeks to bridge the gap between climate science and action, providing more interpretable and decision-relevant regional climate information.

The origin of characteristic increase of continental-scale summer surface temperature in decades over Northern hemisphere was investigated. July-temperature in Europe, East Asia, the North Pacific, and western North America have been undergoing more increase than the global average of about 1ºC increase, especially in recent years. On the contrary, MME of 37 CMIP5 models do not show the focused regional warming hot-spots but exhibits hemispheric surface temperature increase. From the comparison between GISTEMP observation record and CMIP5 MME historical data, we show an evidence that the observed characteristic increase of regional temperature for the recent 43 years is dominated by the Internal Climate Variability (ICV) of decadal time-scale. Performing EOF analysis on the ICV, the four dominant modes are identified as Pacific Decadal Oscillation (PDO), North Pacific Oscillation (NPO), Pacific Meridional Mode (PMM), and Atlantic Multi-decadal Oscillation (AMO). It is also shown that, with only two dominant modes among those modes, large portion of continental-scale temperature increase can be explained: We show that PDO and PMM are dominant modes in Europe and East Asia, NPO and AMO in the North Pacific, and PDO and NP in western North America. The observed sub-trend is nicely reproduced with only these two modes. Quantitatively, the observed sub-trend from ICV explains 73%, 60%, 55%, and 18% of the total variability in North Pacific, East Asia, western North America and Europe, respectively. Note that, despite 82% of temperature fluctuation in Europe is attributed by the external forcing, 18% of the internal variability is still important to explain the increasing number of extreme heat events in Europe in recent several decades.

Interannual to Decadal Rainfall Variability over Greater Horn of Africa: Non-Stationary Teleconnections, Physical Mechanisms, and Predictability

The Atlantic multidecadal variability (AMV) is a coherent pattern of variability of the North Atlantic sea surface temperature field affecting several components of the climate system in the Atlantic region and the surrounding areas. The relatively short observational record severely limits our understanding of the physical mechanisms leading to the AMV. The present study shows that the spatial and temporal characteristics of the AMV, as assessed from the historical records, should also be considered as highly uncertain. Using 11 multi-century preindustrial climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database, we show that the AMV characteristics are not constant along the simulation when assessed from different 200-year-long periods to match the observed period length. An objective method is proposed to test whether the variations of the AMV characteristics are consistent with stochastic internal variability. For 7 out of the 11 models analysed, the results indicate a non-stationary behaviour for the AMV time series. However, the possibility that the non-stationarity arises from sampling errors can be excluded with high confidence only for one of the 7 models. Therefore, longer time series are needed to robustly assess the AMV characteristics. In addition to any changes imposed to the AMV by external forcings, the detected dependence on the time interval identified in most models suggests that the character of the observed AMV may undergo significant changes in the future.

The terrestrial carbon dioxide (CO2) budget interacts with the Earth's climate system on diurnal to centennial and longer time scales, making it critical for climatic prediction and stabilization. Atmospheric observations and global syntheses of CO2 data indicate that the terrestrial biosphere is one the major sources of interannual variability, but the underlying mechanisms operating on different time-scales and the potential impacts of this on future projections remain unclear. Here it is shown that the El Nino and Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) regime affect temporal variability in the terrestrial carbon budget with different time scales. The terrestrial carbon budget, estimated using a process-based model (VISIT) for the period 1910–2005, was correlated with the indices of PDO, AMO, and ENSO with various smoothing periods and lag lengths. ENSO showed the highest short-term correlation, corresponding to interannual terrestrial variability, whereas PDO and AMO had higher correlations at the decadal time scale. Such correlations with the meteorological regimes occurred heterogeneously over the land surface. This study suggests that long-term monitoring is needed to elucidate the temporal variability, and that decadal predictability of climate and terrestrial models should be improved further.

The Atlantic Multidecadal Variability (AMV) is a prominent mode of low-frequency climate variability, characterized by basin-scale sea surface temperature (SST) variations in the North Atlantic and strong global impacts. AMV can be forced externally by surface radiative fluxes or internally generated. The latter generation mechanism is commonly attributed to variations in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that a robust AMV can arise and be sustained by large-scale atmosphere–ocean interactions, even in the absence of a dominant role for AMOC variations, in a fully coupled model—the Department of Energy’s Energy Exascale Earth System Model version 2 (E3SMv2). The simulated AMV is driven primarily by surface shortwave and turbulent heat fluxes across the North Atlantic. Essentially, an initial warming over the Gulf Stream region strengthens and spreads by reducing low cloud cover and enhancing surface shortwave radiation. This mechanism is enabled by the relatively narrow width of the North Atlantic, compared to the North Pacific. Our results broaden the conceptual understanding of AMV physics and underscore the importance of atmosphere–ocean interactions in sustaining it.

The semi-enclosed European seas (Baltic Sea, North Sea, Mediterranean Sea, and Black Sea) have been among the fastest warming seas worldwide during the last decades. Projections of future temperature trends can provide essential information for decision makers, but they are subject to a number of uncertainties. One important source of uncertainty is the rather high internal climate variability in that region. Separating internal variability and forced trends is therefore paramount. Particularly powerful tools for this separation are Single-Model Initial-condition Large Ensembles (SMILEs). In such ensembles, the same model is run multiple times with similar forcing but varying initial conditions, resulting in different realizations of internal variability. This study employs two SMILEs to decompose European sea surface temperatures (SSTs) into forced trends and internal variability and to investigate whether and how the latter is affected by the Atlantic multidecadal variability (AMV) in the historical period and different future emission scenarios. It is shown that on a decadal scale, the effect of the AMV on the European SSTs will stay important, especially in lower emission scenarios. However, on longer time scales, forced trends will be much higher and in all but the lowest emission scenario considered, more than 2 ∘ C warming compared to 1950 are very likely to be reached until the end of the century in all semi-enclosed European seas.

<p>There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV). However, decadal-scale variability embedded within the AMV has received less attention, despite being a prominent feature of observed North Atlantic sea surface temperature (SST) and important for the climate of adjacent continents. These decadal fluctuations in the North Atlantic Ocean are also a key source of skill in decadal climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations and pre-industrial control simulations, we identify a decadal mode of atmosphere-ocean variability in the North Atlantic with a dominant time scale of 13-18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air-sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This strongly suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability, and that we might expect it to become less pronounced over the next few years.</p>

Arctic precipitation has increased in recent decades (hereafter, Arctic wetting), but the drivers remain uncertain. Using observations, reanalyses, and single-model initial-condition large ensembles (SMILEs), we show that enhanced evaporation due to sea ice loss has been the primary driver of Arctic wetting during 1979–2024, especially in the Atlantic sector. However, the externally forced component in most SMILEs explains only ~69% of sea ice loss and 75% of wetting in the observations and reanalyses. Further analysis reveals that the observed transition of one of the Northern Hemisphere’s interdecadal internal variability—Atlantic multidecadal variability (AMV)—from a negative to a positive phase substantially enhanced Arctic sea ice loss, thereby accelerating wetting by about 31%. Under SSP3-7.0, if the AMV switches phase in the near future from the current +1 to a −1 standard deviation anomaly, then the rates of Arctic sea ice loss and wetting would slow by nearly 29 and 33%, respectively, relative to the externally forced response alone. These results underscore the pivotal role of AMV in modulating Arctic sea ice loss and wetting and highlight the need to account for AMV phase changes in near-term Arctic climate projections.

The Atlantic Multidecadal Oscillation (AMO) significantly influences the climate of the surrounding continents and has previously been attributed to variations in the Atlantic Meridional Overturning Circulation. Recently, however, similar multidecadal variability was reported in climate models without ocean circulation variability. We analyze the relationship between turbulent heat fluxes and sea surface temperatures (SSTs) over the midlatitude North Atlantic in observations and coupled climate model simulations, both with and without ocean circulation variability. SST anomalies associated with the AMO are positively correlated with heat fluxes on decadal time scales in both observations and models with varying ocean circulation, whereas in models without ocean circulation variability the anomalies are negatively correlated when heat flux anomalies lead. These relationships are captured in a simple stochastic model and rely crucially on low‐frequency forcing of SST. The fully coupled models that better capture this signature more effectively reproduce the observed impact of the AMO on European summertime temperatures.

Winter Central Asian precipitation (WCAP) is increasingly replacing snowfall as a critical water resource under global warming. Observations show a decline in WCAP from 1891 to 1946, followed by a recovery from 1947 to the recent decade. However, the relative contributions of external forcing and internal variability to these changes remain unclear. By analyzing observations and climate model simulations, this study finds that greenhouse gas forcing favors increasing WCAP, potentially offsetting drying trends driven by anthropogenic aerosols. Internal variability, primarily the phase transition of Atlantic Multidecadal Variability (AMV), plays a dominant role in shaping WCAP trends. The AMV‐induced Rossby wave train, sustained by extracting baroclinic energy from the background mean flow, triggers barotropic atmospheric circulation anomalies that modulate WCAP. The cold‐to‐warm AMV phase transition (1891–1946) weakened the externally forced upward precipitation trend, reducing it from 0.19 to −0.20 mm month −1 decade −1 . In contrast, the warm‐to‐cold phase transition (1947–1997) amplified the externally forced precipitation trend, increasing it from 0.28 to 0.99 mm month −1 decade −1 . Under the high‐emission future scenario, the time of emergence of externally driven WCAP increases is projected to occur between 2030 and 2060, at least a decade earlier than the post‐2060 timeline projected under the medium‐emission scenario. These findings underscore the critical role of AMV in shaping WCAP variability and highlight the necessity of emission reductions to delay the time when externally driven precipitation increases exceed the region's adaptive capacity.

Against the backdrop of warming of the Northern Hemisphere it has recently been acknowledged that North Atlantic temperature changes undergo considerable variability over multidecadal periods. The leading component of natural low-frequency temperature variability has been termed the Atlantic Multidecadal Oscillation (AMO). Presently, correlative studies on the biological impact of the AMO on marine ecosystems over the duration of a whole AMO cycle (∼60 years) is largely unknown due to the rarity of continuously sustained biological observations at the same time period. To test whether there is multidecadal cyclic behaviour in biological time-series in the North Atlantic we used one of the world's longest continuously sustained marine biological time-series in oceanic waters, long-term fisheries data and historical records over the last century and beyond. Our findings suggest that the AMO is far from a trivial presence against the backdrop of continued temperature warming in the North Atlantic and accounts for the second most important macro-trend in North Atlantic plankton records; responsible for habitat switching (abrupt ecosystem/regime shifts) over multidecadal scales and influences the fortunes of various fisheries over many centuries.