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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 for, This research has been supported by the Departament de Recerca i Universitats de la Generalitat de Catalunya for the Climate Variability and Change (CVC) Research Group (reference: 2021 SGR 00786). Roberto Bilbao was supported by the European Union's Horizon 2020 research project CONFESS (grant no. 101004156). Pablo Ortega was supported by the Spanish Ministry of Economy, Industry and Competitiveness through Ramon y Cajal (grant no. RYC-2017-22772). Stephen Yeager acknowledges the support of the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the Office of Biological and Environmental Research (BER) (award no. DE-SC0022070)., Peer Reviewed, "Article signat per 17 autors/es: Roberto Bilbao, Pablo Ortega, Didier Swingedouw, Leon Hermanson, Panos Athanasiadis, Rosie Eade, Marion Devilliers, Francisco Doblas-Reyes, Nick Dunstone, An-Chi Ho, William Merryfield, Juliette Mignot, Dario Nicolì, Margarida Samsó, Reinel Sospedra-Alfonso, Xian Wu, and Stephen Yeager", Postprint (published version)

Due to large northward transports of heat, the Atlantic Ocean circulation is strongly affecting the climate of various regions. Its internal variability has been shown to be predictable decades ahead within climate models, providing the hope that synchronizing ocean circulation with observations can improve decadal predictions, notably of the North Atlantic subpolar gyre (SPG). Climate predictions require a starting point which is a reconstruction of the past climate. This is usually done with data assimilation methods that blend available observations and climate model states together. There is no unique method to derive initial conditions. Moreover, initialization can be implemented with full-field observations or their anomalies superimposed on the model's climatology to avoid strong drifts in predictions. How critical ocean circulation drifts (following the initialization step) are for prediction skill has not been assessed yet. We analyze this possible connection using the dataset of twelve prediction systems from the World Meteorological Organization Lead Centre for Annual-to-Decadal Climate Prediction. We find a wide variety of initial errors for the Atlantic meridional overturning circulation (AMOC) related to a dynamically imbalanced AMOC cell leading to strongly displaced or multiple maxima in the overturning structures. This likely results in a blend of model drift and initial shock. We identify that the AMOC initialization influences the quality of SPG predictions. When predictions show a large initial error in their AMOC, they usually have low skill for predicting internal variability of the SPG for a time horizon of 6-10 years. Full-field initialized predictions with low AMOC drift show better prediction skill of the SPG than those with a large AMOC drift. Nevertheless, while the anomaly-initialized predictions do not experience large drifts, they show low SPG skill when skill also present in historical runs is removed using a residual correlation metr, The author(s) declare financial support was received for the research, authorship, and/or publication of this article. Funding for IP was provided by the Deutsche Forschungsgemeinschaft, Project number 436413914. LH and DSm were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. IB was supported by the Trond Mohn Foundation (Grant BFS2018TMT01; Bjerknes Climate Prediction Unit) and the Research Council of Norway (Grant 309562; Climate Futures). PA, DN, HP, and MK acknowledge funding from the European Union's Horizon Europe Research and Innovation Programme through the ASPECT project under grant agreement No 101081460. DSw received financial support from the French government in the framework of the University of Bordeaux's IdEx Investments for the Future program / RRI Tackling Global Change. DSw also acknowledges funding from UKRI Decadal project (MR/W009641/1). RM would like to acknowledge funding support from CSIRO DCFP (Decadal Climate Forecasting Project). HT and TK were supported by MEXT program for the advanced studies of climate change projection (SENTAN) Grant Number JPMXD0722680395., Peer Reviewed, "Article signat per 26 autors/es: Iuliia Polkova, Didier Swingedouw, Leon Hermanson, Armin Köhl, Detlef Stammer, Doug Smith, Jürgen Kröger, Ingo Bethke, Xiaosong Yang, Liping Zhang, Dario Nicolì, Panos J. Athanasiadis, Pasha Karami, Klaus Pankatz, Holger Pohlmann, Bo Wu, Roberto Bilbao, Pablo Ortega, Shuting Yang, Reinel Sospedra-Alfonso, William Merryfield, Takahito Kataoka, Hiroaki Tatebe, Yukiko Imada, Masayohi Ishii, Richard J. Matear", Postprint (published version)

As climate change accelerates, societies and climate-sensitive socioeconomic sectors cannot continue to rely on the past as a guide to possible future climate hazards. Operational decadal predictions offer the potential to inform current adaptation and increase resilience by filling the important gap between seasonal forecasts and climate projections. The World Meteorological Organization (WMO) has recognized this and in 2017 established the WMO Lead Centre for Annual to Decadal Climate Predictions (shortened to “Lead Centre” below), which annually provides a large multimodel ensemble of predictions covering the next 5 years. This international collaboration produces a prediction that is more skillful and useful than any single center can achieve. One of the main outputs of the Lead Centre is the Global Annual to Decadal Climate Update (GADCU), a consensus forecast based on these predictions. This update includes maps showing key variables, discussion on forecast skill, and predictions of climate indices such as the global mean near-surface temperature and Atlantic multidecadal variability. it also estimates the probability of the global mean temperature exceeding 1.5°C above preindustrial levels for at least 1 year in the next 5 years, which helps policy-makers understand how closely the world is approaching this goal of the Paris Agreement. This paper, written by the authors of the GADCU, introduces the GADCU, presents its key outputs, and briefly discusses its role in providing vital climate information for society now and in the future., LH and AS were supported by the EUCP project funded by the European Commission’s Horizon 2020 programme, Grant Agreement 776613 and supported by the U.K.–China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund. AS, DS, and MS were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. The EC-Earth simulations at SMHI were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at PDC and NSC and have been performed as part of the NordForsk-funded Nordic Centre of Excellence project (Award 76654) Arctic Climate Predictions: Pathways to Resilient, Sustainable Societies (ARCPATH). SY and TT were supported by the ARCPATH (NordForsk Award 76654) and the Danish state-funded National Centre for Climate Research [Nationalt Center for Klimaforskning (NCFK)]. SY was also partly supported by the EUCP project (Horizon 2020 Grant Agreement 776613). The EC-Earth simulations at BSC were performed using resources from the Red Española de Supercomputación. HP and WM received funding from the German Federal Ministry of Education and Research (BMBF) project MiKlip (FKZ 01LP1519A). Takahito Kataoka and Hiroaki Tatebe were supported by the Integrated Research Program for Advancing Climate Models (TOUGOU) Grant JPMXD0717935457 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. MIROC simulations were performed on the Earth Simulator at JAMSTEC. NK, YW, FC, and IB were supported by the Trond Mohn Foundation (Grant BFS2018TMT01; Bjerknes Climate Prediction Unit)., Peer Reviewed, "Article signat per 45 autors/es: Leon Hermanson, Doug Smith, Melissa Seabrook, Roberto Bilbao, Francisco Doblas-Reyes, Etienne Tourigny, Vladimir Lapin, Viatcheslav V. Kharin, William J. Merryfield, Reinel Sospedra-Alfonso, Panos Athanasiadis, Dario Nicoli, Silvio Gualdi, Nick Dunstone, Rosie Eade, Adam Scaife, Mark Collier, Terence O’Kane, Vassili Kitsios, Paul Sandery, Klaus Pankatz, Barbara Früh, Holger Pohlmann, Wolfgang Müller, Takahito Kataoka, Hiroaki Tatebe, Masayoshi Ishii, Yukiko Imada, Tim Kruschke, Torben Koenigk, Mehdi Pasha Karami, Shuting Yang, Tian Tian, Liping Zhang, Tom Delworth, Xiaosong Yang, Fanrong Zeng, Yiguo Wang, François Counillon, Noel Keenlyside, Ingo Bethke, Judith Lean, Jürg Luterbacher, Rupa Kumar Kolli, and Arun Kumar", Postprint (published version)

Understanding the influence of volcanism on ENSO and associated climatic impacts is of great scientific and social importance. Although many studies on the volcano–ENSO nexus are available, a thorough review of ENSO sensitivity to explosive eruptions is still missing. Therefore, this study aims to provide an in-depth assessment of the ENSO response to volcanism. Most past studies suggest an emerging consensus in models, with the vast majority showing an El Niño-like SST response during the eruption year and a La Niña-like response a few years later. RCP8.5-based climate model projections also suggest strong El Niño conditions and significant monsoonal rainfall reduction following strong tropical volcanism. However, some studies involving climate reconstructions and model simulations still raise concerns about the ENSO–volcano link and suggest a weak ENSO response to volcanism. This happens because ENSO response to volcanism seems very sensitive to reconstruction methods, ENSO preconditioning, eruption timing, position and amplitude. We noticed that some response mechanisms are still unclear, for instance, how the tropical volcanic forcing with nearly uniform radiative cooling projects onto ENSO when coincidental ENSO events are underway. Moreover, there are very less observational and proxy records for assessing the extratropical volcanism impact on ENSO. Nevertheless, model-based studies suggest that Northern (Southern) Hemispheric extratropical eruptions may lead to an El Niño (La Niña)-like response. We further noticed that the origin of post-eruption winter warming is still elusive; however, recent findings suggest that the large-scale circulation changes concurrently occurring during volcanism are the potential source of high-latitude winter warming. Existing uncertainties in the simulated ENSO response to volcanism could be reduced by considering a synchronized modeling approach with large ensembles.

Understanding the influence of volcanism on ENSO and associated climatic impacts is of great scientific and social importance. Although many studies on the volcano–ENSO nexus are available, a thorough review of ENSO sensitivity to explosive eruptions is still missing. Therefore, this study aims to provide an in-depth assessment of the ENSO response to volcanism. Most past studies suggest an emerging consensus in models, with the vast majority showing an El Niño-like SST response during the eruption year and a La Niña-like response a few years later. RCP8.5-based climate model projections also suggest strong El Niño conditions and significant monsoonal rainfall reduction following strong tropical volcanism. However, some studies involving climate reconstructions and model simulations still raise concerns about the ENSO–volcano link and suggest a weak ENSO response to volcanism. This happens because ENSO response to volcanism seems very sensitive to reconstruction methods, ENSO preconditioning, eruption timing, position and amplitude. We noticed that some response mechanisms are still unclear, for instance, how the tropical volcanic forcing with nearly uniform radiative cooling projects onto ENSO when coincidental ENSO events are underway. Moreover, there are very less observational and proxy records for assessing the extratropical volcanism impact on ENSO. Nevertheless, model-based studies suggest that Northern (Southern) Hemispheric extratropical eruptions may lead to an El Niño (La Niña)-like response. We further noticed that the origin of post-eruption winter warming is still elusive; however, recent findings suggest that the large-scale circulation changes concurrently occurring during volcanism are the potential source of high-latitude winter warming. Existing uncertainties in the simulated ENSO response to volcanism could be reduced by considering a synchronized modeling approach with large ensembles.

The subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large-scale ocean circulation changes. This article analyses the interrelationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the subpolar gyre (SPG) circulation, and the upper-ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked to the strength of the subpolar AMOC and gyre circulation, and it is also linked to the subtropical AMOC, although the strength of this relationship is model dependent and affected by the inclusion of the Ekman component. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including the mean density stratification in the Labrador Sea, the strength and depth of the AMOC, and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link to the subtropical AMOC might be more realistic.

The subpolar North Atlantic (SPNA) is a region with prominent decadal variability that has experienced remarkable warming and cooling trends in the last few decades. These observed trends have been preceded by slow-paced increases and decreases in the Labrador Sea density (LSD), which are thought to be a precursor of large-scale ocean circulation changes. This article analyses the interrelationships between the LSD and the wider North Atlantic across an ensemble of coupled climate model simulations. In particular, it analyses the link between subsurface density and the deep boundary density, the Atlantic Meridional Overturning Circulation (AMOC), the subpolar gyre (SPG) circulation, and the upper-ocean temperature in the eastern SPNA. All simulations exhibit considerable multidecadal variability in the LSD and the ocean circulation indices, which are found to be interrelated. LSD is strongly linked to the strength of the subpolar AMOC and gyre circulation, and it is also linked to the subtropical AMOC, although the strength of this relationship is model-dependent and affected by the inclusion of the Ekman component. The connectivity of LSD with the subtropics is found to be sensitive to different model features, including the mean density stratification in the Labrador Sea, the strength and depth of the AMOC, and the depth at which the LSD propagates southward along the western boundary. Several of these quantities can also be computed from observations, and comparison with these observation-based quantities suggests that models representing a weaker link to the subtropical AMOC might be more realistic., This research has been supported by the Natural Environment Research Council (grant nos. M005127/1, NE/M005097/1, NE/T013516/1, NE/T013540/1, and NE/N018044/1) and the Ministerio de Economía, Industria y Competitividad, Gobierno de España (grant no. RYC-2017-22772)., Peer Reviewed, Postprint (published version)

Atlantic multidecadal variability (AMV) has been linked to the observed slowdown of global warming over 1998–2012 through its impact on the tropical Pacific. Given the global importance of tropical Pacific variability, better understanding this Atlantic–Pacific teleconnection is key for improving climate predictions, but the robustness and strength of this link are uncertain. Analyzing a multi-model set of sensitivity experiments, we find that models differ by a factor of 10 in simulating the amplitude of the Equatorial Pacific cooling response to observed AMV warming. The inter-model spread is mainly driven by different amounts of moist static energy injection from the tropical Atlantic surface into the upper troposphere. We reduce this inter-model uncertainty by analytically correcting models for their mean precipitation biases and we quantify that, following an observed 0.26 °C AMV warming, the equatorial Pacific cools by 0.11 °C with an inter-model standard deviation of 0.03 °C