Variabilité climatique de l'Atlantique Nord aux échelles de temps décennale à multidécennale : mécanismes et prévisibilité

Aux échelles de temps décennale à multidécennale, l'Atlantique Nord se caractérise par une modulation de sa température de surface à grande échelle modifiant les conditions climatiques des continents alentours, en particulier le Sahel, l'Amérique du Nord et l'Europe. En raison d'une faible couverture temporelle aux regards des échelles de temps considérées et d'un faible échantillonnage de la structure tridimensionnelle de l'océan, les observations ne permettent pas d'analyser en détail les origines de cette variabilité connue sous le nom d'Oscillation ou Variabilité Atlantique Multidécennale (AMV). Dans cette thèse, nous avons utilisé le modèle de climat CNRM-CM5 comme laboratoire numérique pour étudier d'une part les mécanismes physiques internes (par opposition à ceux forcés par les facteurs externes comme l'activité solaire, les gaz à effet de serre etc.) qui engendrent cette variabilité et d'autre part la prévisibilité associée à cette variabilité. L'analyse d'une simulation de 1000 ans dite de contrôle (tous les forçages externes au système climatique sont maintenus constants) met en évidence que l'AMV simulée par ce modèle est principalement contrôlée par les fluctuations multidécennales de la circulation océanique méridienne de retournement (AMOC) et du transport de chaleur méridien associé. La variabilité de l'AMOC répond à l'excitation de modes de variabilité atmosphérique de type Est Atlantique (EAP) et Oscillation Nord Atlantique (NAO) en hiver. Ceux-ci déclenchent une réaction en chaîne de processus océaniques conduisant in fine ~30 ans plus tard à un événement d'AMOC/AMV. La nature même de ces processus contrôle l'échelle de temps de la variabilité. Plus précisément, nous avons mis en évidence le rôle crucial joué par les anomalies de densité océanique des 500 premiers mètres du gyre subpolaire sur les fluctuations de l'AMOC. Dans une deuxième partie, nous nous sommes intéressés à l'estimation de la prévisibilité associée à l'AMV dans CNRM-CM5. Nous avons pour cela suivi une approche type " modèle parfait " en cherchant à " reprévoir ", par une méthode ensembliste, les variations de la simulation de contrôle. Nous avons montré à partir d'une série de métriques et de modèles statistiques, que, dans CNRM-CM5, l'AMOC et l'AMV sont prévisibles jusqu'à une échéance allant de 15 à plus de 30 ans en fonction des conditions initiales océaniques. Cette prévisibilité conditionnelle provient des anomalies de la densité du gyre subpolaire - plus précisément de sa composante salinité - et de leur évolution selon le mécanisme proposé. Finalement, nous trouvons que la prévisibilité océanique est associée à une prévisibilité sur les continents en termes de température de surface et circulation atmosphérique.At decadal to multidecadal timescale, the North Atlantic Ocean is characterized by a large-scale modulation of its surface temperature and heat/salt content. The latter, known as Atlantic Multidecadal Oscillation or Variability (AMV), is associated with anomalous climate conditions over the adjacent continents, especially over the Sahel, the north American continent and Europe. It is impossible from the sole observations to assess the origin of such a variability because of their short temporal coverage with respect to the involved timescale and because of their critical undersampling of the three dimensional states of the ocean. In this thesis, we have used the CNRM-CM5 climate model as a numerical lab to first investigate the internal mechanisms (as opposed to forced by external factors such as solar, greenhouse gazes etc.) at the origin of the AMV and second to quantify the associated predictability. The analysis of a 1000-yr control simulation (external climate forcing maintained to a constant level) shows that the model AMV is mainly controlled by the multidecadal evolution of the Atlantic meridional overturning circulation (AMOC) and associated heat/salt transport. The AMOC low-frequency variability is forced by the excitation of wintertime atmospheric modes over the Atlantic, namely the East Atlantic Pattern and the North Atlantic Oscillation. Those kick a chain reaction of oceanic processes leading in fine about 30 years later to an AMOC/AMV event. Such a timescale is controlled by the ocean dynamics and thermodynamics intrinsic properties. More specifically, we insist on the critical role played by the density anomalies of the first 500-meter of the subpolar gyre in controlling a large part of the AMOC fluctuations. We then focus on the estimation of the predictability level of the AMV in CNRM-CM5. To do so, we adopt the so-called perfect model approach that consists in reforecasting the model itself via an ensemblist method. Based on the use of a series of metrics and simple statistical models, we show that the AMOC/AMV in CNRM-CM5 is predictable for leadtimes ranging from 15 to 30 years as a function of the oceanic initial conditions. Such a conditional predictability is linked to the evolution of the density anomalies of the subpolar gyre and more specifically its salinity component, in line with the above-documented proposed mechanism. The oceanic predictability is associated to some predictability over the continents in terms of surface temperature and atmospheric circulation

Organic particle export, remineralization and advection in the North Atlantic mesopelagic layer

The mesopelagic layer of the oceans extends between ~200 and 1000 m depth and plays a fundamental role in global biogeochemical cycles and climate. In addition, it hosts a massive biomass of zooplankton and small fish, what is essential to regulate marine resources. However, scientific understanding and predictive capacity of biogeochemical processes in the mesopelagic zone are still underdeveloped. This lack of information has societal and economic costs because there is uncertainty in estimates of oceanic carbon storage (which inform policies for the reduction of carbon dioxide emissions), and it hampers the management of mesopelagic biological resources. In this way, this thesis would address the study of the carbon cycle and the associated biogeochemical processes. The main objective is to analyse the variability of the transport and transformation of particles that carry organic carbon from the surface to the deep sea in the North Atlantic. For this purpose, this study will focus on the analysis of simulations from dynamical (NEMO4 (Madec & NEMO team, 2008)) and biogeochemical (PISCES-v2 (Amount et al., 2015)) models, together with observations from bio-ARGO floats and satellite data

Atlantic Multidecadal Variability modulates the climate impacts of El Niño-Southern Oscillation in Australia

Atlantic Multidecadal Variability (AMV) modulates El Niño-Southern Oscillation (ENSO) dynamics. Here, we explore the effect of warm (AMV+) and cold (AMV-) AMV conditions on the austral summer teleconnection of ENSO to Australia using idealized simulations performed with the NCAR-CESM1 model. AMV+ strengthens the mean and extreme precipitation and temperature responses to El Niño in south-western Australia and weakens the mean precipitation and temperature impacts in north-eastern Australia. The modulation of La Niña impacts by AMV is asymmetric to El Niño, with a weakening of the mean and extreme precipitation and temperature responses in eastern Australia. Decomposing the total difference in ENSO response between AMV phases, we find that the signals are mainly explained by the direct AMV modulation of ENSO and its teleconnections rather than by changes in background climate induced by AMV. The exception is ENSO-driven fire impacts, where there is a significant increase in burned area in south-eastern Australia only when El Niño and AMV+ co-occur. However, modulation of ENSO between AMV+ and AMV- does offset ~37% of the decrease in burned area extent during La Niña summers. The altered surface climate response to ENSO in Australia by AMV is attributed to variations in large-scale atmospheric circulation. Under AMV+, there is increased subsidence over western Australia during El Niño associated with a westward shift of the local Walker circulation. A weakening of the upwelling branch of the local Hadley circulation over north-eastern Australia is responsible for the weakening of La Niña impacts in AMV+, accompanied by a strengthening of subsidence in south central Australia due to a weakening of the local Hadley circulation, amplifying La Niña impacts over this region. The results suggest the potential for AMV to drive multidecadal variability in ENSO impacts over Australia.P.T.-C. was supported by a PhD scholarship from the Natural Environment Research Council PANORAMA Doctoral Training Partnership (NE/S007458/1). Y.R.-R. received the support of a fellowship from ”la Caixa” Foundation (ID 100010434) and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847648. The fellowship code is LCF/BQ/PR21/11840016. A.C.M. was supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 820829 (CONSTRAIN project) and The Leverhulme Trust (PLP-2018-278). M.T. acknowledges funding by the Spanish Ministry of Science, Innovation and Universities through the Ramón y Cajal Grant Reference RYC2019-027115-I and through the project ONFIRE, grant PID2021-123193OB-I00, funded by MCIN/AEI/10.13039/501100011033. Computing facilities were provided by the Barcelona Supercomputing Center and the University of LeedsPeer ReviewedPostprint (author's final draft

Role of the Atlantic Multidecadal Variability in modulating the climate response to a Pinatubo-like volcanic eruption

The modulation by the Atlantic multidecadal variability (AMV) of the dynamical climate response to a Pinatubo-like eruption is investigated for the boreal winter season based on a suite of large ensemble experiments using the CNRM-CM5 Coupled Global Circulation Model. The volcanic eruption induces a strong reduction and retraction of the Hadley cell during 2 years following the eruption and independently of the phase of the AMV. The mean extratropical westerly circulation simultaneously weakens throughout the entire atmospheric column, except at polar Northern latitudes where the zonal circulation is slightly strengthened. Yet, there are no significant changes in the modes of variability of the surface atmospheric circulation, such as the North Atlantic Oscillation (NAO), in the first and the second winters after the eruption. Significant modifications over the North Atlantic sector are only found during the third winter. Using clustering techniques, we decompose the atmospheric circulation into weather regimes and provide evidence for inhibition of the occurrence of negative NAO-type circulation in response to volcanic forcing. This forced signal is amplified in cold AMV conditions and is related to sea ice/atmosphere feedbacks in the Arctic and to tropical-extratropical teleconnections. Finally, we demonstrate that large ensembles of simulations are required to make volcanic fingerprints emerge from climate noise at mid-latitudes. Using small size ensemble could easily lead to misleading conclusions especially those related to the extratropical dynamics, and specifically the NAO.This research was carried out within the pro- jects: (i) MORDICUS funded by the French Agence Nationale de la Recherche (ANR-13-SENV-0002-02); (ii) SPECS funded by the European Commission’s Seventh Framework Research Programme under the grant agreement 308378; (iii) VOLCADEC funded by the Spanish program MINECO/FEDER (ref. CGL2015-70177-R). We thank Javier Garcia-Serrano for its comments about the NAO precursors, Omar Bellprat for its suggestions concerning the statistical analysis and François Massonnet for its recommendations in terms of graphical presentation. CC is grateful to Marie-Pierre Moine, Laure Coquart and Isabelle Dast for technical help to run the model. Computer resources have been provided by Cerfacs. We thank the two anonymous referees for their useful comments and suggestions to improve this manuscript.Peer ReviewedPostprint (author's final draft

Contrasting responses of Atlantic and Pacific tropical cyclone activity to Atlantic Multidecadal Variability

This research assesses the influences of Atlantic Multidecadal Variability (AMV) on global tropical cyclones (TCs) using two large ensembles of idealized global climate model simulations with opposite signs of AMV forcings superimposed (i.e., AMV+ and AMV–). We first detect TCs and then compare TC activity by basin in the two AMV experiments. We find contrasting responses of Atlantic and Pacific TC frequency to the AMV anomalies. Compared to AMV–, AMV+ significantly increases TC frequency in the North Atlantic, including those making landfalls. The increase is explained by warmer sea surface temperature, higher relative humidity, increased relative vorticity, and weaker vertical wind shear under AMV+. By contrast, AMV+ decreases TC occurrence over the western North Pacific and South Pacific, which is tied to stronger vertical wind shear and lower relative humidity. The opposite responses of TC activity to AMV+ are attributed to strengthened Walker Circulation between the Atlantic and Pacific

The EC-Earth3 Earth system model for the Coupled Model Intercomparison Project 6

The Earth system model EC-Earth3 for contributions to CMIP6 is documented here, with its flexible coupling framework, major model configurations, a methodology for ensuring the simulations are comparable across different high-performance computing (HPC) systems, and with the physical performance of base configurations over the historical period. The variety of possible configurations and sub-models reflects the broad interests in the EC-Earth community. EC-Earth3 key performance metrics demonstrate physical behavior and biases well within the frame known from recent CMIP models. With improved physical and dynamic features, new Earth system model (ESM) components, community tools, and largely improved physical performance compared to the CMIP5 version, EC-Earth3 represents a clear step forward for the only European community ESM. We demonstrate here that EC-Earth3 is suited for a range of tasks in CMIP6 and beyond.The development of EC-Earth3 was supported by the European Union's Horizon 2020 research and innovation program under project IS-ENES3, the third phase of the distributed e-infrastructure of the European Network for Earth System Modelling (ENES) (grant agreement no. 824084, PRIMAVERA grant no. 641727, and CRESCENDO grant no. 641816). Etienne Tourigny and Raffaele Bernardello have received funding from the European Union’s Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement nos. 748750 (SPFireSD project) and 708063 (NeTNPPAO project). Ivana Cvijanovic was supported by Generalitat de Catalunya (Secretaria d'Universitats i Recerca del Departament d’Empresa i Coneixement) through the Beatriu de Pinós program. Yohan Ruprich-Robert was funded by the European Union's Horizon 2020 research and innovation program in the framework of Marie Skłodowska-Curie grant INADEC (grant agreement 800154). Paul A. Miller, Lars Nieradzik, David Wårlind, Roland Schrödner, and Benjamin Smith acknowledge financial support from the strategic research area “Modeling the Regional and Global Earth System” (MERGE) and the Lund University Centre for Studies of Carbon Cycle and Climate Interactions (LUCCI). Paul A. Miller, David Wårlind, and Benjamin Smith acknowledge financial support from the Swedish national strategic e-science research program eSSENCE. Paul A. Miller further acknowledges financial support from the Swedish Research Council (Vetenskapsrådet) under project no. 621-2013-5487. Shuting Yang acknowledges financial support from a Synergy Grant from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC (grant agreement 610055) as part of the ice2ice project and the NordForsk-funded Nordic Centre of Excellence project (award 76654) ARCPATH. Marianne Sloth Madsen acknowledges financial support from the Danish National Center for Climate Research (NCKF). Andrea Alessandri and Peter Anthoni acknowledge funding from the Helmholtz Association in its ATMO program. Thomas Arsouze, Arthur Ramos, and Valentina Sicardi received funding from the Ministerio de Ciencia, Innovación y Universidades as part of the DeCUSO project (CGL2017-84493-R).​​​​​​​Peer Reviewed"Article signat per 61 autors/es: Ralf Döscher, Mario Acosta, Andrea Alessandri, Peter Anthoni, Thomas Arsouze, Tommi Bergman, Raffaele Bernardello, Souhail Boussetta, Louis-Philippe Caron, Glenn Carver, Miguel Castrillo, Franco Catalano, Ivana Cvijanovic, Paolo Davini, Evelien Dekker, Francisco J. Doblas-Reyes, David Docquier, Pablo Echevarria, Uwe Fladrich, Ramon Fuentes-Franco, Matthias Gröger, Jost v. Hardenberg, Jenny Hieronymus, M. Pasha Karami, Jukka-Pekka Keskinen, Torben Koenigk, Risto Makkonen, François Massonnet, Martin Ménégoz, Paul A. Miller, Eduardo Moreno-Chamarro, Lars Nieradzik, Twan van Noije, Paul Nolan, Declan O'Donnell, Pirkka Ollinaho11, Gijs van den Oord, Pablo Ortega, Oriol Tintó Prims, Arthur Ramos, Thomas Reerink, Clement Rousset, Yohan Ruprich-Robert, Philippe Le Sager, Torben Schmith, Roland Schrödner, Federico Serva, Valentina Sicardi, Marianne Sloth Madsen, Benjamin Smith, Tian Tian, Etienne Tourigny, Petteri Uotila, Martin Vancoppenolle, Shiyu Wang, David Wårlind, Ulrika Willén, Klaus Wyser, Shuting Yang, Xavier Yepes-Arbós, and Qiong Zhang"Postprint (author's final draft

Tropical cyclone integrated kinetic energy in an ensemble of HighResMIP simulations

This study investigates tropical cyclone integrated kinetic energy, a measure which takes into account the intensity and the size of the storms and which is closely associated with their damage potential, in three different global climate models integrated following the HighResMIP protocol. In particular, the impact of horizontal resolution and of the ocean coupling are assessed. We find that, while the increase in resolution results in smaller and more intense storms, the integrated kinetic energy of individual cyclones remains relatively similar between the two configurations. On the other hand, atmosphere-ocean coupling tends to reduce the size and the intensity of the storms, resulting in lower integrated kinetic energy in that configuration. Comparing cyclone integrated kinetic energy between a present and a future scenario did not reveal significant differences between the two periods

Atlantic multidecadal variability and North Atlantic jet: a multi-model view from the decadal climate prediction project

The influence of the Atlantic Multidecadal Variability (AMV) on the North Atlantic storm track and eddy–driven jet in the winter season is assessed via a coordinated analysis of idealised simulations with state-of-the-art coupled models. Data used are obtained from a multi-model ensemble of AMV± experiments conducted in the framework of the Decadal Climate Prediction Project component C. These experiments are performed by nudging the surface of the Atlantic ocean to states defined by the superimposition of observed AMV± anomalies onto the model climatology. A robust extra-tropical response is found in the form of a wave-train extending from the Pacific to the Nordic seas. In the warm phase of the AMV compared to cold phase, the Atlantic storm track is typically contracted and less extended poleward and the low-level jet is shifted towards the equator in the Eastern Atlantic. Despite some robust features, the picture of an uncertain and model-dependent response of the Atlantic jet emerges and we demonstrate a link between model bias and the character of the jet response

Coupled climate response to Atlantic Multidecadal Variability in a multi-model multi-resolution ensemble

North Atlantic sea surface temperatures (SSTs) underwent pronounced multidecadal variability during the 20th and early 21st century. We examine the impacts of this Atlantic Multidecadal Variability (AMV), also referred to as the Atlantic Multidecadal Oscillation (AMO), on climate in an ensemble of five coupled climate models at both low and high spatial resolution. We use a SST nudging scheme specified by the Coupled Model Intercomparision Project's Decadal Climate Prediction Project Component C (CMIP6 DCPP-C) to impose a persistent positive/negative phase of the AMV in the North Atlantic in coupled model simulations; SSTs are free to evolve outside this region. The large-scale seasonal mean response to the positive AMV involves widespread warming over Eurasia and the Americas, with a pattern of cooling over the Pacific Ocean similar to the Pacific Decadal Oscillation (PDO), together with a northward displacement of the inter-tropical convergence zone (ITCZ). The accompanying changes in global atmospheric circulation lead to widespread changes in precipitation. We use Analysis of Variance (ANOVA) to demonstrate that this large-scale climate response is accompanied by significant differences between models in how they respond to the common AMV forcing, particularly in the tropics. These differences may arise from variations in North Atlantic air-sea heat fluxes between models despite a common North Atlantic SST forcing pattern. We cannot detect a widespread effect of increased model horizontal resolution in this climate response, with the exception of the ITCZ, which shifts further northwards in the positive phase of the AMV in the higher resolution configurations

Evaluation of global ocean–sea-ice model simulations based on the experimental protocols of the Ocean Model Intercomparison Project phase 2 (OMIP-2)

We present a new framework for global ocean- sea-ice model simulations based on phase 2 of the Ocean Model Intercomparison Project (OMIP-2), making use of the surface dataset based on the Japanese 55-year atmospheric reanalysis for driving ocean-sea-ice models (JRA55-do).We motivate the use of OMIP-2 over the framework for the first phase of OMIP (OMIP-1), previously referred to as the Coordinated Ocean-ice Reference Experiments (COREs), via the evaluation of OMIP-1 and OMIP-2 simulations from 11 state-of-the-science global ocean-sea-ice models. In the present evaluation, multi-model ensemble means and spreads are calculated separately for the OMIP-1 and OMIP-2 simulations and overall performance is assessed considering metrics commonly used by ocean modelers. Both OMIP-1 and OMIP-2 multi-model ensemble ranges capture observations in more than 80% of the time and region for most metrics, with the multi-model ensemble spread greatly exceeding the difference between the means of the two datasets. Many features, including some climatologically relevant ocean circulation indices, are very similar between OMIP-1 and OMIP- 2 simulations, and yet we could also identify key qualitative improvements in transitioning from OMIP-1 to OMIP- 2. For example, the sea surface temperatures of the OMIP- 2 simulations reproduce the observed global warming during the 1980s and 1990s, as well as the warming slowdown in the 2000s and the more recent accelerated warming, which were absent in OMIP-1, noting that the last feature is part of the design of OMIP-2 because OMIP-1 forcing stopped in 2009. A negative bias in the sea-ice concentration in summer of both hemispheres in OMIP-1 is significantly reduced in OMIP-2. The overall reproducibility of both seasonal and interannual variations in sea surface temperature and sea surface height (dynamic sea level) is improved in OMIP-2. These improvements represent a new capability of the OMIP-2 framework for evaluating processlevel responses using simulation results. Regarding the sensitivity of individual models to the change in forcing, the models show well-ordered responses for the metrics that are directly forced, while they show less organized responses for those that require complex model adjustments. Many of the remaining common model biases may be attributed either to errors in representing important processes in ocean-sea-ice models, some of which are expected to be reduced by using finer horizontal and/or vertical resolutions, or to shared biases and limitations in the atmospheric forcing. In particular, further efforts are warranted to resolve remaining issues in OMIP-2 such as the warm bias in the upper layer, the mismatch between the observed and simulated variability of heat content and thermosteric sea level before 1990s, and the erroneous representation of deep and bottom water formations and circulations. We suggest that such problems can be resolved through collaboration between those developing models (including parameterizations) and forcing datasets. Overall, the present assessment justifies our recommendation that future model development and analysis studies use the OMIP-2 framework