Importance du couplage océan-atmosphère sur la sensibilité au réchauffement climatique - Impact sur les ouragans.

Ocean-atmosphere interactions are essential for tropical cyclones. The ocean provides the energy required to sustain tropical cyclones. The simulation of interactions between the ocean and tropical cyclones is therefore crucial and is the focus of this thesis.The first section introduces the rotated-stretched and coupled version of the general circulation model ARPEGE-Climate, developed for this thesis. This rotated-stretched version achieves a spatial resolution between 60 and 100 km over the North Atlantic basin, resolution required to study tropical cyclones. The coupling of ARPEGE-Climate with oceanic general circulation model NEMO has also allowed taking in consideration ocean-atmosphere of tropical cyclones. Two types of simulations are performed, on the one hand coupled simulations and on the other hand simulations with prescribed sea surface temperatures from the coupled simulation.The second section focuses on the ability of general circulation models from the Intercomparison program, TC-MIP, to represent tropical cyclone activity and its precursors on the main development region of the North Atlantic Ocean. General circulation models have strong difficulties to represent tropical cyclone activity in this region, certainly related to their low spatial resolution, between 1° and 2.5°. Although, the following precursors of tropical cyclone activity have been identified in this region :• African easterly waves activity,• Sahelian rainfall and especially those that occurs over the Fouta-Djallon, 11 ° N,•sea surface temperatures and vertical wind shear horizontal on the main development region,• tropospheric humidity over the African west coast.The third section of the thesis presents sensitivity tests that were performed on ARPEGE-Climate rotated-stretched and coupled with NEMO. These tests show the importance of the stretching pole position, the setting of the convection scheme parameters and the coupling frequency.In the fourth section, the configurations of ARPEGE-Climate coupled and forced are compared to assess the impact of ocean-atmosphere coupling on tropical cyclone activity in the North Atlantic basin. It appears that the coupling :• modifies the geographical distribution of cyclone activity over the North Atlantic basin;•modifies the activity of African easterly waves, which in turn affects cyclone activity over the main development region;•changes the seasonal cycle of tropical cyclone activity over the main development region, and thus on the entire North Atlantic basin.This section also presents the similarities and differences of the sensitivity to global warming of forced and coupled simulations. Both configurations present :• an overall decrease, although small, of tropical cyclone activity over the North Atlantic basin,• a decrease in tropical cyclone activity in the southern Gulf of Mexico and Caribbean Sea,• an increase of tropical cyclone activity in the Sargasso Sea,• an intensification of tropical cyclones in terms of pressure and maximum wind,• an increase in tropical cyclonic precipitations.The coupled and forced simulations also show some differences, especially regarding the evolution of the distribution of cyclone activity in the Sargasso Sea or the main development region. Another important difference is the nature of tropical cyclone in the future climate. The coupled configuration shows much greater differences on the changes of nature of tropical cyclones.Les interactions océan-atmosphère sont primordiales pour les cyclones tropicaux. L’océan fourni aux cyclones tropicaux l’énergie thermique nécessaire a leur maintient. La simulation des interactions entre l’océan et les cyclones tropicaux est donc cruciale et constitue le point central de cette thèse.La première partie présente la version basculée-étirée et couplée du modèle de circulation générale ARPEGE-Climat, développée pour cette thèse. Cette version basculée-étirée sur le bassin Nord Atlantique permet d’atteindre une résolution spatiale comprise entre 60 et 100 km sur cette région, résolution nécessaire quand on souhaite étudier les cyclones tropicaux. Le couplage d’ARPEGE-Climat avec le modèle de circulation général océanique NEMO a de plus permis la prise en compte des interactions océan-atmosphère présentes dans les cyclones tropicaux. Deux types de simulations ont été réalisées, d’une part des simulations couplées et d’autre part des simulations forcées par les températures de surface de la mer de la configuration couplée.La seconde partie s’intéresse à la capacité des modèles de circulation générale du programme d’intercomparaison TC-MIP à représenter l’activité cyclonique et ses précurseurs sur la région principale de développement (Main Development Region en anglais) de l’océan Atlantique Nord. Les modèles de circulation générale éprouvent de réelles difficultés à représenter les cyclones tropicaux dans cette région, certainement en partie à cause de leur faible résolution spatiale, comprise entre 1° et 2.5°. Des précurseurs de l’activité cyclonique sur la main development region ont tout de même pu être identifiés. Ainsi, on trouve : • l’activité des ondes d’est Africaines,• les précipitations sahéliennes et plus particulièrement celles qui se produisent sur le Fouta-Djallon, à 11°N, • les températures de surface de la mer et le cisaillement vertical du vent horizontal sur la main development region,• l’humidité troposphérique au niveau des côtes ouest Africaines.Dans la troisième partie de la thèse, des tests de sensibilité ont été réalisés sur ARPEGE-Climat basculé-étiré et couplé avec NEMO. Ces tests montrent l’importance de la position du pôle d’étirement, du réglage des paramètres du schéma de convection et de la fréquence de couplage. Dans la quatrième partie de cette thèse, les configurations couplée et forcée d’ARPEGE-Climat sont comparées afin d’évaluer l’impact du couplage océan-atmosphère sur l’activité cyclonique dans le bassin Nord Atlantique. Il apparaît que le couplage :•modifie la distribution géographique de l’activité cyclonique sur le bassin Nord Atlantique ;•modifie l’activité des ondes d’est Africaines, qui elle-même influe sur l’activité cyclonique sur la main development region ;•modifie le cycle saisonnier de l’activité cyclonique sur la main development region et, par conséquent, sur la totalité du bassin Nord Atlantique.Cette dernière partie présente aussi les points communs et les différences de sensibilité au réchauffement climatique des configurations couplée et forcée. Les deux configurations présentent :•une diminution généralisée, bien que faible, de l’activité cyclonique sur le bassin Nord Atlantique,•une diminution de l’activité cyclonique sur le sud du Golfe du Mexique et la mer des Caraïbes,•une augmentation de l’activité sur la mer des Sargasses,•une intensification des cyclones tropicaux en termes de pression et de vent maximum,•une augmentation des pluies cycloniques.Les configurations couplée et forcée présentent aussi certaines différences, notamment en ce qui concerne l’évolution de la distribution de l’activité cyclonique sur la mer des Sargasses ou sur la main development region. Un autre point important est la différence de nature des cyclones tropicaux dans le futur. La configuration couplée montre des différences beaucoup plus marquées dans les changements de nature des cyclones tropicaux

Is the poleward migration of tropical cyclone maximum intensity associated with a poleward migration of tropical cyclone genesis?

A recent study showed that the global average latitude where tropical cyclones achieve their lifetime-maximum intensity has been migrating poleward at a rate of about one-half degree of latitude per decade over the last 30 years in each hemisphere. However, it does not answer a critical question: is the poleward migration of tropical cyclone lifetime-maximum intensity associated with a poleward migration of tropical cyclone genesis? In this study we will examine this question. First we analyze changes in the environmental variables associated with tropical cyclone genesis, namely entropy deficit, potential intensity, vertical wind shear, vorticity, skin temperature and specific humidity at 500 hPa in reanalysis datasets between 1980 and 2013. Then, a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large-scale variables. We find a shift toward greater (smaller) average potential number of genesis at higher (lower) latitudes over most regions of the Pacific Ocean, which is consistent with a migration of tropical cyclone genesis towards higher latitudes. We then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the Pacific Ocean basins

Land–atmosphere interactions in sub-polar and alpine climates in the CORDEX Flagship Pilot Study Land Use and Climate Across Scales (LUCAS) models – Part 2: The role of changing vegetation

International audienceAbstract. Land cover in sub-polar and alpine regions of northern and eastern Europe have already begun changing due to natural and anthropogenic changes such as afforestation. This will impact the regional climate and hydrology upon which societies in these regions are highly reliant. This study aims to identify the impacts of afforestation/reforestation (hereafter afforestation) on snow and the snow-albedo effect and highlight potential improvements for future model development. The study uses an ensemble of nine regional climate models for two different idealised experiments covering a 30-year period; one experiment replaces most land cover in Europe with forest, while the other experiment replaces all forested areas with grass. The ensemble consists of nine regional climate models composed of different combinations of five regional atmospheric models and six land surface models. Results show that afforestation reduces the snow-albedo sensitivity index and enhances snowmelt. While the direction of change is robustly modelled, there is still uncertainty in the magnitude of change. The greatest differences between models emerge in the snowmelt season. One regional climate model uses different land surface models which shows consistent changes between the three simulations during the accumulation period but differs in the snowmelt season. Together these results point to the need for further model development in representing both grass–snow and forest–snow interactions during the snowmelt season. Pathways to accomplishing this include (1) a more sophisticated representation of forest structure, (2) kilometre-scale simulations, and (3) more observational studies on vegetation–snow interactions in northern Europe

Land–atmosphere interactions in sub-polar and alpine climates in the CORDEX Flagship Pilot Study Land Use and Climate Across Scales (LUCAS) models – Part 2: The role of changing vegetation

Land cover in sub-polar and alpine regions of northern and eastern Europe have already begun changing due to natural and anthropogenic changes such as afforestation. This will impact the regional climate and hydrology upon which societies in these regions are highly reliant. This study aims to identify the impacts of afforestation/reforestation (hereafter afforestation) on snow and the snow-albedo effect and highlight potential improvements for future model development. The study uses an ensemble of nine regional climate models for two different idealised experiments covering a 30-year period; one experiment replaces most land cover in Europe with forest, while the other experiment replaces all forested areas with grass. The ensemble consists of nine regional climate models composed of different combinations of five regional atmospheric models and six land surface models. Results show that afforestation reduces the snow-albedo sensitivity index and enhances snowmelt. While the direction of change is robustly modelled, there is still uncertainty in the magnitude of change. The greatest differences between models emerge in the snowmelt season. One regional climate model uses different land surface models which shows consistent changes between the three simulations during the accumulation period but differs in the snowmelt season. Together these results point to the need for further model development in representing both grass–snow and forest–snow interactions during the snowmelt season. Pathways to accomplishing this include (1) a more sophisticated representation of forest structure, (2) kilometre-scale simulations, and (3) more observational studies on vegetation–snow interactions in northern Europe

Land–atmosphere interactions in sub-polar and alpine climates in the CORDEX flagship pilot study Land Use and Climate Across Scales (LUCAS) models – Part 1: Evaluation of the snow-albedo effect

Seasonal snow cover plays a major role in the climate system of the Northern Hemisphere via its effect on land surface albedo and fluxes. In climate models the parameterization of interactions between snow and atmosphere remains a source of uncertainty and biases in the representation of local and global climate. Here, we evaluate the ability of an ensemble of regional climate models (RCMs) coupled with different land surface models to simulate snow–atmosphere interactions over Europe in winter and spring. We use a previously defined index, the snow-albedo sensitivity index (SASI), to quantify the radiative forcing associated with snow cover anomalies. By comparing RCM-derived SASI values with SASI calculated from reanalyses and satellite retrievals, we show that an accurate simulation of snow cover is essential for correctly reproducing the observed forcing over middle and high latitudes in Europe. The choice of parameterizations, and primarily the choice of the land surface model, strongly influences the representation of SASI as it affects the ability of climate models to simulate snow cover accurately. The degree of agreement between the datasets differs between the accumulation and ablation periods, with the latter one presenting the greatest challenge for the RCMs. Given the dominant role of land surface processes in the simulation of snow cover during the ablation period, the results suggest that, during this time period, the choice of the land surface model is more critical for the representation of SASI than the atmospheric model

Hurricanes and Climate: the U.S. CLIVAR Working Group on Hurricanes

While a quantitative climate theory of tropical cyclone formation remains elusive, considerable progress has been made recently in our ability to simulate tropical cyclone climatologies and understand the relationship between climate and tropical cyclone formation. Climate models are now able to simulate a realistic rate of global tropical cyclone formation, although simulation of the Atlantic tropical cyclone climatology remains challenging unless horizontal resolutions finer than 50 km are employed. The idealized experiments of the Hurricane Working Group of U.S. CLIVAR, combined with results from other model simulations, have suggested relationships between tropical cyclone formation rates and climate variables such as mid-tropospheric vertical velocity. Systematic differences are shown between experiments in which only sea surface temperature is increases versus experiments where only atmospheric carbon dioxide is increased, with the carbon dioxide experiments more likely to demonstrate a decrease in numbers. Further experiments are proposed that may improve our understanding of the relationship between climate and tropical cyclone formation, including experiments with two-way interaction between the ocean and the atmosphere and variations in atmospheric aerosols

Cluster analysis of downscaled and explicitly simulated North Atlantic tropical cyclone tracks

A realistic representation of the North Atlantic tropical cyclone tracks is crucial as it allows, for example, explaining potential changes in US landfalling systems. Here we present a tentative study, which examines the ability of recent climate models to represent North Atlantic tropical cyclone tracks. Tracks from two types of climate models are evaluated: explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution (1° to 0.25°), and downscaled tracks are obtained using a downscaling technique with large-scale environmental fields from a subset of these models. For both configurations, tracks are objectively separated into four groups using a cluster technique, leading to a zonal and a meridional separation of the tracks. The meridional separation largely captures the separation between deep tropical and sub-tropical, hybrid or baroclinic cyclones, while the zonal separation segregates Gulf of Mexico and Cape Verde storms. The properties of the tracks’ seasonality, intensity and power dissipation index in each cluster are documented for both configurations. Our results show that except for the seasonality, the downscaled tracks better capture the observed characteristics of the clusters. We also use three different idealized scenarios to examine the possible future changes of tropical cyclone tracks under 1) warming sea surface temperature, 2) increasing carbon dioxide, and 3) a combination of the two. The response to each scenario is highly variable depending on the simulation considered. Finally, we examine the role of each cluster in these future changes and find no preponderant contribution of any single cluster over the others

A decentralized approach to model national and global food and land use systems

The achievement of several sustainable development goals and the Paris Climate Agreement depends on rapid progress towards sustainable food and land systems in all countries. We have built a flexible, collaborative modeling framework to foster the development of national pathways by local research teams and their integration up to global scale. Local researchers independently customize national models to explore mid-century pathways of the food and land use system transformation in collaboration with stakeholders. An online platform connects the national models, iteratively balances global exports and imports, and aggregates results to the global level. Our results show that actions toward greater sustainability in countries could sum up to 1 Mha net forest gain per year, 950 Mha net gain in the land where natural processes predominate, and an increased CO2 sink of 3.7 GtCO2e yr−1 over the period 2020-2050 compared to current trends, while average food consumption per capita remains above the adequate food requirements in all countries. We show examples of how the global linkage impacts national results and how different assumptions in national pathways impact global results. This modeling setup acknowledges the broad heterogeneity of socio-ecological contexts and the fact that people who live in these different contexts should be empowered to design the future they want. But it also demonstrates to local decision-makers the interconnectedness of our food and land use system and the urgent need for more collaboration to converge local and global priorities.Fil: Mosnier, Aline. Sustainable Development Solutions Network; FranciaFil: Javalera Rincon, Valeria. International Institute For Applied Systems Analysis, Laxenburg; AustriaFil: Jones, Sarah K. Alliance of Bioversity International; FranciaFil: Andrew, Robbie. Center for International Climate Research; NoruegaFil: Bai, Zhaohai. Chinese Academy of Sciences; República de ChinaFil: Baker, Justin. North Carolina State University; Estados UnidosFil: Basnet, Shyam. Stockholm Resilience Centre; SueciaFil: Boer, Rizaldi. Bogor Agricultural University; IndonesiaFil: Chavarro, John. Geo-agro-environmental Sciences And Resources Research Center; ColombiaFil: Costa, Wanderson. Centro de Previsao de Tempo e Estudos Climáticos. Instituto Nacional de Pesquisas Espaciais; BrasilFil: Daloz, Anne Sophie. Center for International Climate Research; NoruegaFil: DeClerck, Fabrice A.. Alliance of Bioversity International; Francia. Stockholm Resilience Centre; SueciaFil: Diaz, Maria. Sustainable Development Solutions Network; FranciaFil: Douzal, Clara. Sustainable Development Solutions Network; FranciaFil: Howe Fan, Andrew Chiah. Sunway University; MalasiaFil: Fetzer, Ingo. Stockholm Resilience Centre; SueciaFil: Frank, Federico. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires; ArgentinaFil: Gonzalez Abraham, Charlotte E.. University of California at San Diego; Estados UnidosFil: Habiburrachman, A. H. F.. Universitas Indonesia; IndonesiaFil: Immanuel, Gito. Stockholm Resilience Centre; SueciaFil: Harrison, Paula A.. Centre for Ecology & Hydrology; Reino UnidoFil: Imanirareba, Dative. Uganda Martyrs University; UgandaFil: Jha, Chandan. Indian Institute Of Management Ahmedabad; IndiaFil: Monjeau, Jorge Adrian. Fundación Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Vittis, Yiorgos. International Institute For Applied Systems Analysis; AustriaFil: Wade, Chris. North Carolina State University; Estados UnidosFil: Winarni, Nurul L.. Universitas Indonesia; IndonesiaFil: Woldeyes, Firew Bekele. Ethiopian Development Research Institute; EtiopíaFil: Wu, Grace C.. University of California; Estados UnidosFil: Zerriffi, Hisham. University of British Columbia; Canad

Hurricanes and Climate: The U.S. CLIVAR Working Group on Hurricanes

While a quantitative climate theory of tropical cyclone formation remains elusive, considerable progress has been made recently in our ability to simulate tropical cyclone climatologies and to understand the relationship between climate and tropical cyclone formation. Climate models are now able to simulate a realistic rate of global tropical cyclone formation, although simulation of the Atlantic tropical cyclone climatology remains challenging unless horizontal resolutions finer than 50 km are employed. This article summarizes published research from the idealized experiments of the Hurricane Working Group of U.S. Climate and Ocean: Variability, Predictability and Change (CLIVAR). This work, combined with results from other model simulations, has strengthened relationships between tropical cyclone formation rates and climate variables such as midtropospheric vertical velocity, with decreased climatological vertical velocities leading to decreased tropical cyclone formation. Systematic differences are shown between experiments in which only sea surface temperature is increased compared with experiments where only atmospheric carbon dioxide is increased. Experiments where only carbon dioxide is increased are more likely to demonstrate a decrease in tropical cyclone numbers, similar to the decreases simulated by many climate models for a future, warmer climate. Experiments where the two effects are combined also show decreases in numbers, but these tend to be less for models that demonstrate a strong tropical cyclone response to increased sea surface temperatures. Further experiments are proposed that may improve our understanding of the relationship between climate and tropical cyclone formation, including experiments with two-way interaction between the ocean and the atmosphere and variations in atmospheric aerosols

Challenges in assessing and managing multi-hazard risks: a European stakeholders perspective

The latest evidence suggests that multi-hazards and their interrelationships (e.g., triggering, compound, and consecutive hazards) are becoming more frequent across Europe, underlying a need for resilience building by moving from single-hazard-focused to multi-hazard risk assessment and management. Although significant advancements were made in our understanding of these events, mainstream practice is still focused on risks due to single hazards (e.g., flooding, earthquakes, droughts), with a limited understanding of the stakeholder needs on the ground. To overcome this limitation, this paper sets out to understand the challenges for moving towards multi-hazard risk management through the perspective of European stakeholders. Based on five workshops across different European pilots (Danube Region, Veneto Region, Scandinavia, North Sea, and Canary Islands) and an expert workshop, we identify five prime challenges: i) governance, ii) knowledge of multi-hazards and multi-risks, iii) existing approaches to disaster risk management, iv) translation of science to policy and practice, and v) lack of data. These challenges are inherently linked and cannot be tackled in isolation with path dependency posing a significant hurdle in transitioning from single- to multi-hazard risk management. Going forward, we identify promising approaches for overcoming some of the challenges, including emerging approaches for multi-hazard characterisation, a common understanding of terminology, and a comprehensive framework for guiding multi-hazard risk assessment and management. We argue for a need to think beyond natural hazards and include other threats in creating a comprehensive overview of multi-hazard risks, as well as promoting thinking of multi-hazard risk reduction in the context of larger development goals