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Contrasting dynamics of past climate states and critical transitions via dimensional analysis
No full textInternational audienceWhile major climate transitions in the past can be clearly identified, our understanding of the underlying mechanisms and timescales remains limited. To address this, we conducted a dimensional analysis of benthic stable isotope records across various timescales to explore how Cenozoic climatic fluctuations relate to changes in feedback mechanisms. Our analysis reveals that climate responses to orbital forcing differ significantly between warmer and colder periods. Specifically, during the Icehouse state, a high number of feedback mechanisms predominated at obliquity and eccentricity timescales, whereas, in the Warmhouse and Hothouse states, feedback mechanisms were more prominent at precession timescales. During the Coolhouse state, the number of active feedbacks was low, and no single timescale dominated. Additionally, coupling between climate signals affecting oxygen and carbon isotope records was notably high only during the Icehouse state and low or absent in other states. We also observed that during the Paleocene-Eocene Thermal Maximum (PETM), there was an unusually high number of active feedbacks and very high coupling across all timescales, indicating a significant perturbation of the climate system. In conclusion, our findings suggest that a singular model of interconnected feedbacks may not adequately capture Cenozoic paleoclimate variability. Instead, different numbers of active feedbacks and varying levels of coupling, operating over different timescales, influenced the stability and variability of each climate state
The confidence-noise confidence-boost (CNCB) model of confidence rating data
No full textInternational audienceOver the last decade, different approaches have been proposed to interpret confidence rating judgments obtained after perceptual decisions. One very popular approach is to compute meta-d’ which is a global measure of the sensibility to discriminate the confidence rating distributions for correct and incorrect perceptual decisions. Here, we propose a generative model of confidence based on two main parameters, confidence noise and confidence boost, that we call CNCB model . Confidence noise impairs confidence judgements above and beyond how sensory noise affects perceptual sensitivity. The confidence boost parameter reflects whether confidence uses the same information that was used for perceptual decisions, or some new information. This CNCB model offers a principled way to estimate a confidence efficiency measure that is a theory-driven alternative to the popular M-ratio. We then describe two scenarios to estimate the confidence boost parameter, one where the experiment uses more than two confidence levels, the other where the experiment uses more than two stimulus strengths. We also extend the model to experiments using continuous confidence ratings and describe how the model can be fitted without binning these ratings. The continuous confidence model includes a non-linear mapping between objective and subjective confidence probabilities that can be estimated. Altogether, the CNCB model should help interpret confidence rating data at a deeper level. This manuscript is accompanied by a toolbox that will allow researchers to estimate all the parameters of the CNCB model in confidence ratings datasets. Some examples of re-analyses of previous datasets are provided in S1 File
Distinguishing north and south African Easterly Waves with a spectral method: Implication for tropical cyclogenesis from mergers in the North Atlantic
No full textInternational audienceAfrican Easterly Waves (AEWs) are mixed barotropic–baroclinic instabilities, propagating both north (AEW‐N) and south (AEW‐S) of the African Easterly Jet. They are an important component of the North Atlantic region meteorology, influencing precipitation and convection, and are primary seeds for tropical cyclones (TCs) over the North Atlantic. In this study, a novel tracking protocol is introduced based on filtering of the 700‐hPa vorticity over two frequency ranges, suited for each wave track: 2.95–4.55 days (quicker propagating mode) and 4.55–6.35 days (slower propagating mode). It is applied to a 31‐year reanalysis dataset, the European Centre for Medium‐range Weather Forecasts (ECMWF) Fifth Reanalysis. This method effectively isolates north‐track activity even amidst dominant south‐track signals, enabling better identification and tracking of northward vortices. An analysis of the climatological structure of the two modes is conducted. The slower mode propagates around and transitions from 850 hPa above the continent to 700 hPa above the ocean, consistently with AEW‐N characteristics. The quicker mode propagates around and is located at the African Easterly Jet level, between 700 and 600 hPa above the continent and the ocean, consistently with AEW‐S characteristics. The new method allows the detection of AEW‐N even when AEW‐S activity dominates the 2–10 day signal, thereby enabling the detection of more AEW‐S/AEW‐N mergers than in previous studies. When comparing the usual 2–10 day filtering with this new method, the proportion of TCs related to AEW mergers rises from 11% to 25%. 23.4% of mergers are found to be related to TC genesis, making mergers the most efficient precursors for TC genesis in the Atlantic. TC genesis from mergers occurs more frequently on the eastern side of the basin than TC genesis from other precursors
Experimental validation of scenario-based stochastic model predictive control of nanogrids
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IDENTIFICATION OF THE LATERAL TRACK IRREGULARITIES FROM THE BEHAVIOR OF THE TRAIN’S AXLES ON A SURROGATE RAILWAY DYNAMICS MODEL
No full textInternational audienceThis work aims at the reverse identification of the lateral offset irregularity from the kinematics of the axle on a simplified train surrogate model. The fundamental goal of this study is to have a better understanding of the dynamics of the train which would ultimately allow to identify the track irregularities from axle-boxes acceleration measurements on commercial trains. First, a known surrogate model of the dynamics of the axle from the literature is adapted to include a train carbody and the railway track irregularities and then the inversion of the system is achieved so that a relationship between the kinematics that would be measured on the axle and the lateral track irregularity is established
Transient Slip Rates as a Key Control on Laboratory-and Natural-Earthquake Nucleation
No full textInternational audienceThe transition from slow, stable slip to rapid seismic rupture is a key yet incompletely understood phase of earthquake development. Using laboratory experiments designed to reproduce earthquake-like fault behavior, we document a wide spectrum of nucleation processes preceding dynamic rupture. In our experiments, the onset of nucleation follows a small foreshock, and the slip velocity resulting from this foreshock controls both the duration and width of the nucleation phase. This relationship, observed in the lab and mirrored in data extracted from natural earthquakes, suggests that the pace of early slip is a fundamental parameter governing rupture growth. These results are consistent with a novel framework that couples rate-and-state friction to a fracture energy that evolves with rupture speed, emphasizing two controlling factors: a characteristic slip impulse and the level of overstress. Interestingly, while foreshocks often mark the start of nucleation, we also find that under certain parameter ranges, small slip transients can transition directly into full rupture without a discrete foreshock. Our findings provide new constraints for linking precursory slip and foreshock activity to earthquake nucleation physics, providing estimates of the state evolution slip distance for the nucleation phase of real earthquakes
High resolution air quality simulation in the Himalayan valleys, a case study in Bhutan
No full textInternational audienceAbstract. Our study focuses on Bhutan, a highly mountainous country where government authorities are strengthening air pollution monitoring efforts. To support further analysis and the monitoring strategy, we present the first high-resolution air quality simulations with the chemistry transport model WRF-CHIMERE over the western region of Bhutan at a spatial resolution of roughly 1 km. Increasing the horizontal resolution of the model improves its performance and reduces potential errors caused by excessive spatial averaging of meteorological and emission data with high spatial variability. However, the air pollutant emissions must be improved at a fine scale with better proxy, particularly for industries where improvements are still required. For the first time, we propose high resolution maps of air pollution (concentrations and deposition fields). Our simulations confirm that Bhutan valleys also suffer from air pollution mainly due to PM2.5 (concentrations exceeding 20 µg m−3) dominated by carbonaceous species, largely above the World Health Organization guidelines. Wildfires and anthropogenic activities release large amount of carbonaceous species and can also impact the glaciers by atmospheric fallout. Wildfires can locally contribute to 20 % of the total PM2.5 concentrations over a 15 d period, and theoretically, black carbon can be transported up to the highest peaks. Ecosystems are at risks with deposition fluxes of sulfur and nitrogen species comparable with other locations at risk in the world
Caractérisation expérimentale et modélisation de la dispersion atmosphérique en vent faible et en milieu bâti
No full textAtmospheric dispersion in low-wind conditions is frequently underrepresented or even overlooked in impact studies. Under these conditions, the dispersion mechanisms experience significant alterations: turbulence properties (spectrum, anisotropy) change considerably, and meandering characterized by low-frequency oscillations on the sub-mesoscale of wind direction often becomes dominant, leading to a wide range of concentration variations around the source. Developing models that integrate these processes is especially challenging due to the lack of experimental data for such scenarios, particularly in urban environment.This thesis includes both experimental and modeling components. The experimental work, carried out at SIRTA (Site Instrumental de Recherche par Télédétection Atmosphérique, situated on the campus of École Polytechnique in Palaiseau, a peri-urban area), has two parts. The first part focuses on a statistical analysis of high-frequency wind component measurements over a three-year period to investigate the characteristics of meandering and its interaction with turbulence. The second part involves conducting and analyzing measurement campaigns to acquire experimental data on the dispersion of a tracer gas (Helium) in built environments under low-wind conditions.The modeling component focuses on defining and validating a CFD modeling methodology using the open-source software Code_Saturne, adapted to these specific conditions, and relies on documented cases from the experimental campaigns, especially those featuring meandering. Three approaches of the RANS (Reynolds Averaged Navier Stokes) or URANS (Unsteady Reynolds Averaged Navier Stokes) types are examined:- a steady-state simulation with constant boundary conditions;- a pseudo-transient simulation combining multiple steady cases, conducted with different input conditions to capture the temporal variability of meteorological conditions during the release;- a fully transient simulation with variable boundary conditions.To validate these different approaches, comparisons were made between the modeling results and the measurements, covering both dynamic variables and tracer concentration.La dispersion atmosphérique en conditions de vent faible est souvent mal prise en compte voire négligée dans les études d’impact. Dans de telles conditions, les mécanismes de dispersion subissent des modifications importantes, les propriétés de la turbulence (spectre, anisotropie) sont sensiblement différentes, et le méandrement, caractérisé par des oscillations de basse fréquence à l’échelle sub-meso de la direction du vent, devient souvent prédominant, entraînant une forte variation de la concentration sur une large plage angulaire autour de la source. Le développement de modèles prenant en compte ces processus est d’autant plus difficile que l’on constate un manque de données expérimentales pour ce type de situations, notamment en milieu bâti.La thèse inclut un volet expérimental et un volet modélisation. La partie expérimentale réalisée au SIRTA (Site Instrumental de Recherche par Télédétection Atmosphérique, installé sur le campus de l’école Polytechnique à Palaiseau en milieu péri-urbain) comporte deux parties. La première est dédiée à l’analyse statistique de mesures haute fréquence des composantes du vent sur une période de trois ans pour étudier les propriétés du méandrement et son interaction avec la turbulence. La seconde consiste en la réalisation et l’analyse de campagnes de mesures pour acquérir des données expérimentales de dispersion d’un gaz traceur (Hélium) en milieu bâti et en conditions de vent faible. La partie modélisation porte sur la définition et la validation d’une méthodologie de modélisation CFD avec le code open source code_saturne adaptée à ces situations, en s’appuyant sur les cas documentés pendant les campagnes expérimentales, notamment ceux présentant du méandrement. Trois approches de type RANS (Reynolds Averaged Navier Stokes) ou URANS (Unsteady Reynolds Averaged Navier Stokes) sont étudiées :- une simulation en régime stationnaire intégrant des conditions aux limites constantes ;- une simulation pseudo-instationnaire impliquant une combinaison de cas stationnaires, réalisée avec différentes conditions d'entrée représentatives de la variation temporelle des conditions météorologiques pendant le rejet ;- une simulation en régime instationnaire avec des conditions aux limites variables. Dans le but de valider les différentes approches, des comparaisons entre les résultats de modélisation obtenus et les mesures ont été réalisées, tant pour les variables dynamiques que pour la concentration du traceur
Meltwater, seasonality and economic activity in South Asia
No full textWhile climate change is projected to profoundly alter the processes of solid water melt, the connections between seasonal snowpack melt, glacier shrinkage, and downstream economic activities remain unclear. In this paper, through an econometric analysis of meltwater shocks and overall economic activity at sub-national levels, I aim to enhance our understanding of whether and how hydrological conditions affect economic development. I construct a novel dataset of pixel-level monthly meltwater shocks based on snow and ice coverage and temperature, which I combine with data on nighttime light (NTL) intensity across the entire region of Himalayan-fed watersheds. The dataset spans from 2000 to 2013. I estimate the month-specific annual change in economic activity in a watershed as a function of meltwater shocks. This shock measure accounts for all upstream watersheds’ contributions. The main findings suggest that (i) economic development suffers from both excess and shortage in meltwater volumes, (ii) agriculture serves as the primary channel through which impacts of negative meltwater shocks propagate through the economy, (iii) positive meltwater shocks create torrential floods that act negatively on both the level and the growth rate of economic activities and (iv) groundwater pumping can mitigate the effects of meltwater shocks. The main implications for economic research are that upstream hydrological conditions causally impact downstream economic development.Alors que le changement climatique devrait profondément modifier les processus de fonte de la glace et de la neige, les liens entre la fonte saisonnière du manteau neigeux, le recul des glaciers et les activités économiques situées en aval restent mal compris. Dans cet article, à travers une analyse économétrique des chocs liés aux eaux de fonte et de l’activité économique globale à l’échelle infranationale, j’ai pour objectif d’améliorer notre compréhension de la manière dont les conditions hydrologiques influencent le développement économique.Je construis une nouvelle base de données, en point de grille et à une résolution mensuelle, mesurant les chocs d’eau de fonte, que je combine avec des données sur l’intensité lumineuse nocturne (NTL) pour l’ensemble des bassins versants alimentés par l’Himalaya. L’ensemble couvre la période allant de 2000 à 2013. J’estime la variation annuelle, spécifique à chaque mois, de l’activité économique dans un bassin versant en fonction des chocs d’eau de fonte. Cette mesure de choc tient compte des contributions de tous les bassins versants situés en amont.Les principaux résultats indiquent que (i) le développement économique pâtit à la fois d’un excès et d’un déficit d’eau de fonte, (ii) l’agriculture constitue le principal canal de transmission des effets des chocs négatifs d’eau de fonte à l’ensemble de l’économie, (iii) le pompage des eaux souterraines peut atténuer les effets de ces chocs, et (iv) les chocs positifs d’eau de fonte génèrent des chocs négatifs sur le niveau d’activité économique, ce qui suggère des destructions d’infrastructures. Les principales implications pour la recherche économique sont que les conditions hydrologiques en amont ont un impact causal sur le développement économique en aval et devraient donc être pris en compte dans les modèles projectifs économiques
Tropics-wide intraseasonal oscillations
No full textInternational audienceThe tropical climate variability is characterized by various oscillations across a range of timescales. Oscillations that imprint the tropical mean state are generally attributed to slow processes, such as the seasonal cycle or interannual variability. Here, we identify a pronounced tropics-wide intraseasonal oscillation (TWISO) in satellite observations and reanalyses. This oscillation, with a period of 30 to 60 d, is evident across multiple variables and involves interactions between convection, radiation, surface fluxes, and large-scale circulation. It is primarily manifested as convective perturbations in the tropical Indo-Pacific warm pool accompanied by oscillations in the large-scale tropical overturning circulation. Here, we examine the relationship between TWISO, the Madden–Julian Oscillation (MJO), and the instability of radiative-convective equilibrium. Certain phases of TWISO coincide with specific phases of the MJO, suggesting a potential connection between the two. However, although the MJO can amplify the oscillation amplitude of TWISO, it is not essential for TWISO to occur. Finally, due to its broad manifestation across the tropics, TWISO potentially exerts widespread influence on tropical weather and climate at regional scales