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Setting up the mid-resolution SINTEX-F3 coupled model
No full textA mid-resolution ECHAM(T127)-NEMO(ORCA05) named SINTEX-F3-MR is upgraded with ECHAM atmosphere model 6 th version and the 5 th version of the NEMO ocean model. We details the system parameters choice that gives to the tool the appropriate properties needed for the focused Tropical Pacific variability studies, along with the porting actions needed to fully benefit of our last efficient NEC vector machine. A 80 year long simulation is performed to estimate the long term biases. Global volume trends are well stabilised but additional studies and tuning are needed to better evaluate the impact of a global surface cold bias in our main region of interes
Spatial variation of future trends in Atlantic upwelling cells from two CMIP6 models
No full textInternational audienceEastern Boundary Upwelling Systems (EBUS) are characterized by wind-triggered upwelling of deep waters along the coast. They are hotspots of biological productivity and diversity and therefore have a high economic, ecological and social importance. In the past, different methods using surface data have been used to estimate upwelling. Recently, the IPCC has suggested directly assessing vertical velocities as a promising method. We use this method to study the two Atlantic EBUS from CMIP6 models from the HadGEM3-GC3.1 and the CNRM6-CM6 family, for both the historical period and a high-emission future scenario with spatial resolutions in the ocean component ranging from 1 • to 1/12 • . The two major upwelling regions are divided in subregions depending on their seasonality. The vertical transport index shows similar values to a wind-derived Ekman index. Directly evaluating upwelling from transport processes further provides information about the depth of the upwelling, which has previously been identified as an important factor for nutrient availability. We show that depending on the subregion of the upwelling system, different cell structures can be seen in terms of depth and distance to the coast of maximum velocities. When looking at possible future changes, high interannual variability limits the significance of the trends but could indicate a poleward shift of the upwelling regions. A detailed comparison of the spatial structures and the distinction in subregions is important to explain contradictory trends in previous works.</div
Non-thermal energy release in the post-impulsive phase of the May 9, 2021 event
No full textInternational audienceContext. In the standard model of solar flares, a magnetic flux rope erupts and gets ejected from the Sun. The current sheets that form in its wake are the seat of magnetic reconnection, which is thought to power energy release throughout the long-lasting decay phase of the thermal X-ray emission. This model has been broadly tested with plasma diagnostics at soft X-ray, EUV, and Hα wavelengths.Aims. The primary aim of the present investigation is to shed light on the acceleration of non-thermal electrons in the post-impulsive phase through hard X-ray (HXR) radiation and radio spectroscopic imaging at decimeter-to-meter wavelengths. We focus our study on the case of a C4.0 class flare on May 9, 2021.Methods. This event was fully observed by multiple instruments from three different vantage points in space. We analyzed the spectrum and the source configuration of X-ray emission with the Spectrometer-Telescope for Imaging X-rays (STIX) on board the Solar Orbiter spacecraft, complemented by the Gamma-Ray Burst Monitor (GBM) aboard the Fermi mission, and the radio emission with Nançay Radioheliograph (NRH) and the ORFEES spectrograph. The extreme ultraviolet images from both Solar TErrestrial RElations Observatory (STEREO-A) and Solar Dynamics Observatory (SDO) were applied to trace the evolution of thermal plasma and coronal magnetic structures.Results. The radio spectrum at decimeter-to-meter wavelengths shows broadband continuum emission (type IV burst), which is a well-known radio signature of time-extended electron acceleration in eruptive flares. Both moving and stationary radio sources were identified. Energetic electrons were observed in X-rays up to 20 keV, displaying a significant correlation with the time evolution of the stationary type IV radio burst during the long duration decay phase, which lasted over 50 minutes. The X-ray photon spectral index is relatively steep with a value of around – 7.5 and the integrated electron flux above 30 keV is on the order of 1.6 × 1032 electron s−1.Conclusions. This case study provides for the first time evidence that HXR emission accompanies the onset of a stationary type IV radio burst. It ties together several pieces of evidence to support that non-thermal electrons are released into large-scale magnetic flux ropes during the post-impulsive phase of eruptive solar flares. The energies of the non-thermal electrons inferred from the X-ray spectral analysis confirm indirect estimates from radio observations. Electron acceleration processes appear as a significant signature of post-impulsive energy release, with energies in the range from several to tens of kiloelectron volts (keV)
Exospheric neutral density at the 10 R<sub><i>E</i></sub> subsolar point during solar maximum: estimates from XMM soft X-ray observations
No full textInternational audienceHighly charged ions in the solar wind undergo charge exchange with neutral atoms in the Earth’s exosphere, particularly within the magnetosheath and cusps. This solar wind charge exchange process generates X-rays, which are expected to be crucial for imaging Earth’s dayside magnetosphere in the upcoming Lunar Environment Heliospheric X-ray Imager (LEXI) and Solar Wind Magnetosphere Ionosphere Link Explorer (SMILE) missions. A key parameter in this process is the density of neutral hydrogen in the Earth’s exosphere. This study estimates the exospheric density during solar maximum using soft X-ray data from the XMM-Newton astrophysics observatory. We used the Open Geospace Global Circulation Model (OpenGGCM), a global MHD model, to calculate plasma density, velocity, and temperature, and then extracted the exospheric density from the soft X-ray data by deconvolving plasma contributions. Based on five XMM-Newton observations during the solar maximum period from 2000 to 2003, we estimate the exospheric density at the 10 RE subsolar point to range from 42.5 to 65.1 cm−3 , which is higher than the density used in previous soft X-ray imaging studies. This increased density range suggests stronger X-ray signals for the LEXI and SMILE missions
Quantifying fault-related uncertainty with inverse homogenization
No full textInternational audienceAlthough the ease and accuracy of seismic interpretation are continually increasing thanks to the increasing amount of available data, computing power and new automatic interpretation techniques, it is still challenging to resolve fine-scale geological features at depth (e.g. faults) because of physical limitations. Indeed, seismic imaging techniques are based on frequency band-limited seismic data, and therefore they can only recover a smooth version of the true Earth, which is not suited for a proper geological interpretation below the decametric scale. Uncertainties and pitfalls in the interpretation of these fine-scale features can affect natural hazard mitigation strategies, and lead to overly optimistic model-based forecasts. To make sure that such subtle features are appropriately considered in subsurface uncertainty studies, we propose the use of a downscaling (or inverse homogenization) approach.In this work, the downscaling is used to properly detect faults and quantify the uncertainty associated to fault parameters geometry and displacement. In particular, from a smooth representation of the real complex structures, obtained through seismic techniques, such as the well-known Full Waveform Inversion (FWI), the downscaling inversion aims to recover all the finer scale fault models compatible with the FWI solution. Because this is an ill-posed inverse problem, the inversion is cast into a Bayesian framework, which combines the information at larger scale coming from the data (FWI model) with some a priori knowledge on the fault structures in order to retrieve a probability distribution over the possible fine-scale models. A Markov Chain Monte Carlo (MCMC) algorithm is adopted to sample the model space and numerically evaluate the posterior probability distribution. This involves the stochastic generation of velocity model realizations where fault displacement is computed using a kinematic modeling approach and the fine-layering velocity is obtained through geostatistical simulations.A significant advantage of this technique is that it can be applied to downscale a localized area of interest within a larger FWI dataset, consequently reducing memory consumption and computational cost. This latter is also reduced thanks to the inexpensive forward modeling operator (i.e., the non-periodic homogenization), making the stochastic inversion feasible compared to standard MCMC-based seismic inversion methods. The proposed methodology, validated on a synthetic data-case example, proves to be a reliable approach to resolve and quantify fault-related uncertainty
Mercury compound-specific stable isotope fractionation in high-altitude lake ecosystems of the Bolivian Altiplano
No full textInternational audienceA combination of mercury (Hg) speciation and compound-specific stable isotope analyses was employed to trace the origin and fate of monomethylmercury (MMHg) in the high-altitude food webs of Lakes Titicaca (TTKK) and Uru Uru (UU). Significant MMHg biomagnification was observed, with concentrations reaching up to 2 μg.g−1 Hg in top predators. Hg isotopes lake-specific trends were identified in relation to trophic position (δ15N) and MMHg fractions. In particular, Δ199MMHg increased from 0 ‰ in UU epiphytic biofilm to ≈2 ‰ and ≈4 ‰ in UU and TTKK piscivorous fish, respectively. Both δ202MMHg and Δ199MMHg signatures indicate that the sediment and/or the epibenthic environment is the primary MMHg source in both food webs. However, an additional MMHg pool, associated with photodegraded MMHg, was identified entering the two food webs at a different trophic level. Photodemethylation was estimated to account for 21 % of MMHg degradation before it entered TTKK Lake food web at the fish level, and 16 % before reaching UU Lake food web at the invertebrate level. Even-Hg MIF (Δ200Hg) shows that both Hg(0) atmospheric deposition and geogenic inputs contributed to Hg accumulation in sediments, while the food web adds significant atmospheric Hg(II) signatures, with up to 94 % of Hg(II) contributing to Lake TTKK fish MMHg. These findings underscore the high potential of Hg-CSIA revealing the dominant role of atmospheric Hg(II) deposition and distinct MMHg pools in driving MMHg bioaccumulation in high-altitude lake food webs
Increased hydrogen escape from Mars atmosphere during periods of high obliquity
No full textInternational audienceIt is still unknown how much water has escaped from Mars during its history. Hydrogen escape from Mars’s atmosphere probably played a major role in drying the planet, but present-day H loss rates (~3 × 1026 atoms per second on average) cannot explain the geological evidence for the large volumes of liquid water on ancient Mars. Here we used the three-dimensional Mars-Planetary Climate Model to show that H loss rates could have increased by more than one order of magnitude (6 × 1027 atoms per second) during higher spin axis obliquity periods, notably in the last few million years when Mars’s obliquity was about 35° on average. The resulting accumulated H escape over Mars’s history translates into an ~80 m global equivalent layer, which is close to the lower limit of geological estimates, assessing the major role of atmospheric escape in drying Mars
Unraveling climate targets across the Paris conurbation as a gauge of city ambitions
No full textInternational audienceUrban areas are crucial for emission mitigation, yet achieving reduction targets remains challenging. This paper analyzes CO 2 mitigation measures in the Paris metropolitan area, providing a unique, comprehensive assessment of the city's climate action plan at high spatiotemporal resolution, allowing us to examine emissions trends and the likelihood of meeting the 2030 and 2050 climate targets. Our temporal analysis reveals that planned efforts align with the 2030 targets, but achieving the 2050 goals requires additional measures. Projected district-level emissions reveal significant heterogeneity marked by aggressive climate strategies within the urban core in the traffic sector and east/west gradients pointing to socio-economic factors influencing mitigation potentials. Our analysis links income and emissions, indicating the need for balanced interventions. This study introduces a next-generation quantification tool that rigorously assesses urban climate action plans, highlighting the pivotal role of the building and traffic sectors in meeting climate targets while addressing socioeconomic barriers.To keep global warming below 1.5 degrees Celsius by mid-century, several consortia of cities have committed to a substantial reduction of urban system CO 2 emissions 1 . At the Paris Climate Conference (2015), over 360 cities committed to a reduction of up to 3.7 Gt CO 2 e/year by 2030, while the Global Covenant of Mayors (GCoM), representing over 10,000 cities, pledged to reduce urban Greenhouse Gas (GHG) emissions by 1.4-2.3 Gt CO 2 e/year by 2030 2-4 . City governments play a pivotal role in global climate change mitigation, with an increasing contribution from urban centers to energy-related fossil fuel emissions of about 70% 1 . Urbanization primarily occurs in less developed regions, with a growth rate of 2.3% p.a. compared to 0.5% in developed regions, further concentrating emissions over a small fraction of the globe 5 . Joining the global effort to fight climate change, city mayors and local governments have been invited to lead the first Local Climate Action Summit during COP28 in Dubai 6 . This first-of-its-kind event underscored the critical role of local leaders in reducing emissions, adapting to climate risk, and accelerating national climate efforts 6 . Before this event, the Intergovernmental Panel on Climate Change (IPCC) 2018 1.5 °C Special Report had already emphasized the role of cities as crucial actors in global climate responses 7 .Non-state actors have gathered around alliances, like GCoM and C40 Cities Climate Leadership Group, to share best practices and scale up technology and innovation 8,9 . These consortia promote ambitious climate actions to support local governments, with most cities following the IPCC's recommendation to achieve net-zero emissions by mid-century 7,10 . To concretize their climate pledges, cities are issuing Climate Action Plans (CAPs) based on emission inventories, providing an emission baseline and reference year to design mitigation actions 11 . The therein outlined climate targets frequently exceed national and international emissions reduction ambitions 12 . CAPs include mitigation measures according to political boundaries and governance structures 13 with a focus on territorial emissions. In contrast, the city's metabolism and interactions extend far beyond the city's boundaries, amounting on average to 60% of their total carbon footprints 14 . Many cities have made tangible progress in implementing their CAPs. Copenhagen, for instance, is at the forefront of reaching carbon neutrality, while London has significantly expanded its Ultra Low Emission Zone, now the largest in Europe 15,16 . Paris is actively promoting cycling infrastructure, with the objective of becoming a "100% bikeable city" by 2026, and New York City launched a US$ 1.3 bn investment strategy for green infrastructure and stormwater management 17,18 . However, cities continue to face substantial challenges, including the need for behavioral shifts, such as transitioning to low-carbon transportation options (e.g., walking, biking, public transportation, etc.), securing sufficient funding, and</div
Geomorphic activity and related large wood recruitment during debris flows and debris Floods: Storm Alex in the Vésubie valley (France)
No full textInternational audienceExtreme floods in forested catchments cause geomorphic changes and large wood recruitment, worsening flood hazards. The study examines the geomorphic response and large wood recruitment in the Vésubie River catchment (392 km2, southeast France) during Storm Alex in October 2020. Using high-resolution LiDAR-derived DEM, the research quantifies geomorphic changes in the main branches and 43 active tributaries, estimating sediment mobilization and channel dynamics at (sub-)catchment and 100-meter reach scales. Forest cover changes were analyzed by comparing pre- and post-event canopy height models (CHM), allowing for the estimation of large wood recruitment. The results show significant geomorphic changes, with extreme erosion and deposition rates observed at both the (sub-)catchment and reach scales. A total forest cover loss of 121 ha was estimated, with recruited large wood volumes per unit of catchment surface ranging from 4 – 445 m3/km2 in the tributaries and 45 – 95 m3/km2 in the main branches. A positive correlation was identified between forest cover loss and both erosion and deposition rates, suggesting that forest loss resulted from trees being scoured by erosion or buried by deposition. An empirical approach was developed to predict forest cover loss based on total sediment mobilization by implementing different equations for optimistic, intermediate, and catastrophic scenarios. The research highlights the interconnected nature of geomorphic processes and their role in large wood dynamics. The findings provide valuable insights into large wood recruitment and the impact of intense geomorphic activity on channel dynamics, demonstrating complex and tight interactions between sediment transport, forest cover, and flood hazards in mountain catchments
Spatial distribution of alteration and strength in a lava dome: Implications for large-scale volcano stability modelling
No full textInternational audienceVolcanoes are unstable heterogeneous structures that can host hazardous mass movements. Hydrothermal alteration can create weak zones that promote instability. Volcanic instability can be assessed using large-scale numerical models, which require accurate and reliable physical and mechanical rock input parameters. Volcano stability models are often constructed using discrete zones that are assigned homogeneous parameters, rarely accounting for the heterogeneity of volcanoes. Given that the range and distribution of alteration and rock strength in volcanoes are likely highly variable, these factors should be carefully determined for accurate modelling. Here, we performed an integrated field and laboratory study. We examined a total of 544 variably altered andesites from seven sampling locations at La Soufrière de Guadeloupe (Eastern Caribbean). Based on a visual assessment, we assigned the rocks an alteration grade index, from 1 (least altered) to 5 (most altered), and measured the strength of rocks in the field using a point load tester. The alteration and strength distribution maps we provide highlight the extreme heterogeneity of a volcanic structure. We provide a method for direct on-site conversion from field to laboratory strength. We find that porosity and strength increase and decrease, respectively, as a function of increasing alteration. The most altered rocks were weak regardless of their porosity, suggesting that the alteration is the primary factor governing strength. We conclude that a volcano can be heterogeneous in terms of alteration and strength, between and within the discrete zones. Therefore, if possible, material property heterogeneity should be incorporated in future volcanic stability models