Comment dialoguer sur nos transformations futures ?

06/10/2023 09:00

Forum International de la Météo et du Climat 2023

06/10/2023 09:00

25 ans d'observation au SIRTA

01/06/2023 09:00

Des végétaux aux hotspots microbiens du sol : Comment le statut hydrique du sol influence le devenir des matières organiques ?

13/03/2024 13:00

Séminaire de l’UMR METIS-IPSL.

From microscale observation to macroscopic quantification: Linking microbial dynamics and carbon fluxes in inland waters

06/03/2024 13:00

Séminaire de l’UMR METIS-IPSL.

Atelier interactif sur les violences sexistes et sexuelles

05/03/2024 14:30

Antigoni Alexandrou, directrice de recherche CNRS à l’École Polytechnique, viendra présenter une atelier interactif sur les violences sexistes et sexuelles, et discutera de ce que dit la loi, comment reconnaître ces violences, et comment y faire face.

What controls the 3D distribution of water vapor isotopic composition in East Asia?

20/11/2023 14:00

Unlike polar ice core records, the isotope variations in Tibetan ice cores challenge the interpretation of temperature signals. One of the main reasons is that in monsoon regions at low and middle latitudes, water isotopes are influenced by convective and cloud processes. A deeper understanding of water isotope behavior and the dynamical controls involved in moisture transpiration and convection is needed. Large-scale in-situ observations and vertical profiles of vapor isotopes during convection would be helpful. However, such data are rare. The aim of this thesis was to document horizontal, vertical, and temporal variations in the isotopic composition of water in East Asian monsoon region.

First, to document the horizontal and seasonal variations of water isotopes near the surface across China, we made in-situ observations of near-surface vapor isotopes over a large region (over 10000 km) across China in both pre-monsoon and monsoon seasons, using a newly-designed vehicle-based vapor isotope monitoring system. We found that the observed spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation along air mass trajectories during the pre-monsoon period, but are significantly influenced by different moisture sources, continental recycling processes, and convection along moisture transport during the monsoon period. These results provide an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors and emphasize the need to interpret proxy records in the context of the regional system and moisture sources.

Second, to better understand the physical processes that control the vertical distribution of vapor isotopes and its intra-seasonal and seasonal variability, we observed the vertical profiles of atmosphere vapor isotopes up to the upper troposphere (from the ground surface at 3856m up to 11000m a.s.l.) from June to October in the southeastern Tibetan Plateau using a specially-designed unmanned-aerial-vehicle (UAV) system. For the sampling, we chose to carry air bags on UAVs as a portable sampling device, but encountered the permeability problem commonly associated with these bags. To corrected for this problem, we developed a diffusion model with diffusion parameters calibrated through laboratory experiments. This allows us to document forthe first time the vertical distribution of atmospheric water vapor isotopes across the entire monsoon period up to the upper troposphere, boasting an unprecedented vertical resolution and altitude range. We find that the vertical profiles of water vapor isotopic composition reflect a combination of large-scale processes, in particular deep convection and continental recycling along trajectories, and local convective processes, in particular convective detrainment, and sublimation of ice crystals. The observed seasonal and intra-seasonal variations are generally vertically coherent, due to the strong vertical convective mixing and local convective detrainment of vapor originating from the low levels, and are mainly due to deep convection along trajectories.

Contributions des processus physiques à la variabilité récente (1980-2015) des concentrations en nutriments dans le gyre subpolaire en Atlantique Nord

26/10/2023 14:00

The Subpolar Gyre (SPG) of the North Atlantic plays a key role in the carbon cycle and climate variability. It is the site of a vigorous spring phytoplankton bloom, maintained by the seasonal transport of nutrients in association with light. Macronutrients (NO₃, PO₄, DSi) are supplied predominantly to the mixed layer by lateral transport from high latitudes (mainly through the Davis and Hudson Straits), from lower latitudes by the North Atlantic Current, or by vertical transport from below the mixed layer where higher concentrations are present. These supply processes have been suggested to vary in response to the North Atlantic Oscillation, a major mode of natural climate variability.

When the NAO index is negative, as it was from the mid-1990s to the end of the 2000s, physical conditions are similar to those projected forclimate change (i.e. , warming and freshening, weakening of deep convection, slowing down of the Atlantic Meridional Overturning Circulation, increasing stratification. During the same period, a decline in macronutrient concentrations was observed throughout the region leading to the hypothesis that underlying processes could be similar to those acting under global warming to reduce mixed layer nutrient levels.

The main objective of this thesis was to analyze and quantify the contributions of dynamic processes (i.e., lateral and vertical transport) to the observed variability in macronutrient mixed layer concentrations within the SPG between 1980 and 2015. The analysis used output from a coupled physical-biogeochemical model (NEMO-PISCES) discretized on a quarter-degree grid.

An assessment of the model’s representation of the spatial and temporal variability of macronutrient concentrations and the main physical processes, such as deep convection in the Labrador Sea, and lateral transport of water and nutrients, was carried out in comparison with data from observations. An initial focus was on the Labrador Sea, which is a region characterized by deep, intense, and variable winter convection, making it an ideal laboratory for distinguishing the role of variability in deep winter convection from that of the Subpolar Gyre circulation and inputs from the Arctic Ocean. Despite some weakening, the contribution of changes of Arctic DSi transport through the Davis and Hudson Straits to observed nutrient variability was shown to be negligible (= 3$\%$). Deep convection was identified as the main driver of the decline in pre-bloom DSi concentrations in the Labrador Sea. The study was extended next to the broader SPG, with assessment of the recent variability of macronutrient concentrations in the mixed layer through analysis of the variability of lateral and vertical transports between a period of strong positive NAO, a period of negative NAO, and the period following.

In addition to the temporal variability of concentrations in response to atmospheric forcing, regional differences emerged, with a contribution dominated by vertical transport in the Labrador and Irminger Seas, led by variations in the depth of the mixed layer. Zonally-integrated nutrient transport east and north of the SPG are coherent with the variability of nutrients within the SPG but decoupled from nutrients transported by the Gulf stream via the North Atlantic Current west of 38°N. In this thesis, I highlighted the predominance of vertical mixing in the contemporary variability of nutrient concentrations, compared with lateral inputs.

I show that a slowing of the general circulation associated with stratification of the water column leading to the weakening of vertical nutrient fluxes, as is the case projected for climate change conditions, would lead to a reduction in macronutrient concentrations and potential future primary production in the SPG.

A spectroscopic study of water vapor on Mars

25/10/2023 09:00

Abstract

Water vapor is a minor species in the atmosphere of Mars, yet it has played and plays a significant role in shaping the climate of the planet. Water was once much more abundant, evident today from features such as dry riverbeds, outflow channels and hydrated minerals, prompting extensive research into its disappearance. Atmospheric water vapor has been monitored and studied extensively in the past decades, and most of its chemical and dynamical behaviors are now known. One of the few remaining challenges is related to its vertical distribution. Missions like Mars Express, MAVEN, and the ExoMars Trace gas Orbiter ushered in a new era in water vertical profile studies and have all provided valuable insights, but access to the lowest part of the atmosphere has remained limited. Water vapor is commonly assumed to have a uniform distribution below the cloud condensation layer, but some measurements are challenging this view, suggesting a more complex profile near the surface, where regolith-atmosphere exchanges might be present.

The main work of this thesis is related to water vapor, with the objective of investigating the near-surface water content in both a seasonal and geographical context. To do this, we have explored two unconventional infrared spectroscopic techniques; a spectral synergy method applied to nadir observations, and surface observations. This thesis also contains one chapter about Martian space weather, which contains a collection of smaller projects conducted throughout the duration of this PhD.

The spectral synergy uses near- and thermal-infrared spectra from SPICAM and PFS respectively on Mars Express. Water vapor is retrieved simultaneously from both spectral bands, and since these two wavelength intervals are sensitive to separate altitude regions, the resulting increased degree of freedom allows information on the vertical distribution to be gained. The synergy was applied to almost 200 000 co-located observations, sampled across roughly eight Mars year. Composite climatologies of very accurate column abundances and vertical profiles were assembled. The column abundances were in good agreement with previous studies, but the results exhibited some significant differences from the Mars Climate Database, both with respect to the column abundances and the vertical distribution. The spring sublimation peak was observed to be less extreme, and the sublimation onset occurred later than the model. The vertical confinement is observed to be stronger compared to the model at almost all seasons and latitudes, and the distribution is rarely uniform. The confinement as a function of season and latitude was studied in details, and a latitudinal wave-like behavior was discovered in both hemispheres, as well as a prevailing double-layer structure in the northern hemisphere.

For the surface observations, we make use of the infrared spectrometer part of SuperCam on the Perseverance rover, and conduct so-called passive sky measurements. To date, we have 64 observations taken regularly across one Martian year. In the passive sky technique, infrared spectra are acquired at two elevation angles and then ratioed in order to remove continuum and instrumental effects. The resulting spectrum is mainly sensitive to the atmosphere below 15 km, and can therefore directly probe altitudes rarely accessible from orbit. Here we outline the progress made so far regarding data processing and the development of a retrieval pipeline. A forward model and minimization routine has been composed, and is currently undergoing testing and further maturing.

 

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Résumé

La vapeur d’eau est un composant mineur dans l’atmosphère de Mars, mais elle joue un rôle significatif dans la formation du climat actuel de la planète. L’eau était autrefois beaucoup plus abondante, ce qui se traduit aujourd’hui par des caractéristiques telles que des lits de rivières asséchées, des canaux d’écoulement et des minéraux hydratés, suscitant ainsi des recherches approfondies sur sa disparition. La vapeur d’eau atmosphérique a été largement surveillée et étudiée au cours des dernières décennies, et la plupart de ses caractéristiques chimiques et dynamiques sont désormais connues. Cependant, l’une des rares difficultés restantes concerne sa distribution verticale. Des missions telles que Mars Express, MAVEN et ExoMars Trace gas Orbiter ont ouvert une nouvelle ère dans les études de profils verticaux de l’eau, fournissant des informations précieuses. Cependant, l’accès à la partie la plus basse de l’atmosphère est resté limité. La vapeur d’eau est généralement supposée avoir une distribution uniforme en dessous de la couche de condensation des nuages, mais certaines mesures remettent en question cette hypothèse, suggérant un profil plus complexe près de la surface où les échanges entre le régolithe et l’atmosphère pourraient jouer un rôle.

Le principal travail de cette thèse est lié à la vapeur d’eau, avec pour objectif l’étude de la teneur en eau près de la surface dans un contexte saisonnier et géographique. Pour cela, nous avons exploré deux techniques non conventionnelles : une méthode de synergie spectrale appliquée aux observations au nadir, et des observations spectroscopiques infrarouges de surface. Cette thèse contient également un chapitre sur la météo spatiale martienne, qui présente des travaux secondaires menés tout au long de cette thèse.

La synergie spectrale utilise des spectres infrarouges proches et thermiques provenant respectivement du SPICAM et du PFS sur Mars Express. La vapeur d’eau est récupérée simultanément à partir des deux bandes spectrales, et comme ces deux intervalles de longueurs d’onde sont sensibles à des régions altitudinales atmosphériques distinctes, le degré de liberté accru qui en résulte permet d’obtenir des informations sur la distribution verticale. La synergie a été appliquée à près de 200 000 observations co-localisées, échantillonnées sur environ huit années martiennes. Des climatologies composites d’abondances de colonne très précises et de profils verticaux ont été rassemblées. Les abondances de colonne s’accordaient avec les études précédentes, mais les résultats présentaient des différences significatives avec la base de données climatiques de Mars, autant pour les abondances de colonne que la distribution verticale. Le pic de sublimation printanier a été observé comme moins extrême et son apparition était plus tardive par rapport au modèle. Le confinement vertical est observé comme étant plus fort par rapport au modèle dans presque toutes les saisons et latitudes, et la distribution est rarement uniforme. Le confinement en fonction de la saison et de la latitude a été étudié en détail, et un comportement de type ondulation latitudinale a été découvert.

Pour les observations de surface, nous utilisons le spectromètre infrarouge de SuperCam sur le rover Perseverance, et réalisons des mesures dites « passives du ciel ». À ce jour, nous avons effectué 64 observations régulières sur une année martienne. Dans la technique du ciel passif, des spectres infrarouges sont acquis à deux angles d’élévation puis rationnés afin d’éliminer les effets continus et instrumentaux. Le spectre résultant est principalement sensible à l’atmosphère en dessous de 15 km et peut donc sonder directement des altitudes peu accessibles depuis l’orbite. Nous présentons ici les progrès réalisés en matière de traitement des données et de développement d’un outil de récupération. Un modèle direct et une routine de minimisation ont été composés et sont en cours de test et de d’amélioration.