Le numérique responsable

15/04/2022 12:30

(In)Justice climatique : éclairages apportés par le WGIII du GIEC

12/04/2022 18:00

Journée scientifique du groupe SAMA-IPSL

11/04/2022 08:45

L'engagement des chercheurs face à la déstabilisation écologique du monde

04/07/2023 11:00

Claire Nouvian est fondatrice et directrice générale de l’association de protection de l’océan BLOOM

Climat : la responsabilité des médias

29/06/2023 14:00

Cycle de séminaires sur l’éthique et la responsabilité de l’engagement public des scientifiques.

Phytoplankton community response to atmospheric dust addition in the Arabian Sea under different CO2 levels : An experimental Approach

28/06/2023 11:00

The Indian Ocean receives nearly one-fourth of the global atmospheric dust deposition which is about 113 Mt. yr− 1 and the Arabian Sea is vulnerable to dust supply which can potentially modulate its surface water biogeochemical processes, particularly, phytoplankton communities.

Ocean ventilation at the mesoscale

18/07/2022 14:00

Within the Earth’s climate system, the ocean is engaged as a huge reservoir of important properties such as heat and carbon, predominantly resulting from exchanges with the atmosphere on timescales from hours to millennia. Such large volume of storage in the ocean interior thus questions the mechanisms of water property transport and distribution, leading to the concept of ocean ventilation, a process that connects ocean surface waters with the interior. Commonly associated with an increase in density of surface waters, ventilation is typically interpreted as a downward transfer of water masses due to stability and other fine-scale processes. Understanding the dynamics and thermodynamics of water mass formation, ventilation and dissipation, is therefore one of the key scientific challenges confronting the entire climate community.

In this thesis, several processes related to ventilation have been discussed and a specific attention has been given to the mesoscale whose typical length is less than 100 km and timescale spans on the order of a month. The largest proportion of mesoscale kinetic energy is contained by coherent vortices, known as mesoscale eddies, which are nearly geostrophic and can have the vertical extent down to the thermocline. Aimed at a combination between the ventilation theory and mesoscale dynamics, the first part of this thesis has been devoted to a revisit to the theory of subduction at the bottom of mixed layer that quantifies long-term (permanent) transport of surface water masses into the main thermocline. Interpreted as a transient state in the subduction process, mode waters are a specific type of water mass homogeneous in properties (i.e., characterized by low potential vorticity) and residing between the seasonal and main thermoclines.

Such transiency of mode waters is associated with their formation mechanism largely due to surface buoyancy forcing that is season-dependent. The second part of this thesis is thus related to an algorithm development to detect more precisely than other available methods the surface mixed layers and mode waters from several profiling databases. By co-locating mode waters with mesoscale eddies identified from the satellite altimetry, it is possible to quantify 1) the percentage of mode waters carried by eddies in an Eulerian sense, and 2) anomalies of temperature, salinity and others transported within eddies in a Lagrangian framework. Accordingly, a revisit to global mode water distribution has been provided, in terms of their dynamics and thermodynamics at the mesoscale. The South Atlantic Subtropical Mode Water has been considered as a special example and brought into details in the last chapter, since it not only forms according to the typical baroclinity at the western boundary, but also develops due to a large amount of inter-basin transport carried by anticyclonic Agulhas Rings shedding from the Indian Ocean.

Apart from the thermohaline perspective of ocean circulation and ventilation, i.e., surface convection and its significance on mode water formation and renewal, this thesis also provides an assessment on the wind-driven aspect and a combination of these two components. In specific, we extended the Ekman dynamics to allow for an influence from geostrophic motions and self-advection. A brief discussion on diapycnal and more complex physics of ventilation at the mesoscale is also presented.

Boundary-layer processes impacting the surface energy balance in the Arctic

07/07/2022 14:00

The Arctic is warming at two to three times as fast as the rest of the Earth, and it is therefore a crucial area of study for atmospheric scientists. However, the logistical difficulty of leading measure campaigns at high latitudes means that some key boundary-layer processes are still poorly understood. This thesis aimed to gain insight on two characteristics of the Arctic boundary-layer (clouds and surface based temperature inversions) and to determine their impact on the surface energy balance through a combination of novel measurements and modelling.

First, a novel statistic of cloud frequency and characteristics over the Arctic sea-ice was derived from a set of 1777 lidar profiles obtained during the 5-year Ice, Atmosphere, Ocean Observation Systems (IAOOS) campaign. Clouds were found to occur more than 85% of the time from May to October and single cloud layers were optically and geometrically thickest in October, possibly linked to moisture intrusions in autumn. Total cloud radiative forcing over a typical summer cycle was estimated to be negative for optically thin clouds, but positive for optically thick clouds.

Second, the impact of wind speeds on the development of surface based temperature inversions (SBI) in the continental Arctic was investigated. The analysis of measurements from the pre-ALPACA winter 2019 campaign that took place in Fairbanks, Alaska, showed that a local, likely topographically driven flow developed under anticyclonic conditions. This flow inhibited the development of strong SBIs by sustaining significant turbulence even under very strong radiative cooling. A transitional wind speed between weakly and strongly stable regimes was evidenced; this was coherent with the predictions of Minimum Wind speed for Sustainable Turbulence (MWST) theory. The modelling of clear-sky surface layer temperature inversions and their dependence on wind speed was then studied, with a focus on forest areas. A 2-layer analytical model of the vegetated surface layer was developed. This model exhibited a slower decrease of the SBI strength with wind speed compared to a 1-layer model, which was shown to be coherent with observations at an Ameriflux site close to Fairbanks. These models were then compared to two WRF (Weather Research and Forecasting) surface layer schemes, which were found to place excessive limits on the turbulence, preventing the development of large temperature gradients. The Arctic boundary-layer has become an active field of research in recent years. In this context, modelling advances and numerous planned campaigns open many perspectives for furthering the work presented in this thesis.

Reconstruction multi-centenaire des variations d'indicateurs hydro-climatiques en Patagonie à partir de la composition isotopique des cernes d'Araucaria araucana

13/07/2022 14:00

Depuis une centaine d’années, la partie Ouest de l’Amérique du Sud, de l’Altiplano à la Patagonie du Nord, connait des périodes de sécheresse de plus en plus longues et de plus en plus fréquentes. Cette tendance doit se poursuivre d’après les modélisations climatiques. Or, on la soupçonne être en partie liée à l’expansion de la cellule atmosphérique de Hadley associée à une phase positive dominante de l’Oscillation Antarctique ces dernières décennies. L’objectif de ma thèse est de contribuer à améliorer la compréhension des processus responsables de cette évolution en reconstituant les variations hydro-climatiques passées en Patagonie du Nord. J’utilise pour cela la composition isotopique en carbone (δ¹³C) et en oxygène (δ¹⁸O) de la cellulose des cernes d’Araucaria araucana, une espèce endémique de Patagonie dont l’aire de répartition est comprise entre 37°20’S et 40°20’S. Plusieurs résultats se dégagent de cette étude.

Tout d’abord, j’ai montré que la composition isotopique de la cellulose des A. araucana reflète les conditions climatiques de la saison de croissance en cours tandis que les largeurs de cernes dépendent davantage des conditions climatiques de la saison de croissance précédente. La remobilisation des réserves n’intervient donc pas dans la fabrication des sucres utilisés pour produire les cernes. Puis j’ai montré que les variations du δ¹³C et le comportement physiologique des arbres sont liés à l’humidité du milieu dans lequel ils évoluent. Il existe donc une différence entre les arbres poussant à l’Ouest des Andes, où les précipitations sont abondantes, plutôt sensibles aux variations de luminosité, et ceux poussant à l’Est dans un environnement plus sec, plutôt sensibles aux variations d’humidité. Au cours de cette thèse, j’ai également mis en évidence le lien entre les variations de δ¹³C et de δ¹⁸O des cernes d’A. araucana évoluant dans un milieu sec, et celles de la température et de l’humidité, elles-mêmes contrôlées par l’Oscillation Antarctique et la position de la branche descendante de la cellule de Hadley.

Le fort potentiel du δ¹³C de la cellulose des cernes d’A. araucana à enregistrer les variations de température et d’aridité d’été permet de reconstituer les variations climatiques de Patagonie sur plusieurs siècles (315 ans) et met en avant une forte augmentation des températures maximales (+1°C) à la fin du XVIIIème siècle. A l’échelle régionale, les reconstructions climatiques révèlent une uniformisation des variations de température sur les dernières décennies, probablement en lien avec le changement climatique global qui devient le principal facteur de contrôle de la variabilité climatique. Le δ18O de la cellulose, contrôlé par les δ¹⁸O des précipitations et du sol, a lui aussi enregistré un changement climatique à la fin du XVIII^ème siècle qui serait imputable, du moins en partie, à l’Oscillation Antarctique. Les déplacements de la branche descendante de la cellule de Hadley semblent également enregistrés par le δ¹⁸O jusqu’au milieu des années 1990, date à laquelle la cellule se serait probablement trop étendue pour affecter les A. araucana à ces latitudes.

Ces outils isotopiques s’avèrent donc prometteurs pour mieux comprendre la variabilité spatio-temporelle des phénomènes qui touchent la Patagonie, notamment la contribution de la cellule de Hadley dans le changement global actuel du climat.