Journée des stagiaires de l'IPSL

12/06/2025 09:00

ICOLMDZ Limited Area Model (LAM) Day

03/06/2025 09:30

Au cœur de l'actualité du monde de la recherche : libertés académiques et engagements publics des chercheur·es

23/05/2025 10:00

Laser-based mass spectrometry in the planetary sciences: convergence of emerging priorities and enabling technologies

17/06/2025 11:30

Séminaire du LATMOS.

Big Data Assimilation Revolutionizing Numerical Weather Prediction Using Fugaku

13/06/2025 14:00

Séminaire du LMD.

Simulated climatologies of Northern Hemisphere blocking and storm tracks in AGCMs

12/06/2025 14:30

Séminaire du LMD à l’ENS.

Emergence of temperature and salinity changes in the ocean interior in response to climate change: timescales and mechanisms

03/02/2022 14:30

Human-induced climate change is already affecting every inhabited region of the planet. Yet, over 90% of the excess heat associated with human activities has been absorbed by the ocean since the 1970s, which acts to largely damp atmospheric warming, but has large impacts on human societies and marine life. In this thesis, I explore when and where thermohaline changes in the ocean interior become large enough to be unambiguously set apart from internal variability and investigate their associated physical drivers, using ensembles of climate models and dedicated numerical experiments. We find that the climate signal in the upper ocean water-masses emerges between the late 20th century and the first decades of the 21st. The Southern Hemisphere mid-latitude Mode Waters emerge before their Northern Hemisphere counterparts. The associated warming at these timescales is mostly caused by the uptake of heat from the atmosphere, passively transported into the ocean interior. In the deeper parts of the ocean, circulation changes play a more important role in the emergence timescales of the climate signals. Increased buoyancy gain at the surface in the subpolar areas cause a slowdown in the meridional overturning circulation. This warms the subsurface and abyssal waters in the Southern Ocean as soon as the mid-20th century, adding up to the weaker passive uptake of heat, but counteracts it in the deep North Atlantic over the 21st, delaying the emergence. Although climate models miss some important aspects of the ocean response to climate change, they allow to shed light on the balance of processes at play, and suggest anthropogenic influence has already spread to large parts of the ocean.

 

Lieu : présentiel et/ou lien de connexion: https://t.co/SxkaH6vt8v (meeting ID: 944 3024 6191; password : 7eCes6)

Variability and Changes of Hydrography and Circulation in the Subpolar Southern Ocean

31/01/2022 15:00

The Southern Ocean is central to the global oceanic circulation and climate. This region is however on the frontline of human-induced climate change, through intense uptake of anthropogenic heat and carbon. Consequently, the Southern Ocean has experienced important changes in its hydrography and circulation over the last decades.

Its subpolar part, south of the Antarctic Circumpolar Current, hosts large circulation systems of importance for the production of water masses and their associate heat and carbon content, for ocean interactions with sea-ice and ice-shelves, and consequently for global mean sea level. Observations are still sparse in that region, particularly in wintertime when it is covered by sea ice. Thus, the regional response of the subpolar Southern Ocean hydrography and circulation to interactions with the atmosphere, cryosphere, and background circulation at various spatial and time scales is still under active research.

In this thesis, I contribute to observing the variability and long-term changes of the hydrography and circulation of the subpolar Southern Ocean, and to identifying the mechanisms driving their variability. I first observe the long-term temperature changes in the upper layer of the Southern Ocean, from repeated ship-based measurement transects over 25 years. Besides previously documented trends, I refine the monitoring on the still poorly observed warming and shallowing of the warm subsurface water of the Southern Ocean. The long-term warming is stronger than interannual variability, and the shallowing rate is 3 to 9 times the previously estimated one. In a second part, I develop and exploit an ocean topography dataset, spanning six years of measurements over the whole Southern Ocean south of 50°S. This dataset allows me to explore the variability of the subpolar Southern Ocean circulation, particularly the seasonal cycle of the large-scale circulation and the mesoscale variability under sea ice. At the seasonal scale, the circulation of the Weddell and Ross gyres, and the Antarctic Slope Current are mainly dictated by three modes of variability, principally linked to the surface stress of the wind on the surface of the ocean and its modulation by the sea ice. The mesoscale variability is weak outside the energetic Antarctic slope current in the pack ice, while the marginal ice zone seems to be a region with enhanced cyclonic eddies generation.

The implications of these results on the physical processes of the Southern Ocean and its long-term changes are discussed.

 

Lien visio : https://www.us02web.zoom.us/j/81861065877

Modéliser l'évolution des plantes à fleurs au Crétacé et leurs rétroactions avec le climat

28/01/2022 14:00

Au Crétacé, l’évolution foliaire des plantes à fleurs, ou Angiospermes, vers de fortes densités de nervures et de stomates, suggère une augmentation de la conductance stomatique et des flux d’évapotranspiration sans précédent. Cependant, ces paléo-traits ne sont pas pris en compte dans les modèles de végétation qui visent justement à déterminer les effets de l’évapotranspiration sur le climat.

L’objectif de ma thèse est donc de modéliser l’évolution de la conductance stomatique des plantes à fleurs au cours du Crétacé et d’en évaluer ses effets sur les interactions et rétroactions climat-végétation. En combinant des données fossiles et des modèles écophysiologiques, je développe une paramétrisation innovante de la végétation proto-angiosperme dans le modèle de végétation ORCHIDEE qui considère une réduction conjointe de leurs capacités hydrauliques et photosynthétiques.

Avec le modèle couplé atmosphère-végétation dynamique LMDZOR, je montre que la radiation des Angiospermes génère des boucles de rétroactions positives. Dans un contexte de baisse de la teneur en CO₂ atmosphérique au cours du Crétacé, l’augmentation des capacités hydrauliques et photosynthétiques des plantes à fleurs constitue un avantage sélectif qui leur permet de (i) maintenir leur productivité, (ii) développer des forêts tropicales et remplacer les conifères dans les forêts tempérées et boréales et (iii) renforcer les précipitations et limiter ainsi l’effet du stress hydrique sur leur propre essor.

***

During the Cretaceous, the leaf evolution of flowering plants, or angiosperms, towards high vein and stomata densities, suggests an unprecedented increase in stomatal conductance and evapotranspiration fluxes. Yet, these paleo-traits are not included in vegetation models which aim at evaluating the effects of evapotranspiration on climate.

The purpose of this study is to simulate the stomatal conductance evolution of flowering plants through the Cretaceous and assess their effects on climate-vegetation interaction and feedback. By combining fossil data and ecophysiological models, I develop a new parameterization of proto-angiosperm vegetation in the ORCHIDEE vegetation model which considers a reduction of both hydraulic and photosynthetic capacities.

With the LMDZOR coupled atmosphere-dynamic vegetation model, I show that angiosperm radiation triggers positive feedback loops. In a context of decreasing atmospheric CO₂ content during the Cretaceous, the rise in flowering plant hydraulic and photosynthetic capacities constitutes a selective advantage by allowing them to (i) sustain their productivity, (ii) develop tropical forests and replace conifers in temperate and boreal forests and (iii) enhance rainfall thus preventing water stress effect on their own development.

Informations pratiques

Laboratoire des Sciences du Climat et de l’Environnement
Orme des Merisiers
91190 Saint-Aubin
Bâtiment 714, pièce 1129

En ligne :
https://cnrs.zoom.us/j/96220812549?pwd=anZaclpPNmJsVUcxMGtZM01jSEtQdz09
ID de réunion : 962 2081 2549
Code secret : 0tfTm4