Impacts des forçages radiatifs atmosphériques sur le climat tropical

The radiative budget within the atmosphere is critical for the Earth s climate, since the atmospheric radiative cooling is an important driver of atmospheric circulations and the global mean hydrological cycle. Recently, it has been shown that changes in the radiative cooling of the atmosphere have profound impacts on atmospheric circulations and regional precipitation and that these impacts occur within just a few days. The radiative effects of clouds (or CRE) strongly modulate the radiative budget at the top of the atmosphere but also, within the atmosphere. By modulating the atmospheric radiative heating rates, clouds have the potential to affect the atmospheric circulation and the global hydrological cycle. While early studies focused on the impacts of tropical high-clouds atmospheric cloud radiative effects (ACRE), the ACRE of low-clouds, which are the prevailing type of clouds over the ocean and in the tropics, have received much less attention. The goal of this thesis is to better understand the role that low-cloud ACRE play in the tropical climate. For this purpose, we perform simulations in which planetary boundary layer (PBL) clouds are made transparent to the radiation scheme in a climate model. When PBL ACRE is removed, both the hydrological cycle and atmospheric circulation are weakened over tropical oceans. The analysis of the moist static energy (MSE) and water budgets reveals that PBL ACRE affects tropical precipitation and circulation through the strong coupling with surface heat fluxes. To investigate the time-scales involved, we run the model in numerical weather prediction mode. Weather forecasts simulations show that the impacts of low-cloud radiative effects on the tropical climate occur in days, which allows us to use short-term simulations to explore the sensitivity of our results to model formulation. Sensitivity tests and long-term simulations of different configurations of the model, show that our results are robust in this model. Experiments performed with other state-of-the-art climate models confirm the robustness of our results in the present-day climate. To explore the role that PBL ACRE play in the response of PBL clouds to climate change, we use simulations from different climate models in which PBL clouds are made transparent to radiation and sea surface temperature (SST) is increased by 4K. A multi-model analysis of the role of PBL ACRE in climate change show that PBL clouds are as efficient as high-clouds in counteracting the increase of radiative cooling produced by a warmer atmosphere. Surprisingly, PBL clouds are shown to decrease the spread in hydrological sensitivity estimates among climate models.Le bilan radiatif atmosphérique est essentiel pour le climat de la Terre, parce que le refroidissement radiatif de l atmosphére est un facteur important de la circulation atmosphérique et du cycle hydrologique moyenne global. Récemment, il a été montré que des changements dans le refroidissement radiatif de l atmosphère ont des répercussions profondes sur la circulation et les précipitations et ces impacts se produisent dans quelques jours. Les effets radiatifs des nuages (ou CRE en anglais) modulent fortement le bilan radiatif au sommet de l atmosphére, mais aussi, dans l atmosph ere. En modulant les taux de chauffage radiatif de l atmosphère, les nuages ont le potentiel d affecter la circulation atmosphérique et du cycle hydrologique global. Alors que les premières études ont porté sur les effets radiatifs atmosphériques (ACRE) des nuages hautes, l ACRE des nuages bas, qui sont le type dominant de nuages sur l océan, ont reçu moins d attention. L objectif de cette thèse est de mieux comprendre le rôle que jouent l ACRE des nuages bas dans le climat tropical. A cet effet, nous effectuons des simulations dans lesquelles les nuages de la couche limite planétaire (PBL) sont rendues transparentes au rayonnement dans un modèle de climat. Lorsque PBL ACRE est enlevé, à la fois le cycle hydrologique et la circulation atmosphérique sont affaiblis sur des océans tropicaux. L analyse du bilans de l énergie statique humide (MSE) et de l eau révèle que le PBL ACRE affecte des précipitations tropicales et de la circulation à travers du couplage fort avec les flux de chaleur de la surface. Pour étudier les échelles de temps impliquées, nous lançons le modèle de climat en mode de prévision météorologique. Les simulations de court temps montrent que les impacts de PBL ACRE sur le climat tropical se produisent en quelques jours, ce qui nous permet d utiliser des simulations à court terme pour explorer la sensibilité de nos résultats à la physique du modèle de climat. Des tests de sensibilité et des simulations à long terme des différentes configurations du modèle, montrent que nos résultats sont robustes dans ce modèle. Les expériences réalisées avec d autres modèles confirment la robustesse de nos résultats dans le climat actuel. Pour explorer le rôle que l ACRE PBL joue dans la réponse des nuages bas au changement climatique, nous utilisons des simulations de modèles climatiques différents dans lesquels les nuages bas sont transparent au rayonnement et la température de surface de la mer (SST) est augmentée en 4K. Une analyse multi-modèle du rôle de l PBL ACRE dans le changement climatique montre que les nuages bas sont aussi efficaces que des nuages hautes en s opposer à l augmentation de refroidissement radiatif produit par une atmosphère plus chaude. Étonnamment, les nuages bas contribuent à réduire la dispersion des estimations de sensibilité hydrologiques entre les modèles climatiques.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Influence of low-cloud radiative effects on tropical circulation and precipitation

International audienceLow-level clouds, which constitute the most prevalent cloud type over tropical oceans, exert a radiative cooling within the planetary boundary layer. By using an atmospheric general circulation model, we investigate the role that this cloud radiative cooling plays in the present-day climate. Low-cloud radiative effects are found to increase the tropics-wide precipitation, to strengthen the winds at the surface of the tropical oceans, and to amplify the atmospheric overturning circulation. An analysis of the water and energy budgets of the atmosphere reveals that most of these effects arises from the strong coupling of cloud-radiative cooling with turbulent fluxes at the ocean surface. The impact of cloud-radiative effects on atmospheric dynamics and precipitation is shown to occur on very short time scales (a few days). Therefore, short-term atmospheric forecasts constitute a valuable framework for evaluating the interactions between cloud processes and atmospheric dynamics, and for assessing their dependence on model physics

Robust direct effect of carbon dioxide on tropical circulation and regional precipitation

International audiencePredicting the response of tropical rainfall to climate change remains a challenge(1). Rising concentrations of carbon dioxide are expected to affect the hydrological cycle through increases in global mean temperature and the water vapour content of the atmosphere(2-4). However, regional precipitation changes also closely depend on the atmospheric circulation, which is expected to weaken in a warmer world(4-6). Here, we assess the effect of a rise in atmospheric carbon dioxide concentrations on tropical circulation and precipitation by analysing results from a suite of simulations from multiple state-of-the-art climate models, and an operational numerical weather prediction model. In a scenario in which humans continue to use fossil fuels unabated, about half the tropical circulation change projected by the end of the twenty-first century, and consequently a large fraction of the regional precipitation change, is independent of global surfacewarming. Instead, these robust circulation and precipitation changes are a consequence of the weaker net radiative cooling of the atmosphere associated with higher atmospheric carbon dioxide levels, which affects the strength of atmospheric vertical motions. This implies that geo-engineering schemes aimed at reducing global warming without removing carbon dioxide from the atmosphere would fail to fully mitigate precipitation changes in the tropics. Strategies that may help constrain rainfall projections are suggested