Intéractions aérosols-rayonnement-nuages et variabilité climatique en méditerranée - Approche par la modelisation régionale couplée

Le bassin méditerranéen est sujet à de nombreuses sources d'aérosols présentant une variabilité spatio-temporelle élevée. Ces aérosols interagissent de manière directe avec les rayonnements solaire et thermique, et de manière indirecte avec les nuages et la dynamique atmosphérique. Ils peuvent donc avoir un impact important sur le climat de cette région. Ce travail de thèse, à la frontière entre les projets HyMeX et ChArMEx, considère une approche par la modélisation régionale couplée pour répondre aux questions des interactions aérosols-rayonnement-nuages par rapport à la variabilité climatique de la région méditerranéenne. Afin de mieux caractériser les aérosols méditerranéens, une nouvelle climatologie mensuelle et interannuelle d'épaisseur optique a été développée à partir d'une combinaison de produits satellites et de modèles. Ce jeu de données, disponible pour tous les modèles régionaux de climat en Méditerranée sur la période 1979-2012, a été mis au point dans le but d'obtenir la meilleure estimation possible du contenu atmosphérique en aérosols pour les cinq types considérés (sulfates, carbone suie et organique, poussières désertiques et sels marins). Des ensembles de simulations réalisées sur la période 2003-2009 avec et sans aérosols montrent un impact majeur sur le climat régional. Cet impact se caractérise par un forçage radiatif négatif en surface (dû à la diffusion et l'absorption du rayonnement solaire incident) de -15 W.m−2 en moyenne annuelle sur la mer Méditerranée, par un refroidissement induit en surface à la fois sur mer et sur terre de l'ordre de 0.5◦C en moyenne annuelle, par une diminution moyenne des précipitations ainsi que par des changements de nébulosité. Le cycle saisonnier et les structures spatiales du climat méditerranéen sont ainsi significativement modifiés, ainsi que certaines situations spécifiques comme la canicule de juillet 2006 qui a été renforcée par la présence d'aérosols désertiques. Le rôle essentiel de la température de surface de la mer Méditerranée dans la réponse du climat aux aérosols est mis en évidence, et permet de comprendre les modifications induites des flux air-mer (notamment la diminution de la perte en chaleur latente) et ses conséquences sur le climat régional. La convection océanique en mer Méditerranée est également renforcée par la présence d'aérosols. En outre, on démontre que la diminution des aérosols anthropiques observée depuis plus de trente ans a contribué significativement aux tendances climatiques de rayonnement (représentant 81 ± 15 % de l'éclaircissement) et de température (représentant 23 ± 5 % du réchauffement) observées en Europe et en Méditerranée. D'autre part, un schéma interactif d'aérosols a été mis en place dans le modèle atmosphérique ALADIN-Climat afin de pouvoir comprendre les processus liés aux aérosols à l'échelle quotidienne. On montre ici la capacité de ce schéma de simuler de manière réaliste les aérosols présents en Méditerranée, notamment dans le cas des intrusions de poussières désertiques observées pendant la campagne de mesures ChArMEx/TRAQA. Un exercice d'intercomparaison avec d'autres modèles intégrant les aérosols désertiques confirme la performance du nouveau schéma. De plus, utiliser un schéma prognostique d'aérosols au lieu d'une climatologie mensuelle permet de mieux reproduire les variations quotidiennes et en particulier les extrêmes de rayonnement et de température en surface. Cela induit aussi une modification du climat moyen, dans la mesure où les variations des aérosols et de leurs effets dépendent des régimes de temps et de la nébulosité. Cette thèse conclut ainsi à la nécessité pour les systèmes climatiques de modélisation régionale en Méditerranée de bien prendre en compte les effets radiatifs des aérosols et leur variabilité spatiotemporelle, y compris à haute fréquence. Les impacts de ces effets radiatifs sur de nombreux paramètres (rayonnement, température, humidité, flux air-mer, circulation océanique, etc.) sont en effet démontrés à différentes échelles d'espace et de temps (variabilité quotidienne, cycle saisonnier, tendances climatiques, extrêmes, structures spatiales). Ce travail a aussi montré que le couplage entre l'atmosphère et la mer Méditerranée est indispensable pour des études aérosols-climat dans cette région. ABSTRACT : The Mediterranean basin is affected by numerous and various aerosols which have a high spatiotemporal variability. These aerosols directly interact with solar and thermal radiation, and indirectly with clouds and atmospheric dynamics. Therefore they can have an important impact on the regional climate. This work, located at the boundary between the ChArMEx and HyMeX programs, considers a coupled regional modeling approach in order to address the questions of the aerosol-radiation-cloud interactions with regards to the climate variability over the Mediterranean. In order to improve the characterization of Mediterranean aerosols, a new interannual monthly climatology of aerosol optical depth has been developed from a blended product based on both satellitederived and model-simulated datasets. This dataset, available for every regional climate model over the Mediterranean for the 1979-2012 period, has been built to obtain the best possible estimate of the atmospheric aerosol content for the five species at stake (sulfate, black carbon, organic matter, desert dust and sea salt particles). Simulation ensembles, which have been carried out over the 2003-2009 period with and without aerosols, show a major impact on the regional climate. This impact is characterized by a negative surface radiative forcing (due to the absorption and the scattering of the solar incident radiation) of -15 W.m−2 on annual average over the Mediterranean Sea, an induced surface cooling both over land and sea of about -0.5◦C on annual average, a decrease in precipitation as well as cloud cover changes. The seasonal cycle and the spatial patterns of the Mediterranean climate are significantly modified, as well as some specific situations such as the heat wave in July 2006 strengthened by the presence of desert dust particles. The essential role of the Mediterranean sea surface temperature is highlighted, and enables to understand the induced changes on air-sea fluxes (notably the decrease in the latent heat loss) and the consequences on regional climate. Oceanic convection is also strengthened by aerosols. In addition, the decrease in anthropogenic aerosols observed for more than thirty years is shown to significantly contribute to the observed Euro-Mediterranean climatic trends in terms of surface radiation (representing 81 ± 15 % of the brightening) and temperature (representing 23 ± 5 % of the warming). Besides, an interactive aerosol scheme has been developed in the atmospheric model ALADINClimate in order to better understand aerosol processes at the daily scale. This scheme shows its ability to represent correctly the aerosol patterns over the Mediterranean, especially with regards to dust outbreaks that were measured during the ChArMEx/TRAQA field campaign. An intercomparison exercise with several dust models confirms the performance of the new scheme. Moreover, the use of a prognostic aerosol scheme instead of a monthly climatology enables to better reproduce the daily variations of surface radiation and temperature and related extremes. This also leads to changes in the mean climate, insofar as aerosol variations and their effects depend on weather regimes and cloud cover. Finally this study concludes with the need for regional climate system models over the Mediterranean to take into account the radiative aerosol effects and their spatio-temporal variability, including at high frequency. The impacts of these radiative effects on numerous parameters (radiation, temperature, humidity, ocean-atmosphere fluxes, oceanic circulation, etc.) are indeed shown and understood at different space and time scales (daily variability, seasonal cycle, climate trends, spatial structures). This work has also shown the importance of the coupling between the atmosphere and the Mediterranean Sea for aerosol-climate studies in this region

Contribution of anthropogenic sulfate aerosols to the changing Euro-Mediterranean climate since 1980

Since the 1980s anthropogenic aerosols have been considerably reduced in Europe and the Mediterranean area. This decrease is often considered as the likely cause of the brightening effect observed over the same period. This phenomenon is however hardly reproduced by global and regional climate models. Here we use an original approach based on reanalysis-driven coupled regional climate system modeling to show that aerosol changes explain 81±16% of the brightening and 23±5% of the surface warming simulated for the period 1980-2012 over Europe. The direct aerosol effect is found to dominate in the magnitude of the simulated brightening. The comparison between regional simulations and homogenized ground-based observations reveals that observed surface solar radiation and land and sea surface temperature spatiotemporal variations over the Euro-Mediterranean region are only reproduced when simulations include the realistic aerosol variations. Key Points A regional climate system model over the Euro-Mediterranean includes aerosols Aerosol changes are needed to reproduce observed climate trends since 1980 Aerosols play an essential role in the brightening and warming since 1980This work is a contribution to the HyMeX (HYdrological cycle in the Mediterranean EXperiment) and ChArMEx (Chemistry-Aerosol Mediterranean Experiment) program through INSU-MISTRALS support and the Med-CORDEX initiative (COordinated Regional climate Downscaling EXperiment Mediterranean region, www.medcordex.eu). This research has been supported by the French National Research Agency (ANR) project REMEMBER (contract ANR-12-SENV-001). Gridded temperature data sets, GISS and CRUTEM, have been provided, respectively, by the NASA Goddard Institute for Space Studies and the Met Office Hadley Center. HISTALP temperature data sets have been downloaded from http://www.zamg.ac.at/histalp. We also thank Brigitte Dubuisson and Anne-Laure Gibelin for the availability of homogenized temperature series in France, and we acknowledge the data providers in the ECA&D project. A. S. L. was supported by the "Secretaria per a Universitats i Recerca del Departament d'Economia i Coneixement, de la Generalitat de Catalunya i del programa Cofund de les Accions Marie Curie del 7e Programa marc d'R+D de la Unio Europea" (2011 BP-B 00078), the postdoctoral fellowship JCI-2012-12508, and the project NUCLIERSOL (CGL2010-18546

Characterizing, modelling and understanding the climate variability of the deep water formation in the North-Western Mediterranean Sea

Observing, modelling and understanding the climate-scale variability of the deep water formation (DWF) in the North-Western Mediterranean Sea remains today very challenging. In this study, we first characterize the interannual variability of this phenomenon by a thorough reanalysis of observations in order to establish reference time series. These quantitative indicators include 31 observed years for the yearly maximum mixed layer depth over the period 1980–2013 and a detailed multi-indicator description of the period 2007–2013. Then a 1980–2013 hindcast simulation is performed with a fully-coupled regional climate system model including the high-resolution representation of the regional atmosphere, ocean, land-surface and rivers. The simulation reproduces quantitatively well the mean behaviour and the large interannual variability of the DWF phenomenon. The model shows convection deeper than 1000 m in 2/3 of the modelled winters, a mean DWF rate equal to 0.35 Sv with maximum values of 1.7 (resp. 1.6) Sv in 2013 (resp. 2005). Using the model results, the winter-integrated buoyancy loss over the Gulf of Lions is identified as the primary driving factor of the DWF interannual variability and explains, alone, around 50 % of its variance. It is itself explained by the occurrence of few stormy days during winter. At daily scale, the Atlantic ridge weather regime is identified as favourable to strong buoyancy losses and therefore DWF, whereas the positive phase of the North Atlantic oscillation is unfavourable. The driving role of the vertical stratification in autumn, a measure of the water column inhibition to mixing, has also been analyzed. Combining both driving factors allows to explain more than 70 % of the interannual variance of the phenomenon and in particular the occurrence of the five strongest convective years of the model (1981, 1999, 2005, 2009, 2013). The model simulates qualitatively well the trends in the deep waters (warming, saltening, increase in the dense water volume, increase in the bottom water density) despite an underestimation of the salinity and density trends. These deep trends come from a heat and salt accumulation during the 1980s and the 1990s in the surface and intermediate layers of the Gulf of Lions before being transferred stepwise towards the deep layers when very convective years occur in 1999 and later. The salinity increase in the near Atlantic Ocean surface layers seems to be the external forcing that finally leads to these deep trends. In the future, our results may allow to better understand the behaviour of the DWF phenomenon in Mediterranean Sea simulations in hindcast, forecast, reanalysis or future climate change scenario modes. The robustness of the obtained results must be however confirmed in multi-model studies

Evaluation of ocean dimethylsulfide concentration and emission in CMIP6 models

Characteristics and trends of surface ocean dimethylsulfide (DMS) concentrations and fluxes into the atmosphere of four Earth system models (ESMs: CNRM-ESM2-1, MIROC-ES2L, NorESM2-LM, and UKESM1-0-LL) are analysed over the recent past (1980–2009) and into the future, using Coupled Model Intercomparison Project 6 (CMIP6) simulations. The DMS concentrations in historical simulations systematically underestimate the most widely used observed climatology but compare more favourably against two recent observation-based datasets. The models better reproduce observations in mid to high latitudes, as well as in polar and westerlies marine biomes. The resulting multi-model estimate of contemporary global ocean DMS emissions is 16–24 Tg S yr−1, which is narrower than the observational-derived range of 16 to 28 Tg S yr−1. The four models disagree on the sign of the trend of the global DMS flux from 1980 onwards, with two models showing an increase and two models a decrease. At the global scale, these trends are dominated by changes in surface DMS concentrations in all models, irrespective of the air–sea flux parameterisation used. In turn, three models consistently show that changes in DMS concentrations are correlated with changes in marine productivity; however, marine productivity is poorly constrained in the current generation of ESMs, thus limiting the predictive ability of this relationship. In contrast, a consensus is found among all models over polar latitudes where an increasing trend is predominantly driven by the retreating sea-ice extent. However, the magnitude of this trend between models differs by a factor of 3, from 2.9 to 9.2 Gg S decade−1 over the period 1980–2014, which is at the low end of a recent satellite-derived analysis. Similar increasing trends are found in climate projections over the 21st century

Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850 to 2100

Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer-Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions. © Author(s) 2021

Climate and air quality impacts due to mitigation of non-methane near-term climate forcers

It is important to understand how future environmental policies will impact both climate change and air pollution. Although targeting near-term climate forcers (NTCFs), defined here as aerosols, tropospheric ozone, and precursor gases, should improve air quality, NTCF reductions will also impact climate. Prior assessments of the impact of NTCF mitigation on air quality and climate have been limited. This is related to the idealized nature of some prior studies, simplified treatment of aerosols and chemically reactive gases, as well as a lack of a sufficiently large number of models to quantify model diversity and robust responses. Here, we quantify the 2015-2055 climate and air quality effects of non-methane NTCFs using nine state-of-the-art chemistry-climate model simulations conducted for the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). Simulations are driven by two future scenarios featuring similar increases in greenhouse gases (GHGs) but with weak (SSP3-7.0) versus strong (SSP3-7.0-lowNTCF) levels of air quality control measures. As SSP3-7.0 lacks climate policy and has the highest levels of NTCFs, our results (e.g., surface warming) represent an upper bound. Unsurprisingly, we find significant improvements in air quality under NTCF mitigation (strong versus weak air quality controls). Surface fine particulate matter (PM2:5) and ozone (O3) decrease by 2:20:32 ugm3 and 4:60:88 ppb, respectively (changes quoted here are for the entire 2015-2055 time period; uncertainty represents the 95% confidence interval), over global land surfaces, with larger reductions in some regions including south and southeast Asia. Non-methane NTCF mitigation, however, leads to additional climate change due to the removal of aerosol which causes a net warming effect, including global mean surface temperature and precipitation increases of 0:250:12K and 0:030:012mmd1, respectively. Similarly, increases in extreme weather indices, including the hottest and wettest days, also occur. Regionally, the largest warming and wetting occurs over Asia, including central and north Asia (0:660:20K and 0:030:02mmd1), south Asia (0:470:16K and 0:170:09mmd1), and east Asia (0:460:20K and 0:150:06mmd1). Relatively large warming and wetting of the Arctic also occur at 0:590:36K and 0:040:02mmd1, respectively. Similar surface warming occurs in model simulations with aerosol-only mitigation, implying weak cooling due to ozone reductions. Our findings suggest that future policies that aggressively target non-methane NTCF reductions will improve air quality but will lead to additional surface warming, particularly in Asia and the Arctic. Policies that address other NTCFs including methane, as well as carbon dioxide emissions, must also be adopted to meet climate mitigation goals. © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License