Abstract
Abstract. The present work aims at better understanding regional climate–aerosol interactions by studying the relationships between aerosols and synoptic atmospheric circulation over the Euro-Mediterranean region. Two 40-year simulations (1979–2018) have been carried out with version 6.3 of the Centre National de Recherches Météorologiques (National Centre for Meteorological Research) – Aire Limitée Adaptation dynamique Développement InterNational (CNRM-ALADIN) regional climate model, one using interactive aerosols and the other one without any aerosol. The simulation with aerosols has been evaluated in terms of different climate and aerosol parameters. This evaluation shows a good agreement between the model and observations, significant improvements compared to the previous model version and consequently the relevance of using this model for the study of climate–aerosol interactions over this region. A first attempt to explain the climate variability of aerosols is based on the use of the North Atlantic Oscillation (NAO) index. The latter explains a significant part of the interannual variability, notably in winter for the export of dust aerosols over the Atlantic Ocean and the eastern Mediterranean, and in summer for the positive anomalies of anthropogenic aerosols over western Europe. This index is however not sufficient to fully understand the variations of aerosols in this region, notably at daily scale. The use of “weather regimes”, namely persisting meteorological patterns, stable at synoptic scale for a few days, provides a relevant description of atmospheric circulation, which drives the emission, transport and deposition of aerosols. The four weather regimes usually defined in this area in winter and in summer bring significant information to answer this question. The blocking and NAO+ regimes are largely favourable to strong aerosol effects on shortwave surface radiation and near-surface temperature, either because of higher aerosol loads or because of weaker cloud fraction, which reinforces the direct aerosol effect. Inversely, the NAO− and Atlantic Ridge regimes are unfavourable to aerosol radiative effects, because of weaker aerosol concentrations and increased cloud cover. This study thus puts forward the strong dependence of aerosol loads on the synoptic circulation from interannual to daily scales and, as a consequence, the important modulation of the aerosol effects on shortwave surface radiation and near-surface temperature by atmospheric circulation. The role of cloud cover is essential in this modulation as shown by the use of weather regimes.
Highlights
In the climate system, atmospheric aerosols exert a strong influence on the radiative budget and clouds (Kaufman et al, 2002)
Before studying the aerosols and their impact on climate variability, an evaluation of the main simulation ALD-AER is performed in order to ensure the robustness of the subsequent results
Note that the ALD-NO simulation, which is similar to ALD-AER apart from the aerosols (AOD is set to zero in ALD-NO), is not evaluated here, since such a couple of simulations had already been the focus of two previous studies (Nabat et al, 2015a, b)
Summary
Atmospheric aerosols exert a strong influence on the radiative budget and clouds (Kaufman et al, 2002) They absorb and scatter radiation (direct effect), but they interact with cloud microphysics as cloud condensation nuclei (indirect effect). P. Nabat et al.: Aerosols and atmospheric circulation gional climate insofar as they can, for example, modify the radiative budget (Spyrou et al, 2013; Nabat et al, 2012), surface temperature (Zanis et al, 2012; Nabat et al, 2015a), past climate trends (Zubler et al, 2011; Nabat et al, 2014; Boé et al, 2020) and air–sea fluxes (Nabat et al, 2015a). The daily variability of shortwave surface radiation and temperature is better represented in climate models when using a prognostic representation of dust aerosols (Nabat et al, 2015b)
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