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Accueil du site → Doctorat → Allemagne → 2012 → Mineral dust mobilisation, transport, and deposition in different climate epochs

Johannes Gutenberg Universität in Mainz (2012)

Mineral dust mobilisation, transport, and deposition in different climate epochs

Gläser, Gregor

Titre : Mineral dust mobilisation, transport, and deposition in different climate epochs

Auteur : Gläser, Gregor

Université de soutenance : Johannes Gutenberg Universität in Mainz

Grade : Doktor der Naturwissenschaften 2012

Résumé
Mineral dust is an important component of the Earth’s climate system and provides essential nutrients to oceans and rain forests. During atmospheric transport, dust particles directly and indirectly influence weather and climate. The strength of dust sources and characteristics of the transport, in turn, might be subject to climatic changes. Earth system models help for a better understanding of these complex mechanisms. This thesis applies the global climate model ECHAM5/MESSy Atmospheric Chemistry (EMAC) for simulations of the mineral dust cycle under different climatic conditions. The prerequisite for suitable model results is the determination of the model setup reproducing the most realistic dust cycle in the recent climate. Simulations with this setup are used to gain new insights into properties of the transatlantic dust transport from Africa to the Americas and adaptations of the model’s climate forcing factors allow for investigations of the impact of climatic changes on the dust cycle. In the first part, the most appropriate model setup is determined through a number of sensitivity experiments. It uses the dust emission parametrisation from Tegen et al. (2002) and a spectral resolution of T85, corresponding to a horizontal grid spacing of about 155 km. Coarser resolutions are not able to accurately reproduce emissions from important source regions such as the Bod´el´e Depression in Chad or the Taklamakan Desert in Central Asia. Furthermore, the representation of ageing and wet deposition of dust particles in the model requires a basic sulphur chemical mechanism. This setup is recommended for future simulations with EMAC focusing on mineral dust. One major branch of the global dust cycle is the long-range transport from the world’s largest dust source, the Sahara, across the Atlantic Ocean. Seasonal variations of the main transport pathways to the Amazon Basin in boreal winter and to the Caribbean during summer are well known and understood, and corroborated in this thesis. Both Eulerian and Lagrangian methods give estimates on the typical transport times from the source regions to the deposition on the order of nine to viii ten days. Previously, a huge proportion of the dust transported across the Atlantic Ocean has been attributed to emissions from the Bod´el´e Depression. However, the contribution of this hot spot to the total transport is very low in the present results, although the overall emissions from this region are comparable. Both model results and data sets analysed earlier, such as satellite products, involve uncertainties and this controversy about dust transport from the Bod´el´e Depression calls for future investigations and clarification. Aforementioned characteristics of the transatlantic dust transport just slightly change in simulations representing climatic conditions of the Little Ice Age in the middle of the last millennium with mean near-surface cooling of 0.5 to 1 K. However, intensification of the West African summer monsoon during the Little Ice Age is associated with higher dust emissions from North African source regions and wetter conditions in the Sahel. Furthermore, the Indian Monsoon and dust emissions from the Arabian Peninsula, which are affected by this circulation, are intensified during the Little Ice Age, whereas the annual global dust budget is similar in both climate epochs. Simulated dust emission fluxes are particularly influenced by the surface parameters. Modifications of the model do not affect those in this thesis, to be able to ascribe all differences in the results to changed forcing factors, such as greenhouse gas concentrations. Due to meagre comparison data sets, the verifi- cation of results presented here is problematic. Deeper knowledge about the dust cycle during the Little Ice Age can be obtained by future simulations, based on this work, and additionally using improved reconstructions of surface parameters. Better evaluation of such simulations would be possible by refining the temporal resolution of reconstructed dust deposition fluxes from existing ice and marine sediment cores.

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Page publiée le 21 novembre 2013, mise à jour le 12 janvier 2019