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Theses

Influence dynamique de l'Himalaya sur le climat en Extrême-Orient

Abstract : Mountains have an impact on the large scale circulation of the atmosphere, this impact being predominantly a consequence of the forces they exert on the atmosphere : for this reason, the part of the orography which is not resolved explicitly by the general circulation models is taken into account through parametrisations of the forces it applies to the atmosphere. This PhD thesis is focused on understanding the impact of the forces applied by the midlatitude mountains, particularly the Tibetan Plateau, on the atmosphere. For that purpose, we will consider the mountain torque applied by the mountains on the atmosphere, a global scale counterpart of the mountain forces.The major midlatitude mountain ranges can trigger massive synoptic scale cold surges on their eastern flanks. This thesis uncovers the importance of the equatorial mountain torque in triggering the cold surges over east-Asia (impact of the Tibetan Plateau), north-America (impact of the Rockie Mountains) and south-America (impact of the Andes cordillera). Using a simple dynamical model, an interpretation of the mechanism underlying the forcing of the cold surges by the equatorial mountain torque is proposed, showing that the existence of a lift force applied by the mountain on the atmosphere is sufficient for the triggering of cold surges.The dynamical impact of the Tibetan plateau on the east-asian winter monsoon, more specifically on the wintertime convection events on the South China Sea, is important. A typical sequence of events showing this dynamical impact has been identified statistically : a dynamical forcing of the atmospheric circulation by the Tibetan Plateau, materialized by a strong signal on the equatorial mountain torque on the atmosphere is followed quickly by the occurence of a cold surge and, a few days later, by an outburst of deep convection over the South China Sea. This dynamical effect of the Tibetan Plateau on the winter monsoon extends as far south as Indonesia, and westward to the Bay of Bengal. The use of the general circulation model of the Laboratoire de Météorologie Dynamique general circulation model, LMDz, makes it possible to extend the observational results described above. This model closes properly the budget of atmospheric angular momentum, and will be used to show that that sub-grid sale orography plays an important role in the terminal phase of the evolution of cold surges. New results are presented concerning the equatorial atmospheric angular momentum budget are presented, adressing specifically the impact of the equatorial mountain torque and that of the contribution of the Tibetan Plateau. It is shown that the equatorial mountain torque applied by the Tibetan Plateau has only a weak role in the temporal evolution of the equatorial angular momentum, but a significant role concerning its spatial repartition
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Sylvain Mailler. Influence dynamique de l'Himalaya sur le climat en Extrême-Orient. Sciences de la Terre. Université Paris-Est, 2010. Français. ⟨NNT : 2010PEST1075⟩. ⟨tel-00598530⟩

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