PhD Defense: Thomas OUDAR – Atmospheric Circulation Response to Anthropogenic Forcings : from Annular Modes to Synoptic Activity
Climate variability in the mid and high latitudes is very complex due to numerous physical mechanism implied. This climate variability can be decomposed into 2 components : the internal variability associated with internal processes and the forced variability linked to the external forcings which can be natural (volcanism, natural aerosols) or anthropogenic (greenhouse gases, anthropogenic aerosols). These external forcings play a crucial role on the climate and its variability. The challenge in the climate research is to understand their effects on the climate and their roles relatively with the internal variability. The objective of this thesis is a better understanding of the respective roles of internal variability and forced variability on the past and future atmospheric circulation in the both hemispheres characterized by the annular mode and the synoptic activity associated using atmospheric reanalysis and experiments performed with the coupled climate model CNRM-CM5.
First, we focus on the annular mode changes in both hemisphere, namely the NAM (Northern Annular Mode) and the SAM (Southern Annular Mode). We show that the observed positive trend of the SAM in the 1960s in austral summer is well reproduced by the climate model. However, contrarily to other studies which suggest that this positive trend can be explained by only stratospheric ozone depletion, it is reproduced in the CNRM-CM5 model when the ozone depletion and greenhouse gases (GHG) increase are both prescribed.
Then, we investigate the changes in the Northern Hemisphere atmospheric circulation. These are more complex than in the Southern Hemisphere. Indeed, the increase of GHG in the atmosphere causes a general global warming maximum in the tropical high troposphere and over the pole at the surface which is mainly explained by Arctic sea ice loss. So the understanding of the changes is very complex due to several physical processes and retroactions. Thus, we have conducted a protocol with the coupled climate model CNRM-CM5 in order to assess the respective role of Arctic sea ice loss and GHG increase. Arctic sea ice loss is responsible for an increase in the heat flux between the atmosphere and the ocean which modify the atmospheric circulation. We show that Arctic sea ice loss can cause the baroclinic response in the Northern Hemisphere.
Finally, the last part of the thesis is the study of past and future changes in the North Atlantic storm-tracks. There are still several uncertainties because of the complex processes involving the water vapour, the meridional temperature gradient and the static stability. We find the tripolar response, already found by other studies, consisting of a significant decrease in the south of the basin and over the Mediterranean sea, a small increase over the British Isles, and a decrease east of the Greenland. We show that the signal in the historical period is not robust, due to large chaotic variability associated with storms.
Emilia SANCHEZ-GOMEZ, Directeur de thèse
Fabrice Chauvin, Codirecteur de thèse
Gwendal Rivière, Rapporteur
Laurent Li, Rapporteur,
Masa Kageyama, Rapporteur,
Hervé Douville, Examinateur