Atmospheric circulation response to anthropogenic forcings : from annular modes to storm tracks

• Main Author: Oudar, Thomas
• Other Authors: CERFACS Toulouse, Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Université Paul Sabatier - Toulouse III, Laurent Terray, Emilia Sanchez-Gomez, Fabrice Chauvin
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2016
• Subjects: Annular modes; Atmospheric circulation; External forcings; Sea ice; Storm tracks; Modes annulaires; Circulation atmosphérique; Forçages externes; Glace de mer; Dépressions synoptiques; [SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology; Arctic; Austral; Global warming
• Online Access: https://theses.hal.science/tel-01548285; https://theses.hal.science/tel-01548285/document; https://theses.hal.science/tel-01548285/file/2016TOU30212d.pdf

Climate variability in mid and high latitudes is very complex due to numerous physical mecanims 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 natutal (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 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 hemispheres, named 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 ...