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Analyse des facteurs de variabilité de la température dans la stratosphère.

Abstract : The stratospheric processes play an important role in the climate system and addressing the issue of the impact of the stratosphere on climate is of fundamental importance to the scientific community. Observed long term changes in the stratosphere include increase of GHG (greenhouse gases), of lower stratospheric water vapour, decrease of stratospheric ozone and a systematic cooling of the stratosphere during the last two decades (1980-2000). This research was dedicated to the estimation of thermal and dynamical long term changes of the stratosphere and to the attribution of the causes, in particular the role of the ozone in the observed changes. The work was designed to cover the recent past (1980 to 2000), for which good data coverage exists; data analysis has then been coupled with GCM transient simulations to identify the role of the ozone decrease on the observed changes. Three datasets based on monthly-mean satellite and radio sounding product analysis have been considered: the TOVS/3I dataset gives a high resolution picture of the lower stratosphere on a short timescale, the FUB one has a lower resolution but is available for a longer period for the northern hemisphere; finally, the SSU/MSU dataset provides the entire vertical thermal structure of the stratosphere with a coarse vertical resolution (SSU/MSU). Stratospheric temperature trends for the period 1980-2000 have been determined using a multiple linear regression model (AMOUNTS) to separate the effect of the major sources of atmospheric temperature variability from a long-term linear trend. First, a detailed analysis of the impact of the factors of temperature variability is given: the stratospheric Quasi-Biennial Oscillation (QBO), the troposphere pattern of variability described by the El Nino Southern Oscillation (ENSO), the external solar variability and the low-frequency extra tropical mode of variability defined as the Arctic Oscillation. It has been found that the amplitude of the response of temperature to some of these forcing are of the same order of magnitude as the calculated trends. Temperature trends are then described as a function of altitude, latitude and season, showing an unambiguous general cooling of the stratosphere, with a maximum global-mean annual-mean cooling observed in the higher stratosphere northern hemisphere (of the order of 3 K/decade) which decreases in the middle stratosphere and increases again in the lower stratosphere (reaching 1 K/decade in global mean but strongly latitude-dependent). The analysis of temperatures permits also an indirect estimation of a weakening of the stratospheric mean circulation. To investigate the role of ozone change on stratospheric temperature and dynamical fields, two ensemble of GCM transient simulations have been considered (Unified Model, Met office and University of Reading); the first one, representing conditions prior to ozone depletion and the second one, including zonal and monthly mean ozone trends as input of the simulations (ozone run). They have been compared with observed trends. Results confirm the role of the ozone decreases in contributing to the temperature trends in the upper stratosphere (up to 60%) and lower stratosphere (up to 30%). While a general good agreement between observations and ozone run is found in the middle and upper stratosphere, in the lower stratosphere trends are however still underestimated by the ozone run very likely due to the effect of water vapour changes, not taken into account in the simulations. The spring Arctic cooling in the lower stratosphere is captured by the ozone run, but not entirely reproduced. Changes in the wave activity entering the lower stratosphere, estimated through the vertical component of the EP-flux, are also observed in the simulations.
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Submitted on : Wednesday, July 28, 2010 - 4:42:52 PM
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Chiara Cagnazzo. Analyse des facteurs de variabilité de la température dans la stratosphère.. Sciences de l'ingénieur [physics]. Ecole Polytechnique X, 2004. Français. ⟨pastel-00002293⟩



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