Stratospheric circulation changes: investigations using multidecadal observations and simulations of inorganic fluorine

The intense human activity of the past two hundred years has perturbed the subtle balance existing between the spheres of the Earth system. The atmospheric composition has been modified with massive emissions of greenhouse gases and substances depleting the life-essential ozone layer (ODSs). The most known to the general public resulting changes are certainly the global warming of the troposphere and the dramatic formation of the Antarctic ozone hole. However, it is less generally known that the most robustly modelled response to the increase of greenhouse gases, and the resulting global warming, is a speeding-up of the transport circulation occurring in the stratosphere, the atmospheric layer that is situated well above our head, between 10 and 50 km. This transport circulation, referred to as the Brewer-Dobson circulation (BDC), controls the distribution of ozone and other long-lived gaseous constituents of the stratosphere. Therefore, it is crucial to characterize the BDC and its changes to assess precisely the healing of the ozone layer, expected to occur gradually in the twenty-first century as most of ODS emissions have been successfully phased out by the Montreal Protocol on Substances that Deplete the Ozone Layer, including its Amendments and Adjustments. In this work, we investigated BDC changes through their impact on multidecadal time-series of stratospheric fluorine. To this end, we include ground-based Fourier transform infrared time-series from Jungfraujoch (Switzerland, 46°N) and Lauder (New Zealand, 45°S), Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) satellite time-series and five simulations performed by the BASCOE chemical-transport model (CTM). These simulations are driven by the five modern meteorological reanalyses of the atmosphere. Thus, we assess the representation of the investigated BDC changes in state-of-the-art reanalyses which are designed to represent at best the atmospheric state over the past 30 years. We first improved the retrieval strategy of HCFC-22 (CHF2Cl), the most abundant hydrochlorofluorocarbon (HCFC) and the second source of fluorine in the stratosphere, using infrared solar spectra recorded at Jungfraujoch. We showed that HCFC-22 accumulation rates are progressively decreasing in the last decade, highlighting the success of the Montreal Protocol. Furthermore, this first step allowed us to demonstrate the validity of our BASCOE CTM set-up. Indeed, it is the first time that this model is used for such simulations, hence new features were implemented just before and during this thesis project and needed to be validated. The investigations on the impact of BDC changes on the time-series of stratospheric fluorine showed that, for the past twenty years, the BDC has been changing asymmetrically, with the Southern Hemisphere branch getting stronger relative to that of the Northern Hemisphere. Observational datasets and all of the five reanalyses are qualitatively in agreement with this change. However, this important finding is opposed to all model projections, notably used to project ozone recovery, modelling a weakening of the southern branch, in response to increases in greenhouse gases and to decreases in ODSs, calling for further investigations. Superimposed to this 20 year-trend, we have further confirmed a 5-to-7-year variability of the stratosphere, a feature which allows to put into perspective recent studies questioning specific stratospheric variabilities and associated hemispheric asymmetries.

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