Stratospheric fluorine as a tracer of circulation changes: comparison between infrared remote-sensing observations and simulations with five modern reanalyses

Using multidecadal time-series of ground-based and satellite Fourier transform infrared measurements of inorganic fluorine (i.e., total fluorine resident in stratospheric fluorine reservoirs), we investigate stratospheric circulation changes over the past 20 years. The representation of these changes in five modern reanalyses are further analysed through chemical-transport model (CTM) simulations. From the observations but also from all reanalyses, we show that the inorganic fluorine is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere during the twenty-first century. Comparisons with a study evaluating the age-of-air of these reanalyses using the same CTM allow us to link this hemispheric asymmetry to changes in the Brewer-Dobson circulation (BDC), with the age-of-air of the Southern Hemisphere getting younger relative to that of the Northern Hemisphere. Large differences in simulated total columns and absolute trend values are, nevertheless, depicted between our simulations driven by the five reanalyses. Superimposed on this multidecadal change, we, furthermore, confirm a 5-to-7-year variability of the BDC that was first described in a recent study analysing long-term time series of hydrogen chloride (HCl) and nitric acid (HNO3). It is important to stress that our results, based on observations and meteorological reanalyses, are in contrast with the projections of chemistry-climate models in response to the coupled increase of greenhouse gases and decrease of ozone depleting substances, calling for further investigations and the continuation of long-term observations.