Stratospheric Fluorine as a Tracer of Circulation Changes: Comparison Between Infrared Remote‐Sensing Observations and Simulations With Five Modern Reanalyses

Abstract

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 is further analyzed 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 21st 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 ageof-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 5-7-year variability of the BDC that was first described in a recent study analyzing long-term time series of hydrogen chloride (HCl) and nitric acid (HNO 3 ). It is important to stress that our results, based on observations and meteorological reanalyses, are in contrast with the projections of chemistryclimate 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. Plain Language SummaryThe overturning circulation of the stratosphere is projected to change in response to increases in greenhouse gases and decreases in ozone-depleting substances, with the Southern Hemisphere branch expected to get weaker relative to the Northern Hemisphere. Here, we use 30-year time series of observations and simulations of a long-lived tracer to investigate stratospheric circulation changes. The observations analyzed are from ground-based and satellite instruments. We compare them with simulations that use a chemical-transport model driven by winds from meteorological reanalyses of 1990-2019. A reanalysis assimilates meteorological measurements into a forecast model to simulate the best possible representation of the atmospheric state. All five simulations, which use different reanalyses, agree with the observational analysis showing that the analyzed tracer is accumulating less rapidly in the Southern Hemisphere than in the Northern Hemisphere through 2019. This hemispheric asymmetry is attributed to changes in the stratospheric transport circulation, with the Southern Hemisphere branch getting stronger relative to the Northern Hemisphere. Our results support the conclusions of a recent observational study but do not support circulation changes projected by chemistry-climate models. This study highlights the crucial importance of long-term observation time series.

Evaluation of the N2O Rate of Change to Understand the Stratospheric Brewer‐Dobson Circulation in a Chemistry‐Climate Model

“…Those asymmetries are consistent with the results of Strahan et al (2020), who found significantly negative mean AoA trends in the SH compared to the NH using HCl and HNO 3 measured at several ground-based FTIR stations. In addition, the hemispheric differences of the N 2 O trends are also consistent with the results of Prignon et al (2021), who found larger and more significant F y trends from FTIR above Jungfraujoch than above Lauder.…”

“…However, this application of the irregular sampling of ACE-FTS to the model output consistently enhances the N 2 O trends, both positive and negative. Using continuous time sampling on native model grids, ERAI, and ERA5 to a lesser extent, still simulate the meridional dipole in the N 2 O trends, consistently with a large number of modeling studies using both idealized and real tracers (e.g., Chabrillat et al, 2018;Ploeger & Garny, 2022;Prignon et al, 2021), but MERRA2, JRA55 and both WACCM versions fail to reproduce the meridional dipole. The inherently limited spatial and temporal sampling of ACE-FTS, and its effect on N 2 O trends, highlight the necessity to carry out the N 2 O trend analysis discussed here using a satellite instrument with a more regular coverage.…”

“…N 2 O is emitted in the troposphere while F y is produced in the stratosphere, and, as a consequence of the poleward transport of the BDC, N 2 O is removed and F y is increased in the stratospheric mid-latitudes. In the light of this relationship between N 2 O and F y , the underestimated N 2 O columns above Lauder and Jungfraujoch in MERRA2 are consistent with larger stratospheric F y columns in MERRA2 compared to the other reanalyses above those stations (Prignon et al, 2021).…”

“…In this study, we use the BASCOE CTM driven by four dynamical reanalyses: the European Center for Medium-Range Weather Forecast Interim reanalysis (ERAI, Dee et al, 2011), and its newer version ERA5 (Hersbach et al, 2020), the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA2, Gelaro et al, 2017), and the Japanese 55-year Reanalysis (JRA55, Kobayashi et al, 2015). In the following, we provide a brief overview of the BASCOE CTM and the ERAI, MERRA2 and JRA55 reanalyses, as more detailed information can be found in such companion studies: Chabrillat et al (2018); Prignon et al (2019Prignon et al ( , 2021 and M2020. Since ERA5 is not detailed in these publications, we provide a more detailed description.…”

“…The meridional dipole in the N 2 O trends derived from ACE-FTS is generally reproduced by the CTM simulations, with ERAI and ERA5 delivering trends that are most similar to the satellite measurements. Prignon et al (2021) used the same simulations as the present study to investigate global stratospheric trends of total inorganic fluorine F y . The dipoles obtained here in the N 2 O trends from the ECMWF reanalyses are consistent with the opposite trends of F y for almost the same period (Prignon et al, 2021).…”