Evaluation of the N2_2O Rate of Change to Understand the Stratospheric Brewer‐Dobson Circulation in a Chemistry‐Climate Model

The Brewer-Dobson Circulation (BDC) determines the distribution of long-lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We evaluate decadal (2005–2018) trends of nitrous oxide (N$_2$O) in two versions of the Whole Atmosphere Chemistry-Climate Model (WACCM) by comparing them with measurements from four Fourier transform infrared (FTIR) ground-based instruments, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and with a chemistry-transport model (CTM) driven by four different reanalyses. The limited sensitivity of the FTIR instruments can hide negative N$_2$O trends in the mid-stratosphere because of the large increase in the lowermost stratosphere. When applying ACE-FTS measurement sampling on model datasets, the reanalyses from the European Center for Medium Range Weather Forecast (ECMWF) compare best with ACE-FTS, but the N$_2$O trends are consistently exaggerated. The N$_2$O trends obtained with WACCM disagree with those obtained from ACE-FTS, but the new WACCM version performs better than the previous above the Southern Hemisphere in the stratosphere. Model sensitivity tests show that the decadal N$_2$O trends reflect changes in the stratospheric transport. We further investigate the N$_2$O Transformed Eulerian Mean (TEM) budget in WACCM and in the CTM simulation driven by the latest ECMWF reanalysis. The TEM analysis shows that enhanced advection affects the stratospheric N$_2$O trends in the Tropics. While no ideal observational dataset currently exists, this model study of N$_2$O trends still provides new insights about the BDC and its changes because of the contribution from relevant sensitivity tests and the TEM analysis

Observed Hemispheric Asymmetry in Stratospheric Transport Trends From 1994 to 2018

©2020. American Geophysical Union. All Rights Reserved. Total columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5- to 7-year period driven by interactions between the circulation and the quasi-biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions

Improved FTIR retrieval strategy for HCFC-22 (CHClF₂), comparisons with in situ and satellite datasets with the support of models, and determination of its long-term trend above Jungfraujoch

Hydrochlorofluorocarbons (HCFCs) are the first, but temporary, substitution products for the strong ozone-depleting chlorofluorocarbons (CFCs). HCFC consumption and production are currently regulated under the Montreal Protocol on Substances that Deplete the Ozone Layer and their emissions have started to stabilize or even decrease. As HCFC-22 (CHClF2) is by far the most abundant HCFC in today\u27s atmosphere, it is crucial to continue to monitor the evolution of its atmospheric concentration. In this study, we describe an improved HCFC-22 retrieval strategy from ground-based high-resolution Fourier transform infrared (FTIR) solar spectra recorded at the high-altitude scientific station of Jungfraujoch, the Swiss Alps, 3580 m a.m.s.l. (above mean sea level). This new strategy distinguishes tropospheric and lower-stratospheric partial columns. Comparisons with independent datasets, such as the Advanced Global Atmospheric Gases Experiment (AGAGE) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), supported by models, such as the Belgian Assimilation System for Chemical ObErvation (BASCOE) and the Whole Atmosphere Community Climate Model (WACCM), demonstrate the validity of our tropospheric and lower-stratospheric long-term time series. A trend analysis on the datasets used here, now spanning 30 years, confirms the last decade\u27s decline in the HCFC-22 growth rate. This updated retrieval strategy can be adapted for other ozone-depleting substances (ODSs), such as CFC-12. Measuring or retrieving ODS atmospheric concentrations is essential for scrutinizing the fulfilment of the globally ratified Montreal Protocol

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

peer reviewedThe Brewer-Dobson Circulation (BDC) determines the distribution of long-lived tracers in the stratosphere; therefore, their changes can be used to diagnose changes in the BDC. We evaluate decadal (2005–2018) trends of nitrous oxide (N2O) in two versions of the Whole Atmosphere Chemistry-Climate Model (WACCM) by comparing them with measurements from four Fourier transform infrared (FTIR) ground-based instruments, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS), and with a chemistry-transport model (CTM) driven by four different reanalyses. The limited sensitivity of the FTIR instruments can hide negative N2O trends in the mid-stratosphere because of the large increase in the lowermost stratosphere. When applying ACE-FTS measurement sampling on model datasets, the reanalyses from the European Center for Medium Range Weather Forecast (ECMWF) compare best with ACE-FTS, but the N2O trends are consistently exaggerated. The N2O trends obtained with WACCM disagree with those obtained from ACE-FTS, but the new WACCM version performs better than the previous above the Southern Hemisphere in the stratosphere. Model sensitivity tests show that the decadal N2O trends reflect changes in the stratospheric transport. We further investigate the N2O Transformed Eulerian Mean (TEM) budget in WACCM and in the CTM simulation driven by the latest ECMWF reanalysis. The TEM analysis shows that enhanced advection affects the stratospheric N2O trends in the Tropics. While no ideal observational dataset currently exists, this model study of N2O trends still provides new insights about the BDC and its changes because of the contribution from relevant sensitivity tests and the TEM analysis

Impact of lower stratospheric dynamical variability on total inorganic fluorine derived from ground-based FTIR, satellite and model data

ACCROS

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.Au cours des 200 dernières années, l’activité humaine a perturbé l’équilibre subtil qui existait entre les sphères de notre système planétaire. Notamment, la composition atmosphérique a été altérée par des émissions massives de gaz à effet de serre et de substances réduisant la couche d’ozone qui, pourtant, est essentielle à la vie. Les conséquences les plus connues du grand public sont certainement le réchauffement global de la troposphère et la formation du trou d’ozone au-dessus de l’Antarctique. Par contre, la réponse atmosphérique à l’augmentation des gaz à effet de serre, et au réchauffement climatique en résultant, reste très largement méconnue, en particulier la projection unanime par les modèles d’une accélération de la circulation de transport de la stratosphère, la couche atmosphérique se situant entre 10 et 50 km. Cette circulation, appelée la circulation de Brewer-Dobson (BDC), contrôle la distribution d’ozone ainsi que celle d’autres constituants à longue durée de vie dans la stratosphère. Il est donc crucial de caractériser au mieux la BDC et ses changements afin d’évaluer précisément le rétablissement de la couche d’ozone. Son recouvrement progressif est effectivement attendu avant la fin du 21ème siècle grâce à la suppression par le Protocole de Montréal, ainsi que ses Amendements et Ajustements, des émissions de la plupart des substances qui appauvrissent la couche d’ozone. Dans ce travail, nous avons investigué, à l’aide de séries temporelles multidécennales, les changements de la BDC à travers leur impact sur la distribution du fluor dans la stratosphère. Dans ce but, nous incluons des séries temporelles générées par des spectromètres par transformée de Fourier (FTS) situés au Jungfraujoch (Suisse, 46°N) et à Lauder (Nouvelle-Zélande, 45°S), des séries temporelles d’un senseur satellitaire (ACE-FTS) ainsi que cinq simulations réalisées par le modèle de chimie et transport BASCOE (BASCOE CTM). Ces simulations sont conduites par les réanalyses météorologiques de l’atmosphère les plus récentes. Ainsi, il nous est possible d’évaluer leur aptitude à représenter fidèlement les changements de circulation qui ont affecté l’atmosphère terrestre ces 30 dernières années. Au début de cette thèse, nous avons amélioré la stratégie d’inversion du HCFC-22 (CHF2Cl), l’hydrochlorofluorocarbure (HCFC) le plus abondant ainsi que la deuxième source actuelle de fluor dans la stratosphère, à l’aide de spectres solaires infrarouges enregistrés au Jungfraujoch. Nous avons alors montré que le HCFC-22 s’accumule moins rapidement dans l’atmosphère durant cette dernière décennie, mettant en exergue le succès du Protocol de Montréal. Cette première étape a aussi permis de démontrer la validité de notre configuration de BASCOE CTM, après implémentation de nouvelles fonctionnalités indispensables à la réalisation de nos simulations. Nos investigations, fondées sur les séries observationnelles et les simulations du fluor stratosphérique, ont démontré que la BDC a changé de manière asymétrique au cours des vingt dernières années, avec la branche de l’hémisphère Sud s’intensifiant par rapport à celle de l’hémisphère Nord. Il est important de noter que cette conclusion est en contradiction avec nombre de projections qui anticipent un affaiblissement de la branche de l’hémisphère Sud en réponse à l’augmentation des gaz à effet de serre et à la diminution des substances appauvrissant l’ozone. Enfin, nous avons confirmé de manière indépendante que la variabilité pluriannuelle (5 à 7 ans) de la BDC identifiée très récemment est superposée à ce changement multidécennal.ACCROS

ACCROSS Project: Investigating the impacts of circulation changes on stratospheric tracers

ACCROS

ACCROSS Project: Investigating the impacts of circulation changes on stratospheric tracers

The ACCROSS project (Atmospheric Composition and Circulation investigated with meteorological Reanalyses, Observational datasets and models for the Study of the Stratosphere and its changes) aims to improve our understanding of the circulation changes in the stratosphere during the past three decades. To achieve this objective, miscellaneous observational datasets and model simulations are used. Here are presented the main aspects and the first preliminary results of the project. In the framework of the recent studies demonstrating the influence of stratospheric circulation changes on the trend of long-lived tracers (e.g. hydrogen chlorine), we decided to investigate the impact of these circulation changes on hydrogen fluoride (HF). Fourier-Transform infrared spectroscopy data produced at the Jungfraujoch NDACC site and satellite data (HALOE and ACE) are used to evaluate the HF trends. Moreover, to support our data interpretation, chemistry-transport model BASCOE simulations are also included. These early investigations show a modulation in HF time series at Jungfraujoch occurring around 2007. At the time being, it seems that BASCOE driven by ERA-Interim reanalysis cannot capture this modulation.ACCROS

Update on the FTIR monitoring program at the Jungfraujoch station

We present a report on the status of the FTIR monitoring program at the Jungfraujoch station. Focus is put on the reanalysis of the HCFC-22 (CHClF2) time series and to our first attempt to retrieve PAN from ground-based infrared solar absorption spectra. Ongoing investigations regarding two heavy stable isotopologues of methane (13CH4 and CH3D) are also presented