Evaluation of the N_(2)O rate of change to understand the stratospheric Brewer-Dobson Circulation in a chemistry-climate model

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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.
© 2022. The Authors. We thank P. Bernath for his leadership of the ACE mission, which is supported by the Canadian Space Agency. Measurements at Lauder are core-funded by the National Institute of Water and Atmospheric Research Ltd. (NIWA) through New Zealand's Ministry of Business, Innovation and Employment Strategic Science Investment Fund. The ULiège team is grateful to the International Foundation High Altitude Research Stations Jungfraujoch and Gornergrat (HFSJG, Bern) for supporting the facilities needed to perform the FTIR observations at Jungfraujoch. Funding via Helmholtz ATMO programme has enabled the sustained NDACC FTIR activities at Izaña since the late 1990s. In addition, the Izaña NDACC FTIR observations strongly rely on the support (facilities and operational activities) of the Izaña Atmospheric Research Centre of the Spanish Weather Service (AEMET), with lead contributions of O. E. García. D. Minganti and M. Prignon were financially supported by the Fonds de la Recherche Scientifique (F.R.S.-FNRS, Brussels) through the ACCROSS research project (Grant PDR.T.0040.16). The University of Liège contribution was further supported by the GAW-CH program of MeteoSwiss and by the F.R.S.-FNRS Grant J.0126.21. E. Mahieu is a senior research associate with the F.R.S.-FNRS. D. Kinnison was funded in part by National Aeronautics and Space Administration (NASA) Grant (NNH19ZDA001N-AURAST). This research was enabled by the computational and storage resources of NCAR's Computational and Information Systems Laboratory (CISL), sponsored by the NSF. Cheyenne: HPE/SGI ICE XA System (NCAR Community Computing). Boulder, CO: National Center for Atmospheric Research. D6RX99HX.