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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 (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. Plain Language Summary: The circulation in the stratosphere is characterized by upward motion above the Tropics, followed by poleward and downward motions above the high latitudes. Changes in the pattern of this stratospheric circulation are currently a challenging topic of research. We investigate the decennial changes of this stratospheric circulation using observations and numerical simulations of the long‐lived tracer nitrous oxide. Observations are obtained from ground‐based and satellite instruments. Numerical simulations include complex atmospheric models that reproduce the chemistry and dynamics of the stratosphere. Both observations and models show differences between the hemispheres in the nitrous oxide decennial changes. Unfortunately, the current observations of nitrous oxide are not perfect. The ground‐based instruments cannot correctly measure the changes of nitrous oxide in the northern hemisphere. The satellite does not measure at all times, and it spatially covers more the high latitudes, which negatively affects the measurements of nitrous oxide. On the other hand, model simulations can provide valuable insights into the changes in the stratospheric circulation. They show that changes in the stratospheric circulation cause the differences between hemispheres in the nitrous oxide trends and show that the circulation changes can be associated with different physical processes. Key Points: Sparse sampling of Atmospheric Chemistry Experiment Fourier Transform Spectrometer exaggerates the stratospheric nitrous oxide trendsTransformed Eulerian Mean analysis shows that the residual mean advection contributes to the positive nitrous oxide trend in the TropicsThe Whole Atmosphere Community‐Climate Model simulates weaker hemispheric asymmetries of the nitrous oxide trends compared to reanalyses [ABSTRACT FROM AUTHOR]

The Brewer-Dobson Circulation (BDC) transports chemical tracers from the well-mixed tropical troposphere to the polar stratosphere, with many important implications for climate, chemistry, ozone distribution and recovery. Since the photochemical losses of nitrous oxide (N[sub 2]O) are well-known, model differences in its rate of change are due to transport processes that can be separated in the mean residual advection and the isentropic mixing terms in the Transformed Eulerian Mean (TEM) framework. Here the climatological impact of the stratospheric BDC on the long-lived tracer N2O is evaluated through a comparison of its TEM budget in the Whole Atmosphere Community Climate Model (WACCM), a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a Chemistry-Transport Model (CTM) driven by four modern reanalyses (ERA-Interim, JRA-55, MERRA and MERRA2). The effects of stratospheric transport on the N[sub 2]O rate of change, as depicted in this study, have not been compared across this variety of datasets and never investigated in a chemical reanalysis. We focus 10 on the seasonal means and climatological annual cycles of the two main contributions to the N2O TEM budget: the vertical residual advection and the horizontal mixing terms. The N2O mixing ratio in the CTM experiments has a spread of approximately ~ 20% in the middle stratosphere, reflecting the large diversity in the mean Age of Air obtained with the same experiments. In all datasets the TEM budget is well-closed and the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and good in the Southern Hemisphere except above the Antarctic region. The datasets do not agree as well with respect to the horizontal mixing term, especially in the Northern Hemisphere where horizontal mixing has a smaller contribution in WACCM than in the reanalyses. WACCM is investigated through three model realizations and a sensitivity test where gravity waves are forced differently in the Southern Hemisphere. The internal variability of the horizontal mixing in WACCM is large in the polar regions, and comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but significantly changes the vertical advection term and produces a less realistic N2O annual cycle above the Antarctic. In this region, all reanalyses show a large wintertime N2O decrease, which is mainly due to horizontal mixing. This is not seen with WACCM, where the horizontal mixing term barely contributes to the TEM budget. While we must use caution in the interpretation of the differences in this region, where the reanalyses show large residuals of the TEM budget, they could be due to the fact that the polar jet is stronger and not tilted equatorward in WACCM compared 25 with the reanalyses. We also compare the inter-annual variability in the horizontal mixing and the vertical advection terms. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions . In the Tropics, the inter-annual variability of the vertical advection term is much smaller in WACCM and JRA-55 than in the other experiments. The large residual in the reanalyses and the disagreement between WACCM and the reanalyses in the Antarctic region highlight 30 the need for further investigations on the modeling of transport in this region of the stratosphere. [ABSTRACT FROM AUTHOR]

The stratospheric circulation is investigated over the period 2005-2015 using WACCM4 (Whole Atmosphere Community Climate Model version 4), together with BASCOE CTM (Chemistry-Transport Model; Chabrillat et al., 2018) and a reanalysis of stratospheric composition observed by MLS: BRAM2 (BASCOE Reanalysis of AURA MLS release 2).We use long-lived tracers (N2O, H2O) and the Transformed Eulerian Mean framework (Abalos et al., 2017) to analyse the tracers budget including the mean analysis increment for BRAM2. We focus on the annual and seasonal June-July-August mean of the advection and eddy mixing terms and on the impact of the Quasi-Biennial Oscillation (QBO).For both tracers, the annual mean pattern is in general agreement among the three datasets but larger differences are found in JJA. Eddy mixing in WACCM (free dynamics and free chemistry) is smaller and has less variability than in the CTM (constrained dynamics and free chemistry) and in BRAM2 (constrained dynamics and chemistry). In addition, the SH polar transport barrier in WACCM is weaker with respect to the CTM and BRAM2.The next step of our research is to perform this analysis with the newer version of WACCM (version 6) as well as additional BASCOE CTM runs using different reanalysis products or short-term forecasts. Multi-decadal changes in the terms of the budget, and their space dependence, will be investigated as well.ReferencesChabrillat, S., et al.: Comparison of mean age of air in five reanalyses using the BASCOE transport model, Atmos. Chem. Phys., 18(19), 14715–14735, doi:10.5194/acp-18-14715-2018, 2018.Abalos, M., W.J. Randel, D.E. Kinnison, and R.R. Garcia, 2017: Using the Artificial Tracer e90 to Examine Present and Future UTLS Tracer Transport in WACCM. J. Atmos. Sci., 74, 3383–3403, https://doi.org/10.1175/JAS-D-17-0135.1 [ABSTRACT FROM AUTHOR]