Climatological impact of the Brewer–Dobson circulation on the N<sub>2</sub>O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses

Abstract

Abstract. The Brewer–Dobson circulation (BDC) is a stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. As the photochemical losses of nitrous oxide (N2O) are well known, model differences in its rate of change are due to transport processes that can be separated into 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), in a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a chemistry transport model (CTM) driven by four modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim; Dee et al., 2011), the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al., 2015), and the Modern-Era Retrospective analysis for Research and Applications version 1 (MERRA; Rienecker et al., 2011) and version 2 (MERRA-2; Gelaro et al., 2017). The effects of stratospheric transport on the N2O rate of change, as depicted in this study, have not been compared before across this variety of datasets and have never been investigated in a modern chemical reanalysis. We focus 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 CTM experiments in a previous study. In all datasets, the TEM budget is closed well; the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and it is 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 using the previous version of the gravity wave parameterization. The internal variability of the horizontal mixing in WACCM is large in the polar regions and is comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but it 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 is not tilted equatorward in WACCM compared with the reanalyses. We also compare the interannual variability in the horizontal mixing and the vertical advection terms between the different datasets. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions. In the tropics, the interannual 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 the need for further investigations on the modeling of transport in this region of the stratosphere.

Phase Unlocking and the Modulation of Tropopause‐Level Trace Gas Advection by the Quasibiennial Oscillation

“…Calculations on JRA-55 reanalysis were performed on 6-hourly snapshots and calculations on free-running WACCM output were done on monthly-mean data. As our calculations from WACCM were performed on coarser temporal resolution data, we validated them by reproducing results of the TEM tracer budget from Minganti et al (2020) and found consistent results (not shown).…”

“…Therefore, attempts to account for imprints of dynamical influences on near-surface concentration signals should include stratosphere-troposphere advection modulated by the QBO and its seasonally changing phase relationship. This work augments existing studies that use a TEM tracer budget to quantify stratosphere-troposphere exchange (Abalos et al, 2020;Minganti et al, 2020). It advances the existing body of literature about synchronizations between the annual cycle of ozone concentrations and the QBO (Gray & Dunkerton, 1990) by examining phase unlocking patterns in the lower stratospheric advection of tracers.…”

The stratospheric Brewer–Dobson circulation inferred from age of air in the ERA5 reanalysis

“…Recent studies have stated a substantial uncertainty in the climatological strength and trends of the stratospheric BDC and age of air in current-generation reanalyses (e.g. Abalos et al, 2015;Miyazaki et al, 2016;Chabrillat et al, 2018;Ploeger et al, 2019). Among the reanalyses considered, JRA-55 was shown to have the fastest BDC and MERRA-2 was shown to have the slowest BDC, with ERA-Interim-based results in between.…”

“…Diallo et al, 2012;Monge-Sanz et al, 2012). However, more recent efforts combining different newest-generation reanalysis data sets have shown a large dependency of BDC trend estimates on the reanalysis used, for both residual circulation (Abalos et al, 2015;Miyazaki et al, 2016) and age of air diagnostics (Chabrillat et al, 2018;Ploeger et al, 2019). In particular, no consensus has been reached amongst reanalyses, including ERA-Interim reanalysis from the ECMWF (Dee et al, 2011), JRA-55 from the Japanese Meteorological Agency (Kobayashi et al, 2015), and MERRA-2 from the National Aeronautics and Space Administration (Gelaro et al, 2017) concerning decadal age of air and BDC changes.…”

Satellite data validation: a parametrization of the natural variability of atmospheric mixing ratios

“…While usually used in the context of stratospheric chemical data assimilation (e.g. Errera et al (2019) ), the Chemistry Transport Model (CTM) of the system has been also used to study of the evolution of the stratospheric composition (Chabrillat et al, 2018;Prignon et al, 2019;Minganti et al, 2020). Here, BASCOE was run for the period 25 Sept-1 Oct 2008 where wind and temperature were taken from the ERA-Interim reanalysis (Dee et al, 2011).…”