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

Abstract

<p>This paper investigates the global stratospheric Brewer-Dobson circulation (BDC) in the ERA5 meteorological reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is based on simulations of stratospheric mean age of air, including the full age spectrum, with the Lagrangian transport model CLaMS, driven by winds and total diabatic heating rates from the reanalysis. ERA5-based results are compared to those of the preceding ERA-Interim reanalysis. Our results show a significantly slower BDC for ERA5 than for ERA-Interim, manifesting in weaker diabatic heating rates and larger age of air. In the tropical lower stratosphere, heating rates are 30-40% weaker in ERA5, likely correcting a known bias in ERA-Interim. Above, ERA5 age of air appears slightly high-biased and the BDC slightly slow compared to tracer observations. The age trend in ERA5 over 1989-2018 is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear over the period but exhibits steplike changes which could be caused by muti-annual variability or changes in the assimilation system. Over the 2002-2012 period, ERA5 age shows a similar hemispheric dipole trend pattern as ERA-Interim, with age increasing in the NH and decreasing in the SH. Shifts in the age spectrum peak and residual circulation transit times indicate that reanalysis differences in age are likely caused by differences in the residual circulation. In particular, the shallow BDC branch accelerates similarly in both reanalyses while the deep branch accelerates in ERA5 and decelerates in ERA-Interim.</p>

Stratospheric Moistening After 2000

“…The ERA5‐driven model runs and their postprocessing are exactly the same as in Ploeger et al. (2021) (i.e., including the update named ERA5.1). In addition to ERA5, three other reanalyses are used: ERA‐Interim (Dee et al., 2011), JRA‐55 (Kobayashi et al., 2015), and MERRA‐2 (Gelaro et al., 2017) using the same CLaMS simulations as those described in Tao et al.…”

“…(2019) and Ploeger et al. (2021). The zonal wind records at 50 and 30 hPa over Singapore used for multi‐linear regression (MLR) are provided by: https://www.geo.fu-berlin.de/met/ag/strat/produkte/qbo/qbo.dat.…”

Variability of Water Vapor in the Tropical Middle Atmosphere Observed From Satellites and Interpreted Using SD‐WACCM Simulations

“…The AOA calculation in SD‐WACCM is largely dependent on the MERRA2 wind field it nudges to. Previous studies have compared the AOA derived from several reanalysis data and concluded that the mean age of MERRA2 is similar to that from ERA5, and is longer than from ERA‐interim and JRA‐55 (Ploeger et al., 2019, 2021).…”

Effects of reanalysis forcing fields on ozone trends and age of air from a chemical transport model

“…The comparison also shows that, before 1998, anomalies from B_ERA5 are relatively smaller than from A_ERAI (up to ∼ −20 DU (Dobson units) biases; shaded green regions) but are larger during later years (up to ∼ +20 DU biases; shaded yellow regions). The relatively better agreement between B_ERA5 and C3S in the NH mid-high-latitude regions could be due to improvements in the representation of dynamical processes in ERA5 reanalysis data, such as convection in the upper troposphere and lower stratosphere (D. and residual mean mass circulation of the Brewer-Dobson circulation (BDC) in the stratosphere (Diallo et al, 2021;Ploeger et al, 2021).…”

“…Fusco and Salby, 1999;Weber et al, 2003;Dhomse et al, 2006). Ploeger et al (2021) analysed the global stratospheric BDC using simulations of stratospheric mean AoA with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by reanalysis (ERA5/ERA-Interim) winds and total diabatic heating rates. They found that ERA5-based results exhibit older AoA compared to results based on ERA-Interim, which is indicative of a significantly slower BDC for ERA5.…”