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

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.