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

Abstract

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 (Chemical Lagrangian Model of the Stratosphere), driven by reanalysis winds and total diabatic heating rates. ERA5-based results are compared to results based on 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 higher age of air. In the tropical lower stratosphere, heating rates are 30 %–40 % weaker in ERA5, likely correcting a bias in ERA-Interim. At 20 km and in the Northern Hemisphere (NH) stratosphere, ERA5 age values are around the upper margin of the uncertainty range from historical tracer observations, indicating a somewhat slow–biased BDC. The age trend in ERA5 over the 1989–2018 period is negative throughout the stratosphere, as climate models predict in response to global warming. However, the age decrease is not linear but steplike, potentially caused by multi-annual variability or changes in the observations included in the assimilation. During the 2002–2012 period, the ERA5 age shows a similar hemispheric dipole trend pattern as ERA-Interim, with age increasing in the NH and decreasing in the Southern Hemisphere (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 in both reanalyses, whereas the deep branch accelerates in ERA5 and decelerates in ERA-Interim.