Abstract
Abstract. The Brewer–Dobson circulation (BDC) is a key feature of the stratosphere that models need to accurately represent in order to simulate surface climate variability and change adequately. For the first time, the Climate Model Intercomparison Project includes in its phase 6 (CMIP6) a set of diagnostics that allow for careful evaluation of the BDC. Here, the BDC is evaluated against observations and reanalyses using historical simulations. CMIP6 results confirm the well-known inconsistency in the sign of BDC trends between observations and models in the middle and upper stratosphere. Nevertheless, the large uncertainty in the observational trend estimates opens the door to compatibility. In particular, when accounting for the limited sampling of the observations, model and observational trend error bars overlap in 40 % of the simulations with available output. The increasing CO2 simulations feature an acceleration of the BDC but reveal a large spread in the middle-to-upper stratospheric trends, possibly related to the parameterized gravity wave forcing. The very close connection between the shallow branch of the residual circulation and surface temperature is highlighted, which is absent in the deep branch. The trends in mean age of air are shown to be more robust throughout the stratosphere than those in the residual circulation.
Highlights
The Brewer–Dobson circulation (BDC) describes the net transport of mass, heat and tracers in the stratosphere and plays a primary role in its chemical composition and radiative transfer properties (Butchart, 2014)
Given that the longest observational age of air (AoA) estimates cover more than 30 years, natural variability cannot explain the discrepancy between the negative trends in the models and the positive trend in AoA derived from observations in the middle stratosphere
The present paper examines the BDC in CMIP6 models, focusing on the residual circulation and the mean age of air
Summary
The Brewer–Dobson circulation (BDC) describes the net transport of mass, heat and tracers in the stratosphere and plays a primary role in its chemical composition and radiative transfer properties (Butchart, 2014). The mean age of air (AoA) transport diagnostic quantifies the elapsed time since an air parcel entered the stratosphere, and it can be estimated from observations of long-lived tracers such as SF6 or CO2 (e.g., Engel et al, 2017) It integrates the effect of both residual circulation and mixing. This allows for a more detailed analysis based on a consistent set of diagnostics as compared to previous assessments (e.g., Manzini et al, 2014; Hardiman et al, 2014) We use this TEM and AoA output to evaluate the past climatology and trends in the BDC against reanalyses and observations using historical simulations and to assess the BDC response to an idealized 1 % yr−1 CO2 increase.
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