Abstract
Climate models consistently predict an acceleration of the Brewer-Dobson circulation (BDC) due to climate change in the 21st century. However, the strength of this acceleration varies considerably among individual models, which constitutes a notable source of uncertainty for future climate projections. To shed more light upon the magnitude of this uncertainty and on its causes, we analyze the stratospheric mean age of air (AoA) of 10 climate projection simulations from the Chemistry Climate Model Initiative phase 1 (CCMI-I), covering the period between 1960 and 2100. In agreement with previous multi-model studies, we find a large model spread in the magnitude of the AoA trend over the simulation period. Differences between future and past AoA are found to be predominantly due to differences in mixing (reduced aging by mixing and recirculation) rather than differences in residual mean transport. We furthermore analyze the mixing efficiency, a measure of the relative strength of mixing for given residual mean transport, which was previously hypothesized to be a model constant. Here, the mixing efficiency is found to vary not only across models, but also over time in all models. Changes in mixing efficiency are shown to be closely related to changes in AoA and quantified to roughly contribute 10% to the long-term AoA decrease over the 21st century. Additionally, mixing efficiency variations are shown to considerably enhance model spread in AoA changes. To understand these mixing efficiency variations, we also present a consistent dynamical framework based on diffusive closure, which highlights the role of basic state potential vorticity gradients in controlling mixing efficiency and therefore aging by mixing.
Highlights
Air mostly enters the stratosphere through the tropical tropopause and it ascends within the tropical pipe
We focus on the age of air (AoA) differences between future and past simulated by 10 chemistry–climate models that participated in the Chemistry-Climate Model Initiative phase 1 (CCMI-1; Morgenstern et al, 2017)
The tropical upwelling in the 70 hPa pressure level has been shown to be a good measure for the strength of the residual circulation throughout the stratosphere. To see if this relationship holds true for AoA in our set of the CCMI model simulations, we present in Fig. 4 the inter-model correlations between AoA and residual circulation transit times (RCTTs), as well as between the AoA and the differences in tropical upwelling in 70 hPa
Summary
Air mostly enters the stratosphere through the tropical tropopause and it ascends within the tropical pipe. Thereafter, it is transported poleward before descending to the extratropical lower stratosphere and back to the troposphere (Butchart, 2014). This stratospheric overturning cycle has been named the Brewer–Dobson circulation (BDC), referring to the early work of Dobson et al (1929), Brewer (1949) and Dobson (1956), who first postulated this transport pattern on the basis of trace gas observations. The representation of the strength, the structure and the predicted future changes of the stratospheric overturning circulation differ vastly among today’s state-of-the-art climate models (SPARC, 2010; Dietmüller et al, 2018), the same models that are applied to making predictions of surface climate conditions across the 21st century
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