H. Garny, T. Birner, H. Boenisch

Residual circulation transit times as diagnostic of the structure of mean meridional transport

Transport in the stratosphere is determined both by the mean meridional circulation and two-way mixing. The transport time scales given by the mean meridional circulation can be diagnosed by trajectories driven only by the residual circulation, thus giving the hypothetical 'age' that air would have if it was transported only by the residual circulation. This quantity is referred to as 'residual circulation transit time' (RCTT). The RCTTs can provide valuable information on the structure of the circulation: it can be clearly distinguished between a shallow and a deep branch of the circulation, with air that travels along the deep branch entering the stratosphere in the deep tropics and traveling relatively far in the vertical compared to the horizontal direction. Using results from a suit of Global Climate Model (GCM) simulations, it is found that transit times along the deep branch are controlled by the strength of tropical upwelling, while transit times along the shallow branch are controlled by the shape of residual circulation trajectories. Furthermore, changes in the tropopause height influence transit times in particular along the shallow branch. Given a consistent estimate of age of air (AoA) together with the RCTTs, as it can be obtained in GCMs, the difference between AoA and RCTTs allows the quantification of effects of mixing on AoA.
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