Quantifying the effect of mixing on the mean age of air in CCMVal-2 and CCMI-1 models
2017
Abstract. The stratospheric age of air (AoA) is a useful measure of the overall
capabilities of a general circulation model (GCM) to simulate stratospheric
transport. Previous studies have reported a large spread in the simulation of
AoA by GCMs and coupled chemistry–climate models (CCMs). Compared to
observational estimates, simulated AoA is mostly too low. Here we attempt to
untangle the processes that lead to the AoA differences between the models
and between models and observations. AoA is influenced by both mean transport
by the residual circulation and two-way mixing; we quantify the effects of
these processes using data from the CCM inter-comparison projects CCMVal-2 (Chemistry–Climate
Model Validation Activity 2) and CCMI-1 (Chemistry–Climate Model Initiative, phase 1).
Transport along the residual circulation is measured by the
residual circulation transit time (RCTT). We interpret the difference between
AoA and RCTT as additional aging by mixing. Aging by mixing thus includes
mixing on both the resolved and subgrid scale. We find that the spread in AoA
between the models is primarily caused by differences in the effects of
mixing and only to some extent by differences in residual circulation
strength. These effects are quantified by the mixing efficiency, a measure of
the relative increase in AoA by mixing. The mixing efficiency varies strongly
between the models from 0.24 to 1.02. We show that the mixing efficiency is
not only controlled by horizontal mixing, but by vertical mixing and vertical
diffusion as well. Possible causes for the differences in the models' mixing
efficiencies are discussed. Differences in subgrid-scale mixing (including
differences in advection schemes and model resolutions) likely contribute to
the differences in mixing efficiency. However, differences in the relative
contribution of resolved versus parameterized wave forcing do not appear to be
related to differences in mixing efficiency or AoA.
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