Abstract
Abstract. Understanding and modeling the large-scale transport of trace gases and aerosols is important for interpreting past (and projecting future) changes in atmospheric composition. Here we show that there are large differences in the global-scale atmospheric transport properties among the models participating in the IGAC SPARC Chemistry–Climate Model Initiative (CCMI). Specifically, we find up to 40 % differences in the transport timescales connecting the Northern Hemisphere (NH) midlatitude surface to the Arctic and to Southern Hemisphere high latitudes, where the mean age ranges between 1.7 and 2.6 years. We show that these differences are related to large differences in vertical transport among the simulations, in particular to differences in parameterized convection over the oceans. While stronger convection over NH midlatitudes is associated with slower transport to the Arctic, stronger convection in the tropics and subtropics is associated with faster interhemispheric transport. We also show that the differences among simulations constrained with fields derived from the same reanalysis products are as large as (and in some cases larger than) the differences among free-running simulations, most likely due to larger differences in parameterized convection. Our results indicate that care must be taken when using simulations constrained with analyzed winds to interpret the influence of meteorology on tropospheric composition.
Highlights
The distributions of greenhouse gases (GHGs) and ozonedepleting substances (ODSs) are strongly influenced by large-scale atmospheric transport
Note that these differences are overall much larger than the differences among the simulations presented in Orbe et al (2017), which feature consistently larger concentrations of χ 5 and χ 50 over northern middle and high latitudes compared to the other simulations, indicative of more efficient poleward transport in those models
Stronger tropical and subtropical convection is associated with faster interhemispheric transport
Summary
The distributions of greenhouse gases (GHGs) and ozonedepleting substances (ODSs) are strongly influenced by large-scale atmospheric transport. In the extratropics the midlatitude jet stream influences the long-range transport of pollutants and water vapor into the Arctic (e.g., Eckhardt et al, 2003; Shindell et al, 2008; Liu and Barnes, 2015) and surface ozone variability over the western United States (Lin et al, 2015). There are large uncertainties in our understanding of how large-scale atmospheric transport influences tropospheric composition. This is largely because transport is difficult to constrain directly from observations and because globalscale tropospheric transport properties differ widely among models. Denning et al (1999) found more than a factor of 2 difference in the interhemispheric exchange rate among simulations produced using both offline chemical transport models (CTMs) and online free-running general circulation models (GCMs)
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