Abstract
Abstract. The impacts of climate change on tropospheric transport, diagnosed from a carbon monoxide (CO)-like tracer species emitted from global CO sources, are evaluated from an ensemble of four chemistry–climate models (CCMs) contributing to the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Model time-slice simulations for present-day and end-of-the-21st-century conditions were performed under the Representative Concentrations Pathway (RCP) climate scenario RCP 8.5. All simulations reveal a strong seasonality in transport, especially over the tropics. The highest CO-tracer mixing ratios aloft occur during boreal winter when strong vertical transport is co-located with biomass burning emission source regions. A consistent and robust decrease in future CO-tracer mixing ratios throughout most of the troposphere, especially in the tropics, and an increase around the tropopause is found across the four CCMs in both winter and summer. Decreases in CO-tracer mixing ratios in the tropical troposphere are associated with reduced convective mass fluxes in this region, which in turn may reflect a weaker Hadley cell circulation in the future climate. Increases in CO-tracer mixing ratios near the tropopause are largely attributable to a rise in tropopause height enabling lofting to higher altitudes, although a poleward shift in the mid-latitude jets may also play a minor role in the extratropical upper troposphere. An increase in CO-tracer mixing ratios also occurs near the Equator, centred over equatorial and Central Africa, extending from the surface to the mid-troposphere. This is most likely related to localised decreases in convection in the vicinity of the Intertropical Convergence Zone (ITCZ), resulting in larger CO-tracer mixing ratios over biomass burning regions and smaller mixing ratios downwind.
Highlights
The transport of pollutants from the atmospheric boundary layer is governed by meteorological processes including deep convection, Hadley-cell-driven overturning in the tropics, and mid-latitude cyclones, as well as slow low-altitude airflow, small-scale turbulent mixing, and other motions (e.g. Cooper et al, 2011; TF-HTAP, 2010)
Since the same parameterisation is used by UM-CAM, HadAM3, and GISS-ER-2, it may be the specific details of its implementation and interactions with internal parameters (Scinocca and McFarlane, 2004) that cause this large difference in magnitudes across the four chemistry–climate models (CCMs)
In response to future increases in greenhouse gases in the 2090s under the Representative Concentrations Pathway (RCP) 8.5 scenario, changes in mixing ratios of a carbon monoxide (CO)-like tracer with a 50-day lifetime exhibit robust features across four chemistry–climate models participating in the ACCMIP model intercomparison
Summary
The transport of pollutants from the atmospheric boundary layer is governed by meteorological processes including deep convection, Hadley-cell-driven overturning in the tropics, and mid-latitude cyclones, as well as slow low-altitude airflow, small-scale turbulent mixing, and other motions (e.g. Cooper et al, 2011; TF-HTAP, 2010). A detailed analysis by Fang et al (2011) used a global CO-like tracer with a first-order 25-day lifetime and global anthropogenic CO emissions to investigate changes in transport under the SRES A1B scenario between 1981–2000 and 2081–2100 using the GFDL AM3 chemistry–climate model (CCM) They found that CO-tracer mixing ratios increased at the surface and decreased in the tropical free troposphere due to reduced convective mass fluxes and that reduced CO-tracer mixing ratios in the Southern Hemisphere were most likely a response to a weaker Hadley circulation and reduced interhemispheric exchange (Fang et al, 2011). Convective mass flux and zonal (u) wind data
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