Abstract
Abstract. The seasonal and interannual variability of transport times from the northern midlatitude surface into the Southern Hemisphere is examined using simulations of three idealized “age” tracers: an ideal age tracer that yields the mean transit time from northern midlatitudes and two tracers with uniform 50- and 5-day decay. For all tracers the largest seasonal and interannual variability occurs near the surface within the tropics and is generally closely coupled to movement of the Intertropical Convergence Zone (ITCZ). There are, however, notable differences in variability between the different tracers. The largest seasonal and interannual variability in the mean age is generally confined to latitudes spanning the ITCZ, with very weak variability in the southern extratropics. In contrast, for tracers subject to spatially uniform exponential loss the peak variability tends to be south of the ITCZ, and there is a smaller contrast between tropical and extratropical variability. These differences in variability occur because the distribution of transit times from northern midlatitudes is very broad and tracers with more rapid loss are more sensitive to changes in fast transit times than the mean age tracer. These simulations suggest that the seasonal–interannual variability in the southern extratropics of trace gases with predominantly NH midlatitude sources may differ depending on the gases' chemical lifetimes.
Highlights
Interhemispheric transport is important for understanding the global distribution of tropospheric trace gases
For all tracers the largest seasonal and interannual variability occurs near the surface within the tropics and is generally closely coupled to variability in the tropical convergence zones (ITCZ, South Pacific Convergence Zone (SPCZ))
The seasonal migration of the Intertropical Convergence Zone (ITCZ) is responsible for the majority of seasonality in the tracer ages, while a large amount of the interannual variability during DJF is due to El Niño–Southern Oscillation (ENSO)-related variations in surface convergence and convection, especially over the Pacific Ocean
Summary
Interhemispheric transport is important for understanding the global distribution of tropospheric trace gases. Most previous studies that have examined interhemispheric transport have used a simple two-box framework to quantify a single interhemispheric exchange time, calculated in terms of the temporal change in the difference between the southern and northern hemispherically integrated tracer mass (e.g., Levin and Hesshaimer, 1996; Geller et al, 1997; Lintner et al, 2004; Maiss et al, 1996; Denning et al, 1999) This metric is useful as it collapses all the transport into a single parameter that can be used for model–observation or inter-model comparisons. Recent studies have used observed and simulated SF6 or simulated idealized mean age tracers to estimate the mean transport time from the NH surface to locations throughout the troposphere (Holzer and Boer, 2001; Waugh et al, 2013) This approach provides a more complete description of interhemispheric transport, quantifying differences in transport into the tropics versus southern extratropics and differences in transport between the lower and upper troposphere.
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