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.
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16 articles.
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