We examine the relative contribution of processes controlling
the interannual variability (IAV) of tropospheric ozone over four sub-regions
of the southern hemispheric tropospheric ozone maximum (SHTOM) over a 20-year
period. Our study is based on hindcast simulations from the National
Aeronautics and Space Administration Global Modeling Initiative chemistry
transport model (NASA GMI-CTM) of tropospheric and stratospheric chemistry,
driven by assimilated Modern Era Retrospective Analysis for Research and
Applications (MERRA) meteorological fields. Our analysis shows that over
SHTOM region, the IAV of the stratospheric contribution is the most important
factor driving the IAV of upper tropospheric ozone (270 hPa), where ozone
has a strong radiative effect. Over the South Atlantic region, the
contribution from surface emissions to the IAV of ozone exceeds that from
stratospheric input at and below 430 hPa. Over the South Indian Ocean, the
IAV of stratospheric ozone makes the largest contribution to the IAV of ozone
with little or no influence from surface emissions at 270 and 430 hPa in
austral winter. Over the tropical South Atlantic region, the contribution
from IAV of stratospheric input dominates in austral winter at 270 hPa and
drops to less than half but is still significant at 430 hPa. Emission
contributions are not significant at these two levels. The IAV of lightning
over this region also contributes to the IAV of ozone in September and
December. Over the tropical southeastern Pacific, the contribution of the IAV
of stratospheric input is significant at 270 and 430 hPa in austral winter,
and emissions have little influence
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