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| Titel |
Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models |
| VerfasserIn |
L. Guo, A. G. Turner, E. J. Highwood |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 11 ; Nr. 15, no. 11 (2015-06-11), S.6367-6378 |
| Datensatznummer |
250119803
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| Publikation (Nr.) |
 copernicus.org/acp-15-6367-2015.pdf |
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| Zusammenfassung |
Comparison of single-forcing varieties of 20th century historical experiments
in a subset of models from the Fifth Coupled Model Intercomparison Project
(CMIP5) reveals that South Asian summer monsoon rainfall increases towards
the present day in Greenhouse Gas (GHG)-only experiments with respect to
pre-industrial levels, while it decreases in anthropogenic aerosol-only
experiments. Comparison of these single-forcing experiments with the
all-forcings historical experiment suggests aerosol emissions have dominated
South Asian monsoon rainfall trends in recent decades, especially during the
1950s to 1970s. The variations in South Asian monsoon rainfall in these
experiments follows approximately the time evolution of inter-hemispheric
temperature gradient over the same period, suggesting a contribution from the
large-scale background state relating to the asymmetric distribution of
aerosol emissions about the equator.
By examining the 24 available all-forcings historical experiments,
we show that models including aerosol indirect effects dominate the negative
rainfall trend. Indeed, models including only the direct radiative effect of
aerosol show an increase in monsoon rainfall, consistent with the dominance
of increasing greenhouse gas emissions and planetary warming on monsoon
rainfall in those models. For South Asia, reduced rainfall in the models with
indirect effects is related to decreased evaporation at the land surface
rather than from anomalies in horizontal moisture flux, suggesting the impact
of indirect effects on local aerosol emissions. This is confirmed by
examination of aerosol loading and cloud droplet number trends over the South
Asia region. Thus, while remote aerosols and their asymmetric distribution
about the equator play a role in setting the inter-hemispheric temperature
distribution on which the South Asian monsoon, as one of the global monsoons,
operates, the addition of indirect aerosol effects acting on very local
aerosol emissions also plays a role in declining monsoon rainfall. The
disparity between the response of monsoon rainfall to increasing aerosol
emissions in models containing direct aerosol effects only and those also
containing indirect effects needs to be urgently investigated since the
suggested future decline in Asian anthropogenic aerosol emissions inherent to
the representative concentration pathways (RCPs) used for future climate
projection may turn out to be optimistic.
In addition, both groups of models show declining rainfall over China, also
relating to local aerosol mechanisms. We hypothesize that aerosol emissions
over China are large enough, in the CMIP5 models, to cause declining monsoon
rainfall even in the absence of indirect aerosol effects. The same is not
true for India. |
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