Journal article

The effect of ENSO-induced rainfall and circulation changes on the direct and indirect radiative forcing from Indonesian biomass-burning aerosols

Chrastansky A, Rotstayn L ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 23 (2012) pp. 11395-11416

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Emissions of biomass-burning aerosols from the Indonesian region are known to
vary in response to rainfall anomalies associated with the El Ni{ñ}o Southern
Oscillation {(ENSO).} However, the effects of these rainfall anomalies on
regional aerosol burdens and radiative forcing have not been investigated. In
this study, we simulate the effects of {ENSO-related} changes in (1) emissions
and (2) rainfall and circulation on the radiative forcing of Indonesian
biomass-burning aerosols. We find that rainfall and circulation anomalies, as
well as emissions, contribute substantially to the direct and first indirect
radiative effects.

We compare two experiments that are performed with the {CSIRO-Mk3.6}
atmospheric global climate model {(GCM).} The first experiment {(AMIP)} consists
of a pair of runs that respectively represent El Ni{ñ}o and La Ni{ñ}a
conditions. In these runs, the distribution of aerosols is simulated under
the influence of realistic Indonesian biomass-burning aerosol emissions and
sea surface temperatures {(SSTs)} for 1997 {(El} Ni{ñ}o) and 2000 {(La} Ni{ñ}a).
The second experiment {(CLIM)} is identical to {AMIP,} but is forced by
climatological {SSTs,} so that in {CLIM} meteorological differences between 1997
and 2000 are suppressed.

The comparison of {AMIP} and {CLIM} shows that the radiative forcing anomalies
associated with {ENSO} {(El} Ni{ñ}o minus La Ni{ñ}a) are substantially stronger
when {ENSO-related} {SST} anomalies are taken into account. This is true for both
for the direct and the first indirect effects. {SST-induced} changes in
rainfall and wind fields enhance the anomaly of aerosol burdens over
Indonesia and the equatorial Indian Ocean. This, in turn, has an indirect
effect on cloud properties due to changes in the concentration and radii of
cloud droplets.

Our results suggest that the direct and indirect radiative effects of
Indonesian biomass-burning emissions would be underestimated if feedbacks of
{ENSO-related} {SST} variations on radiative forcing are not taken into account.

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  • A. Chrastansky

  • L. D. Rotstayn

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