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

by A. Chrastansky, L. D. Rotstayn
Atmospheric Chemistry and Physics ()
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Emissions of biomass-burning aerosols from the Indonesian region are known to\nvary in response to rainfall anomalies associated with the El Ni{ñ}o Southern\nOscillation {(ENSO).} However, the effects of these rainfall anomalies on\nregional aerosol burdens and radiative forcing have not been investigated. In\nthis study, we simulate the effects of {ENSO-related} changes in (1) emissions\nand (2) rainfall and circulation on the radiative forcing of Indonesian\nbiomass-burning aerosols. We find that rainfall and circulation anomalies, as\nwell as emissions, contribute substantially to the direct and first indirect\nradiative effects.\n\n\n\nWe compare two experiments that are performed with the {CSIRO-Mk3.6}\natmospheric global climate model {(GCM).} The first experiment {(AMIP)} consists\nof a pair of runs that respectively represent El Ni{ñ}o and La Ni{ñ}a\nconditions. In these runs, the distribution of aerosols is simulated under\nthe influence of realistic Indonesian biomass-burning aerosol emissions and\nsea surface temperatures {(SSTs)} for 1997 {(El} Ni{ñ}o) and 2000 {(La} Ni{ñ}a).\nThe second experiment {(CLIM)} is identical to {AMIP,} but is forced by\nclimatological {SSTs,} so that in {CLIM} meteorological differences between 1997\nand 2000 are suppressed.\n\n\n\nThe comparison of {AMIP} and {CLIM} shows that the radiative forcing anomalies\nassociated with {ENSO} {(El} Ni{ñ}o minus La Ni{ñ}a) are substantially stronger\nwhen {ENSO-related} {SST} anomalies are taken into account. This is true for both\nfor the direct and the first indirect effects. {SST-induced} changes in\nrainfall and wind fields enhance the anomaly of aerosol burdens over\nIndonesia and the equatorial Indian Ocean. This, in turn, has an indirect\neffect on cloud properties due to changes in the concentration and radii of\ncloud droplets.\n\n\n\nOur results suggest that the direct and indirect radiative effects of\nIndonesian biomass-burning emissions would be underestimated if feedbacks of\n{ENSO-related} {SST} variations on radiative forcing are not taken into account.

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