The effect of ENSO-induced rainfall and circulation changes on the direct and indirect radiative forcing from Indonesian biomass-burning aerosols
CSIRO Marine and Atmospheric Research, Aspendale, Vic, Australia
The Centre for Australian Weather and Climate Research. A partnership between CSIRO and the Bureau of Meteorology, Australia
Abstract. Emissions of biomass-burning aerosols from the Indonesian region are known to vary in response to rainfall anomalies associated with the El Niño 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 Niño and La Niña 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 Niño) and 2000 (La Niña). 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 Niño minus La Niña) 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.