Atmos. Chem. Phys. Discuss., 9, 23319-23348, 2009
www.atmos-chem-phys-discuss.net/9/23319/2009/
doi:10.5194/acpd-9-23319-2009
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Do biomass burning aerosols intensify drought in equatorial Asia during El Niño?
M. G. Tosca1, J. T. Randerson1, C. S. Zender1, M. G. Flanner2, and P. J. Rasch3
1Croul Hall, Department of Earth System Science, University of California, Irvine, CA, USA
2National Center for Atmospheric Research, Boulder, CO, USA
3Pacific Northwest National Laboratory, Richland, WA, USA

Abstract. During El Niño years, fires in tropical forests and peatlands in equatorial Asia create large regional smoke clouds. We characterized the sensitivity of these clouds to regional drought, and we investigated their effects on climate by using an atmospheric general circulation model. Satellite observations during 2000–2006 indicated that El Niño-induced regional drought led to increases in fire emissions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investigate how climate responded to this forcing. We conducted two 30 year simulations in which monthly fire emissions were prescribed for either a high (El Niño, 1997) or low (La Niña, 2000) fire year using a satellite-derived time series of fire emissions. Our simulations included the direct and semi-direct effects of aerosols on the radiation budget within the model. Fire aerosols reduced net shortwave radiation at the surface during August–October by 19.1±12.9 W m−2 (10%) in a region that encompassed most of Sumatra and Borneo (90° E–120° E, 5° S–5° N). The reductions in net radiation cooled sea surface temperatures (SSTs) and land surface temperatures by 0.5±0.3 and 0.4±0.2° C during these months. Tropospheric heating from black carbon (BC) absorption averaged 20.5±9.3 W m−2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and increased atmospheric heating reduced regional precipitation by 0.9±0.6 mm d−1 (10%). The vulnerability of ecosystems to fire was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satellite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensifies drought stress during El Niño.

Citation: Tosca, M. G., Randerson, J. T., Zender, C. S., Flanner, M. G., and Rasch, P. J.: Do biomass burning aerosols intensify drought in equatorial Asia during El Niño?, Atmos. Chem. Phys. Discuss., 9, 23319-23348, doi:10.5194/acpd-9-23319-2009, 2009.
 
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