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 1Department 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 *now at: University of Michigan, Ann Arbor, MI, 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.
We assessed the radiative and climate effects of anthropogenic fire by
analyzing the differences between the high and low fire simulations.
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 shortwave 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., 10, 3515-3528, doi:10.5194/acp-10-3515-2010, 2010.