Do biomass burning aerosols intensify drought in equatorial Asia during El Niño?
During El Ni˜ no 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 cli- mate by using an atmospheric general circulation model. Satellite observations during 2000–2006 indicated that El Ni˜ no-induced regional drought led to increases in fire emis- sions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investi- gate how climate responded to this forcing. We conducted two 30 year simulations in which monthly fire emissions were prescribed for either a high (El Ni˜ no, 1997) or low (La Ni˜ na, 2000) fire year using a satellite-derived time se- ries 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 ef- fects of anthropogenic fire by analyzing the differences be- tween the high and low fire simulations. Fire aerosols re- duced net shortwave radiation at the surface during August– October by 19.1±12.9Wm−2 (10%) in a region that encom- passed 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. Tropo- spheric heating from black carbon (BC) absorption aver- aged 20.5±9.3Wm−2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and in- creased atmospheric heating reduced regional precipitation by 0.9±0.6mmd−1 (10%). The vulnerability of ecosystems to fire was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satel- lite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensifies drought stress during El Ni˜ no.