The influence of the 2006 Indonesian biomass burning aerosols on tropical dynamics studied with the GEOS-5 AGCM

Lesley E. Ott, Bryan Duncan, Steven Pawson, Peter Colarco, Mian Chin, Cynthia Randles, Thomas Diehl, Eric Nielsen

Every year, large quantities of gases and particles (known as aerosols) are released into the atmosphere by fires. In Indonesia, fires are often set deliberately to convert swamp forests to agricultural land. During El Niño episodes, drought conditions in Indonesia allow these fires to spread out of control, consuming large amounts of carbon-rich peat deposits beneath the forests. An Indonesian fire outbreak that occurred between August and November 1997 was estimated to release approximately one third as much carbon monoxide as is released annually from fossil fuel combustion worldwide. Aerosols produced by the fires have been shown in previous studies to strongly influence the transmission of radiation in the region, cooling Earth's surface and the top of the atmosphere while warming the atmosphere itself.

A similar, though smaller, fire outbreak occurred in Indonesia in 2006. In this study, we use NASA's Goddard Earth Observing System, Version 5 (GEOS-5) atmospheric general circulation model to study the impact of aerosols produced by the 2006 Indonesian fires on atmospheric temperature, moisture, and circulation in the region. The influence of aerosols was identified by conducting two sets of simulations, one set which included the direct radiative effects of aerosols produced by the Indonesian fires and one set that did not, and comparing the results. This analysis shows that aerosols produced by the 2006 fires increased temperatures over Indonesia with the greatest increases occurring during October and November between 7.5 and 14 km above the surface. Because warmer air tends to be more buoyant, increases in temperature caused by aerosols strengthen upward motion over Indonesia. Changes in vertical motion are accompanied by changes in horizontal winds. When Indonesian aerosols are present, winds converge over Indonesia at low levels and diverge at upper levels. The aerosol-induced changes in vertical and horizontal winds alter the distribution of trace gases, such as water vapor and carbon monoxide, in the region. The amount of water vapor in the atmosphere increases by as much as 25%% at 7.5 km elevation. Peak increases in carbon monoxide are found just below the tropopause, the boundary between the troposphere and the stratosphere, and can be as large as 50% in October. The results also suggest that aerosol heating can increase the transport of pollution from fires to the lower stratosphere. This research represents a significant contribution to understanding the physical processes that determine the chemistry and composition of the upper troposphere/lower stratosphere region which is critically important in determining Earth's climate.

figure 1 top
figure 1 bottom
Figure 1. Average CO mixing ratio increase due to Indonesian aerosols during September-November 2006 at 150 hPa in the upper troposphere (top) and 100 hPa in the lower stratosphere (bottom). Increases are calculated by subtracting the mean of simulations that did not include the aerosols from the mean of simulations that did (both sets of simulations include CO produced by the Indonesian fires). Only statistically significant grid column differences are shown. Grey contours show mean CO mixing ratios from the set of simulations that included Indonesian aerosols.

Reference

Lesley E. Ott, Bryan Duncan, Steven Pawson, Peter Colarco, Mian Chin, Cynthia Randles, Thomas Diehl, Eric Nielsen, 2009: The influence of the 2006 Indonesian biomass burning aerosols on tropical dynamics studied with the GEOS-5 AGCM, J. Geophys. Res. -Atmospheres (submitted). [PDF]