Analysis of tropical tropospheric ozone, carbon monoxide, and water vapor during the 2006 El Nino using TES observations and the GEOS-Chem model

Elevated levels of tropical tropospheric ozone (O-3) and carbon monoxide (CO) and decreased water (H2O) vapor were observed by the Tropospheric Emission Spectrometer (TES) in the region of Indonesia and the eastern Indian Ocean during the coincident positive phases of the El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) in late 2006. Using the chemical transport model GEOS-Chem, we show that the elevated CO results from increased biomass burning in Indonesia during the ENSO/IOD-induced drought and quantify the effect of the fires and other factors on O-3. In the region of highest CO (similar to 200 ppb), the contribution of the fires to enhanced O-3 is similar to 45% in October, similar to 75% in early November, and only 10% in December. More lightning in late 2006 compared to 2005 causes an increase in O-3 of a few parts per billion. Dynamical changes increase O-3 over a larger region than fire emissions which mainly increase O-3 at 10 degrees N-10 degrees S in October and November. The model matches the O-3 anomaly in October but underestimates it in November and December, which we ascribe to overly active convection in the model in late 2006, based on an analysis of outgoing longwave radiation (OLR) data. An underestimate of NOx emissions from soils may also contribute to the disparity at the end of the year. A dramatic decrease in O-3 in late 2006 in equatorial Africa and the western Indian Ocean is reproduced by the model and is caused by highly enhanced convection in 2006, likely associated with the IOD.