Using NOAA/AVHRR products to monitor El Nino impacts: Focus on Indonesia in 1997-98

The development of the Fl Nino-Southern Oscillation (ENSO) in 1997-98, the most intense in this century, has been monitored by space- and ground-based observations. In this study, the authors present the signatures of ENSO im pacts on the surface-atmosphere system, as detected in satellite products t hat are routinely derived at NOAA from measurements by a single instrument on board NOAA polar-orbiting satellites-the Advanced Very High Resolution R adiometer (AVHRR). The Indonesian archipelago was selected to demonstrate h ow AVHRR products can be synergistically used to monitor interannual variab ility, such as caused by ENSO, on regional and global scales. The authors e xamined month-to-month changes in surface-atmosphere conditions over the re gion during July 1997-June 1998. The major ENSO impact over the Indonesian archipelago was a prolonged dry p eriod with anomalously low amounts of cloud, precipitation, and water vapor . The net effect of these changes was a significant increase in the absorbe d shortwave and outgoing longwave radiation fluxes. ENSO-induced drought co incided with the slash-and-burn agricultural season, which resulted in pers istent fires and smoke from biomass burning, covering larger areas, produci ng more smoke, remaining longer than during most dry seasons, and causing h ealth hazards for millions of people in Southeast Asia. Analysis of the imp act of fires on cloud microphysics confirms earlier suggestions that the ef fective cloud droplet size decreases due to smoke aerosols and cloud reflec tivity increases due to higher concentration of small droplets. Analysis of areas with active fires showed a decrease in both surface albedo and fract ional green vegetation as a result of intensive burning.