Air samples at 8--13 km were collected regularly using a commercial airliner to obtain long-term measurements of carbon monoxide (CO) mixing ratio in the upper troposphere over the western Pacific between Australia and Japan during April 1993--December 1997. The measurements in 1997 clearly reveal an anomalous CO increase during September to November in the Southern Hemisphere, with a maximum of 320--380 ppb around 20 ¿S in October. Tropical biomass burning, not urban/industrial emissions, was the main source for the enhanced CO in 1997. A similar southern-spring increase due to biomass burning was observed in previous years. The peaks showed a large interannual variation associated with the El Ni¿o/Southern Oscillation (ENSO) events. The largest CO spring peak appeared during the strong El Ni¿o even in 1997, while the weak La Ni¿a year of 1996 was marked by a largely suppressed CO spring peak. The outgoing longwave radiation (OLR) anomaly is largest during the El Ni¿o events indicating that the events cause a longer drought in the tropics and significantly influence the enlargement of biomass burning in tropical Southeast Asia. Thus the most likely cause for the ENSO-cycle CO variability is a year-to-year change of biomass-burning emissions mainly from Southeast Asia. The appearance of the CO spring peak in the southern subtropics is discussed on the basis of the possible long-range transport of biomass-burning CO from Southeast Asia to the upper troposphere over the western South Pacific. ¿ 1999 American Geophysical Union