EFFECTS OF 1997-1998 EL-NINO ON TROPOSPHERIC OZONE AND WATER-VAPOR

This paper analyzes the impact of the 1997-1998 El Nino on tropospheri c column ozone and tropospheric water vapor derived respectively from the Total Ozone Mapping Spectrometer (TOMS) on Earth Probe and the Mic rowave Limb Scanning instrument on the Upper Atmosphere Research Satel lite. The 1997-1998 El Nino, characterized by an anomalous increase in sea-surface temperature (SST) across the eastern and central tropical Pacific Ocean, is one of the strongest EI Nino Southern Oscillation ( ENSO) events of the century, comparable in magnitude to the 1982-1983 episode. The major impact of the SST change has been the shift in the convection pattern from the western to the eastern Pacific affecting t he response of rain-producing cumulonimbus. As a result, there has bee n a significant increase in rainfall over the eastern Pacific and a de crease over the western Pacific and Indonesia. The dryness in the Indo nesian region has contributed to large-scale burning by uncontrolled w ildfires in the tropical rainforests of Sumatra and Borneo. Our study shows that tropospheric column ozone decreased by 4-8 Dobson units (DU ) in the eastern Pacific and increased by about 10-20 DU in the wester n Pacific largely as a result of the eastward shift of the tropical co nvective activity as inferred from National Oceanic and Atmospheric Ad ministration (NOAA) outgoing longwave radiation (OLR) data The effect of this shift is also evident in the upper tropospheric water vapor mi xing ratio which varies inversely as ozone (O-3). These conclusions ar e qualitatively consistent with the changes in atmospheric circulation derived from zonal and vertical wind data obtained from the Goddard E arth Observing System data assimilation analyses. The changes in tropo spheric column O-3 during the course of the 1997-1998 El Nino appear t o be caused by a combination of large-scale circulation processes asso ciated with the shift in the tropical convection pattern and surface/b oundary layer processes associated with forest fires in the Indonesian region.