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4. Ozone change in 1997 El Nino

We conducted a simulation on the tropospheric ozone change in September and October 1997. Ozone enhancement in Indonesia during this period was reported in Fujiwara et al.,[1997], and elevated CO levels over Indonesia were also observed(Sawa et al., 1999). Satellite derived data from EP TOMS (Chandra et al., 1998) also indicates these changes(Figure 4).

In addition to the extensive forest fire event occurred in Indonesia, Meteorological changes in convection, humidity, precipitation, and transport from the stratosphere cold be big contributors to observed ozone change. In this simulation, SST data and ECMWF wind and temperature data of 1996, 1997 were used as nudging input to the GCM for a direct comparison to Chandra et al.,1998. Emission change due to extensive forest fire in Indonesia is not included in order to evaluate the effect of meteorological changes itself. Figure 5 shows simulated TCO(Tropospheric Column Ozone) anomaly corresponding to Figure 4. An asymmetrical dipole centered near the date line with TCO is well reproduced. It should be noted that simulated TCO enhancement in Indonesia is underestimated just because emission change associated with the extensive forest fire in Indonesia is not considered in this simulation.

Figure 6 shows the simulated difference of specific humidity (g/kg), ozone chemical lifetime change rate (%) between October 1997 and October 1996. It is clearly seen that chemical life time of ozone over Indonesian region is 2 or 3 times longer in October 1997 than in October 1996. This is just correspondig to much drier condition in Indonesia.

Figure 7 shows ozone concentration (ppbv) which originates in the stratosphere over Indonesian region for October 1997 and October of climatological run of the model. It is clear that more ozone intruded to the lower latitude from the mid latitude in both hemisphere in October 1997 compared to the climatological condition. This could be associated with low convective activity and low humidity in the Indonesian region during this period. These changes in humidity, convection and transport of stratospheric ozone consequently caused the tropospheric ozone change seen in Figure 5.