A possible triggering process of East-West asymmetric anomalies over the Indian Ocean in relation to 1997/98 El Nino

Anomalous east-west asymmetric anomalies were seen in sea surface temperatu re (SST), and convective activity over the equatorial Indian Ocean during O ctober-December 1997. Using NCEP/NCAR reanalysis and sea surface height dat a obtained from TOPEX/POSEIDON satellite altimeter, its triggering process and strengthening mechanism are identified. The climatological wind over the equatorial Indian Ocean exhibits different seasonal cycle between its western and eastern region. During the summer m onsoon season, westerly wind prevails over the western Indian Ocean, on the other hand, easterly wind is dominant over the eastern Indian Ocean at alm ost the same time. During the 1997 summer, divergent easterly wind anomalie s were obvious over the equatorial Indian Ocean due to a warm episode of th e Fl Nino, which weakened (accelerated) the climatological westerly (easter ly) wind over the western (eastern) Indian Ocean. As a result, the east-wes t SST contrast was produced in the succeeding autumn through changing evapo rative cooling and upwelling. Corresponding to these SST changes, the conve ctive activities were enhanced (suppressed) over the western (eastern) Indi an Ocean and actual wind became easterly in place of climatological westerl y wind during October-December 1997. The above easterly anomalies induced westward-moving downwelling Rossby wav es, and led to the maximum SST in January 1998 in the western Indian Ocean. On the other hand, eastward-moving downwelling Kelvin waves were generated after the termination of easterly wind anomalies, which were consistent wi th the SST warming in the eastern Indian Ocean for the period February-June 1998. In this manner, a coupling process between the modulated Walker Circulation associated with the Fl Nino event acid the monsoon circulation from summer to autumn is a crucial factor for inducing the above asymmetric anomalies. Moreover, the oceanic waves are found to be closely related with enhanceme nt of these asymmetric structures.