Interannual variations of the intraseasonal oscillation in the South Asian Summer Monsoon Region

It is noted that the behavior of the intraseasonal oscillation (ISO) of the south Asian monsoon varies from year to year. An index representing seasonally averaged ISO activity is developed using outgoing longwave radiation data for the period 1975-97. Interannual variations in ISO activity are found to be related to year-to-year changes in the number of discrete events rather than to changes in the characteristic period. Summertime ISO activity exhibits a reasonably strong inverse relationship with Indian monsoon strength but not with total south Asian monsoon strength primarily because of a lack of correlation between ISO activity and the Bay of Bengal component of the south Asian monsoon. Over the 22-yr period examined here, the relationship between Indian monsoon strength and ISO activity is comparable to or even stronger than the well-documented relationship with El Niño-Southern Oscillation (ENSO). However, summertime ISO activity is found to be relatively uncorrelated with ENSO except for a weakly positive correlation at the beginning of the south Asian monsoon season. Therefore, the ISO activity-Indian monsoon relationship is essentially independent of the ENSO-Indian monsoon relationship. ISO activity is uncorrelated with any other contemporaneous or leading sea surface temperature variability.