The role of the intraseasonal oscillation (ISO) on the seasonal and interannual variations of the Indian summer monsoon is investigated based on the analysis of observational data. It is shown that the ISO significantly contributes to the establishment of low-level westerlies during the monsoon onset and developing periods. The effect of the ISO on the annual cycle of the monsoon is through nonlinear eddy momentum transport. On the interannual timescale, the Indian summer monsoon rainfall exhibits a significant out-of-phase relationship with the ISO intensity over the Indian monsoon region. In strong ISO years it appears the weak monsoon when there is an abnormal high over the India subcontinent in the lower troposphere. In weak ISO years there exists an abnormal low and the strong monsoon ap-pears.
