Nonstationarity of the intraseasonal oscillations associated with the western North Pacific summer monsoon

The nonstationarity of the intraseasonal oscillations (ISOs) associated with the western North Pacific summer monsoon (WNPSM) is examined using a wavelet analysis of outgoing longwave radiation (OLR). Both the 10-20- and 30-60-day ISOs are found to display significant interannual modulations, and their relative strengths vary with time. The variation of OLR associated with a strong ISO, either 10-20- or 30-60-day, could be as large as 20 W m(-2) in magnitude. Case studies showed that the mechanism for development of low OLR may differ in individual years, and that the 10-20-day ISO, the 30-60-day ISO, and the seasonal cycle may each become dominant in different years.