Abstract:Based on the 1979—2007 daily mean outgoing longwave radiation and 1960—2007 NCEP/NCAR daily reanalysis wind field, the composite analysis and wavenumber-frequency analysis are applied to the characteristics of the strong and weak summer monsoon years in the boreal summer intraseasonal oscillation (BSISO). It is shown that the BSISO energy spectrum is mainly distributed in the wavenumber-1 and wavenumber-2. In the tropics, BSISO mainly propagates eastward in the form of wavenumber-1, and in the higher latitudes, the westward propagation is clear; in the meridional direction it is the northward propagation of 30-60 d periods. In the strong(weak) summer monsoon years in the zonal direction the eastward propagation is enhanced(weakened) in the tropical region, whereas in the higher latitude of the north of 20 °N westward propagating ISO is weakened(strengthened); in the meridional direction the most significant difference is that the northward propagating ISO is strengthened (weakened) over the western Pacific. It is the difference of the propagating ISO in strong and weak summer monsoon years that may lead to the difference of the zonal wind field and convection in western and central Pacific, the propagating ISO causes an apparent difference in this large-scale atmospheric circulation anomalies.