Abstract: Based on spectral analysis and EOFs,the characteristics of low frequency oscillation are studied for the upper cross-equatorial flow and lower equato-rial westerly during 1986 Asian summer monsoon. The main EOFs in the filter series for the two monsoon flows well represent the spatial characteristics of the oscillation. The phase of the time series of EOFs determines the specific time for the active/break periods of the summer monsoon in East Asia. The lag correlation reveals a close relation of the meridional exchange in the 500 hPa transfer of angular momentum in the northern mid-and-low latitudes to the low frequency activity of the summer monsoon in East Asia.