Based on the daily outgoing long-wave radiation(OLR) data of the National Oceanic and Atmospheric Administration(NOAA) from 1982 to 2009,by using EOF method,this paper investigated the intensity changes of the 20—70 d boreal summer(June-September;JJAS) intraseasonal oscillation(BSISO) and winter(December-next February;DJF) intraseasonal oscillation,also known as the Madden-Julian Oscillation(MJO).The results show that BSISO index has a significant intensifying trend during 1982—2009.On the other hand,little trend is found for boreal winter MJO index during this period.The wavenumber-frequency analysis method was applied to separate ISO(intraseasonal oscillation) into westward-propagated and eastward-propagated parts.For the eastward-propagated BSISO,a significant intensified trend is observed over tropical Indian Ocean or its action center.For the eastward-propagated MJO,there is no evident trend observed over its action center,except that a weakened trend is observed over southwestern tropical Indian Ocean(i.e.southwestern part of MJO action center).To gain insight into the different ISO characteristics,the tendencies of sea surface temperature(SST) and the vertical shear of zonal wind were analyzed.The results show that,in both seasons from 1982 to 2009,the global SST trends are similar,and thus they could not be used to explain the intensifying trend of BSISO.However,the lower-tropospheric easterly shear in boreal summer over tropical Indian Ocean has a decreasing trend,while the easterly vertical shear over the maritime continent is enhanced in winter.It is proposed that the reduced easterly vertical shear over tropical Indian Ocean favores the amplification of the eastward-propagated BSISO.The enhanced easterly vertical shear over the maritime continent might weakens the eastward-propagated MJO,but its impact is offset by the enhanced upward motion over the maritime continent.As a result,there is little trend for MJO in boreal winter.