| A new diagnostic approach based on wavenumber-frequency spectral analysis was proposed for the boreal summer intraseasonal oscillation (ISO).; The strongest interannual variations of the ISO are found in the off-equatorial western North Pacific, which is closely linked with ENSO. ENSO affects the northwestward-propagating ISO in the WNP by changing the mean vertical shear in the zonal wind. During July–October in El Niño developing years, the easterly vertical zonal-wind shears over the tropical western Pacific are considerably increased, in contrast to a westerly-vertical-shear mean state in the La Niña years. The easterly vertical shear promotes development and northwestward emanation of Rossby waves away from the equatorial western-central Pacific in the lower troposphere, reinforcing the WNP ISO.; In the Indian summer monsoon region, the life cycle of the northward-propagating ISO was revealed by QuikSCAT and TRMM satellite data. The northward propagation takes place in two steps: At the first step, Rossby waves are emanated from the equatorial convection and propagate northwestward to 15°N, 70°–90°E in less than 10 days. There is also a Rossby-wave response center in the southern hemisphere. During the second step, the southeast-northwestward titled rainband moves northeastward to the inner land region and decays. The second step takes about 20 days.; The weak-ISO years are characterized by isolated intraseasonal convection at the equator and 20°N, whereas the two regions are connected by the Rossby-wave-emanation process in the strong ISO years.; The seasonal activity of the northward-propagating ISO is found significantly influenced by the mean convection in the equatorial eastern Indian Ocean (EEIO). ENSO modifies the northward-propagating ISO mainly through influencing the mean convection in the EEIO. The ISO is significantly weaker in El Niño developing May–Sept, and stronger in the La Niña summer.; The seasonal activity of the northward-propagating ISO is not related to the overall performance of the Indian summer monsoon.; The interannual variations of the MJO have a seasonal dependence. The May–Jul MJO is closely related with ENSO. At El Niño 's developing stage, the MJO is enhanced. This is in a sharp contrast with the wintertime MJO, which is not sensitive to the ENSO condition. |
