Abstract:On the basis of FGGE Ⅲb data, the spatial distribution of 40-50day oscillations and features in their propagation over an extensiveregion (30°S-30°N, 30°E-150°W) during the summer of 1979 are dis-cussed in detail. The results confirm earlier investigations that theselow frequency modes are primarily associated with the zonal windoscillations. These 40-50 day perturbations propagate eastward and northward. However, the monsoon region is characterized by promi-nent spectral peaks in the 850 mb meridional winds with periodsshorter than 10 days, probably reflecting the activities of monsoondisturbances. At the equator,the 40-50 day zonal wind perturbationspropagate systematically eastward (600km/day) and upward (0.7km/day).The phase relationship between the zonal winds, geopotential heights,and large-scale apparent heat sources indicates that these equatoriallow frequency oscillations do not resemble atmospheric Kelvin waves.In contrast to what happens at the equator, the phase propagationof zonal wind perturbations along 15°N, although also being directedeastward, is not so systematic. At this latitude, zonal-wind pertur-bations are pronounced in the lower troposphere over the monsoonregion and also in the upper troposphere over the western Pacific.As an integral part of E-W interaction between these two regimes,there occurs downward progression of westerly (or easterly) pertur-bations over the Arabian Sea region. The downward phase of wes-terly (or easterly)modes corresponds to the commencement of "active"monsoons (or monsoon "breaks") over South and Southeast Asia.