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Based on FGGE Level IIIb data, the structural features of 40-50 day oscillations over ann extensive region (30*S-30*N, 30*E-150*W) during the 1979 summer are detailed.The analysis confirms earlier investigations that these low frequency modes are primarily associated with the zonal wind oscillations.These 40-50 day perturbations propagate northward and eastward, which is most clearly defined over the monsoon region north of the equator from 60* to 150*E. The monsoon region is characterized by prominent spectral peaks in the 850mb meridional winds with periods shorter than 10 days, probably reflecting the activities of monsoon disturbances.However, the local Hadley circulation, as defined by averaging the meridional component of the wind between 60* and 150*E, exhibits a distinct spectral peak in the period range of 40-50 days.Similarly, the square of the meridional winds, which is a measure of synoptic-scale disturbance activity, also changes with a period of 40-50 days. These features, which are similar to the group velocity phenomena, are pronounced only over the central monsoon region (10*-20*N, 60*-150*E). The low frequency modes propagate northward and become most intensified near 10*-20*N through mutual interaction between synoptic-scale disturbances, the local Hadley circulation, and the tonal mean flows over the monsoon region.At the equator, the 40-50 day zonal wind pertubations propagate systematically eastward (500km/day) and upward (0.7km/day). In the equatorial region, the low frequency oscillations owe their existence to a lateral geopotential wave-energy flux from the monsoon region, which represents the major energy source for 40-50 day perturbations via the conversion from potential to kinetic enegy.Compared to the equator, the phase propagation of zonal wind perturbations along 15*N, although moving eastward, is not as systematic.At this latitude, zonal wind perturbations are pronounced in the lower troposphere over the monsoon region, 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) perturbations over to the Arabian Sea region.The downward phase of westerly (easterly) modes corresponds to the commencement of "active" ("break") monsoons over South and Southeast Asia.

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During the early summer of 1979, massive changes occurred in the wind and temperature fields over extensive areas of the Eurasian continent north of 15*N. Among the areas, the most significant one is the Afghanistan-western Tibetan Plateau region which was characterized by an abrupt increase of 300 mb temperature and intensification of 300mb anticyclone at around 4 June; i.e., about two weeks prior to the monsoon onset over India. At the same time, the strong upper jet stream in the westerlies at 300mb exhibited a distinct northward shift from about 30*N to 35*-40*N and the easterlies at 700mb exhibited rapid establishment and intensification over extensive areas near 25*N. A similar northward shift of the 300mb jet stream also took place very far upstream (55*E) of the Tibetan Plateau on about 3 June. But, in contrast to Yin's early finding, this northward shift of the 300mb jet stream was not observed over the eastern Tibetan Plateau.Concurrently, rapid development of upper-tropospheric anticyclone and increase of temperature also occurred in association with the establishment of intense monsoon rains over the East China Sea-Japan region. The computations have shown that the increased temperature in these areas may be primarily caused by the adiabatic heating (*1.5* per day). The above-described changes were also accompanied by rapid weakening and retrograding of a long-persistent blocking high over eastern Europe in May and rapid development of a prominent ridge system over central Siberia. In short, our study has not only shown that the Eurasian continent underwent drastic changes in the circulation and temperature fields during the transitional season from spring to summer of 1979, but also indicated that these large changes could exert some effects on the onset of the summer monsoon over south Asia and east Asia.