Wind and Temperature Changes over Eurasia during the Early Summer of 1979

Abstract

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 300mb temperature and intensification of 300mb anticyclone at around 4 June; i.e., about two weeks prior to the monsoon onset over India. At the sametime, 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°C 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.