On the 40-50 day Oscillations During the 1979 Northern Hemisphere Summer

[摘要] In July 1979, large-scale apparent heat sources exceeded 2°C per day over the eastern Arabian Sea, the northern Bay of Bengal, the central South China Sea, and the equatorial Pacific near the dateline. These heating centers are embedded in the monsoon trough at 850mb and are in good agreement with regions of strong upward motions, apparent moisture sinks, and small outgoing longwave radiation values.In May, the moisture supply for the rainfall near the west coast of Burma and Malaysia comes primarily from the Bay of Bengal, not from the Southern Hemisphere. In midsummer (June to August), the cross-equatorial moisture flux off the east coast of Kenya is not large enough to maintain the rainfall over South and Southeast Asia. Thus, evaporation over the Arabian Sea constitutes the key contribution to the moisture supply for monsoon rains.The northward and eastward passage of 40-50 day perturbations is related to the phase changes between active and break monsoons over central South and Southeast Asia. When active monsoons begin, the large-scale apparent heat sources Q1 and moisture sinks Q2 become above normal over the Arabian Sea. About 5 to 7 days later, both Q1 and Q2 reach their maxima over the Bay of Bengal. This is followed by the intensification of Q1 and Q2 over the South China Sea region about 5 days later. Regions of above normal Q1 and Q2 also propagate northward across the monsoon region. Similarly, regions of break monsoons with below normal Q1 and Q2 propagate eastward and northward.

[学科分类] 大气科学