Introduction

It has been recognized for many years that studies of the monsoon must be intimately related to the study of the seasonal climatology of the general atmospheric and oceanic circulations. As far as the atmospheric component is concerned, the main surface feature of the climate of south Asia is the changeover in early summer from the winter regime with its dry north to northeasterly wind to the summer rain-bringing southwesterly monsoon. The winter regime becomes re-established again during the early autumn. With upper-atmospheric data becoming regularly available from the late 1940s, this surface flow transition has been found to be linked to an equally substantial change in the upper-level flow over the whole of the low-latitude belt. This manifests itself in an intensification and northward movement of the main high-pressure region over southern Asia with the onset of the summer monsoon. Particularly dramatic is the generation and maintenance of an easterly jet-like flow at 10° to 15° N across the whole region from the South China Sea to east Africa with a maximum speed of about 30 m s-1 at a height of about 13 km over Sri Lanka. Thus in the same way as the seasonal changes of the low-level features of the atmosphere on the global scale are largely dominated by the circulations corresponding to the winter and summer Asiatic monsoon regimes, the high-level large-scale features are similarly dominated by the associated monsoonal events.