The present paper contains the results of the study of a synoptic situation over India during a period of 8 days in which normal monsoon conditions were followed by a break and were further followed by a return to normal monsoon. This study has shown that during breaks in the monsoon, a trough in the middle latitude westerlies with a jet embedded in it increases considerably in its amplitude and gets retarded as it moves into the Tibetan Plateau on account of its entering a region of weaker basic current. The large‐amplitude trough weakens or destroys completely the Tibetan high at the 500 mb level and appreciably weakens the easterlies at the higher levels to the south of the Himalayas. Protruding into India and Pakistan, the large amplitude trough contributes to the development of heavy rainfall along and near the foot of the Himalayas which is characteristic of the break conditions. The southward protrusion of the trough and its subsequent movement eastward, is also indirectly responsible for a corresponding shift of the high over Iran and Arabia, which, in its turn, seems to lead to the formation of a secondary jet core in the easterlies west of 80° E between 20° N and 25° N. Consequent on the above developments, the westerly jet which had retreated to the north of the Himalayas at the time of the onset of the monsoon reenters the Indo‐Pakistan sub‐continent during the breaks. Thus we see during break conditions, the remarkable spectacle of two jets of entirely different types—the easterly and the sub‐tropical westerly—within a short latitudinal distance of each other and dynamically interacting with each other.
The above detailed study, supplemented by a general examination of the monthly mean 700 mb charts for the northern hemisphere for a period of 10 years, suggests that active monsoon over India is closely associated with high index circulation in middle latitudes over Asia and neighbourhood, while weak monsoon is closely associated with low index circulation over the same region.
To cite this article: C. Ramaswamy (1962) Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the subtropical westerly jet streams, Tellus,
This paper contains mainly the results of a synoptic and climatological study of the largescale convection in northern India and Pakistan during the three months preceding the onset of the southwest monsoon. It has been shown that the sea-level and lower tropospheric charts give little clue to the development of the large-scale convection and that the latter is overwhelmingly determined by the divergence in the waves in the sub-tropical jet-stream. It has further been shown from detailed synoptic evidence that nor'westers, andhis and the majority of the thunderstorms without squalls in northern India and Pakistan in the pre-monsoon period are fundamentally the same phenomenon. The role of cold-air advection in the middle and upper troposphere in the development of large-scale convection has also been discussed. This study has further revealed that the regions of upper-divergence and convergence can be qualitatively located by identifying certain typical patterns on the high-level maps more than 12 hours before the usual time of commencement of convection and that, consequently, these maps can be used as effective tools in the issue of area-warnings against thunder in general and nor'westers and andhis in particular.A general study has also been made of the large-scale convection in southeast Australia, southeast Brazil and southeast United States and the similarities between the large-scale convection in these countries and in Indo-Pakistan have been brought out. O n the basis of these studies, it has been suggested that the jet stream plays the very important role of producing large-scale convection in the subtropics all over the world wherever it over-runs on its equatorward side, moist air possessing a high degree of latent instability.
The paper contains an analysis of the daily synoptic charts of July 1972 between 50° N and the South Pole and between 50" E and 160° E with reference to the activity of the southwest monsoon (summer monsoon) over India. The month commenced with a spell of normally active monsoon for 8 days and ended with a spell of large-scale break in the monsoon for 9 days with two transitional phases in between. There was nearly simultaneous development in the middlelatitude westerlies in the middle and upper troposphere in both the hemispheres especially during the break period. Blocking ridges over and near the Tasman Sea extended far towards the southsoutheast during the break conditions. The pronounced troughs and ridges observed in the meridional profiles along 35' N and 35O S , especially along the latter during break conditions, have been discussed and certain tentative explanations have been put forward regarding the developments in the Southern Hemisphere.
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