Monsoon is the result of the interaction of regional and planetary factors, both at surface and in the upper troposphere. Recent research has revealed that there seems to be a link between meteorological events which are separated by long distances. They are known as meteorological teleconnection. Teleconnections are defined as linkages over great distances of atmospheric and oceanic variables.
ROLE OF JET STREAMS & TIBETAN PLATEAU
Upper Air Circulations over Tibetan Plateau and position and intensity of sub-tropical westerly jet stream and tropical easterly jet stream play a significant role in the onset, withdrawal and intensity of Indian monsoon.
Upper Air Circulation and Monsoon
During winter season, the sub-tropical westerly jet is bifurcated into two branches. The northern branch occupies a position to the north of the Tibetan Plateau and the southern branch is located over north India to the south of the Himalayas.
In the upper troposphere a ‘high’ pressure system (anticyclonic conditions with clockwise air circulation) develops towards south of southern branch of sub-tropical jet stream over Afghanistan and north-west Pakistan. Consequently, the winds tend to descend over the north-western parts of India, resulting into atmospheric stability and dry conditions.
The sub- tropical westerly jet streams also help western disturbances to enter the Indian sub- continent and affect its weather. On an average 4-6 disturbances (temperate cyclones) per month pass over northern India between November to April. These disturbances result into snow fall in western Himalaya and rainfall in the Great Plains and provide moisture to Rabi season crops.
During the summer season ( during months of April, May and June) due to shift in the overhead position of sun, low pressure areas develop at the surface near Peshawar (Pakistan) and north-west India. The winds descending from the upper air high pressure restrict ascend of winds from the surface low pressure.
This results into warm and dry weather conditions. This is why the months of April and May (hottest month) are dry despite high temperature and evaporation. Contrary to it, upper air low pressure is formed in the eastern Himalayan region due to upper air seasonal easterly jet streams. Due to these conditions the winds coming from southern Myanmar are forced to ascend and produce rainfall in Myanmar, Bangladesh and North East India.
After the first week of June the southern branch of sub-tropical westerly jet steam disappears and only northern branch operates to the north of the Tibetan plateau.
This results into the development of a dynamic depression over north western part of Indo-Pakistan. As this dynamic depression gets established over the thermal depression present in this area, burst of monsoon takes place. M.T. Yin (1949), while explaining the origin of monsoon stated that the burst of monsoon depends on upper air circulations.
P. Koteswaram (1952) established relationship between upper air circulations and atmospheric conditions over Tibetan plateau. He concluded that the fact that northward movement of the sub-tropical jet stream is the first indication of the onset of monsoon over India.
In summer season the Tibetan Plateau gets heated and acts as a high altitude heat source which produces a thermal anti- cyclone over this region (in the upper atmosphere).
This anti-cyclone weakens the westerly sub-tropical jet stream south of the Himalayas, and gives rise to the tropical easterly jet stream at 80º E longitude and intensifies the high pressure cell over the Indian Ocean. Thus a surface pressure gradient is produced from the Indian Ocean towards India and it activates south-west monsoon.