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Jet stream is the most prominent movement in upper level westerly wind flows; irregular, concentrated, meandering bands of geo-strophic wind, travelling at speeds of 300 to 400 kmph. The jet streams are high altitude (9000—12000 m) westerly winds between middle latitudes (summer 35°N-45°N; winter 20°N- 35°N) in the Northern Hemisphere. Recent researches have shown that these winds exert considerable impact on surface weather conditions.
The influence of jet streams on the origin and development of Indian monsoons may be appreciated from the f ollowing description of weather phenomena during the summer and the winter seasons. The upper air westerly jet streams are extended upto 20°N-35°N (Nagpur, Raipur latitudes) due to equator-ward shift of upper air north polar whirl during northern winter (October to February). In the winter season, the upper air westerly jet streams are bifurcated into two branches due to physical obstruction of the Himalayas and Tibetan Plateau. One branch is located to the south of the Himalayas, while the second branch is positioned to the north of the Tibetan Plateau (Fig. 4.7). The upper air high pressure and anticyclonic (with clockwise air circulation) conditions are developed in the troposphere over Afghanistan and Pakistan. Consequently, the winds tend to descend over the north-western parts of India, resulting into the development of atmospheric stability anti dry conditions. Besides, the upper air westerly jet streams also cause periodic changes in general weather conditions because they lie over the temperate low pressure (cyclonic wave) which moves from west to east under the influence of upper air westerly jet streams across the
Mediterranean Sea and reach Afghanistan, Pakistan and north west India. These storms are not frontal cyclones, but waves which move at the height of 2000 metres from the mean sea level, while at the surface they are north east trade winds. The arrival of these temperate storms (western disturbances) causes precipitation leading to an abrupt decrease in air temperature. The weather becomes clear after the western disturbance passes away. On an average, 4 to 6 cyclonic waves reach north-western India between October and April each year. They (the western disturbances) affect weather conditions during the winter season up to Patna (Bihar) and give occasional rainfall which is highly beneficial for the standing rabi crops, (wheat, barley, mustard, gram, lentil, etc.).
The tropical easterly jet stream extends far to the north of Tibet and the air flow is roughly along the Kolkata-Bangalore axis. These upper air easterlies descend into the permanent high pressure area formed over the southern Indian Ocean. This naturally intensifies the ‘High’ already present there. It is from this high pressure cell that the onshore winds start blowing towards the thermally induced low pressure area, developed in the northern part of the Indian subcontinent. After crossing the equator such winds become south-westerly and are known as the south-westerly summer monsoons. These surface winds have vast potentiality for south-westerly summer monsoon and precipitation. It is, therefore, clear that the strength of the easterly jet stream is directly related to the intensification of permanent ‘High’ formed over the southern Indian Ocean. Since this high pressure makes the pressure gradient steeper, so it is the main causative factor for determining the vigour of the summer monsoon.
During the summer season in the Northern Hemisphere, low pressure areas develo p at the ground surface near Peshawar (Pakistan) and north-west India due to intense heating of ground surface during April, May, and June. But as long as the position of the upper air jet stream is maintained above the surface low pressure (to the south of Himalayas), the dynamic cyclonic conditions persist over Iran, Afghanistan, Pakistan, and north-west India. The winds descending from the upper air high pressure obstructs the ascent o I winds from the surface low pressure areas, with the result that the weather remains warm and dry. This is why the months of April and May are generally dry and rainless in spite of high temperatures (low pressure on land) and high evaporation. It may be pointed out that monsoon arrives in Myanmar and north east Indian states in May or early June. Upper-air low pressure is formed to the east of the eastern limit of the Himalayas due to upper air easterly jet streams, with the result that the winds coming from the south of Myanmar are forced to ascend and yield copious rainfall. The Myanmar monsoon also affects Bangladesh and the hilly states of north-east India which receive pre-monsoon showers in the months of April, May, and the first week ofjune.