Indian Monsoon is one of the most prominent monsoon systems around the world. It is depicted from the variation and amount of India’s annual rainfall. Its effects are felt by India, the Indian subcontinent and the neighbouring water bodies of the Arabian Sea, the Bay of Bengal and the Indian Ocean. During the cold months, the direction of the monsoon is from the north-east, known as the north east monsoon in India, while during the time of warmest months of the year the monsoon wind blows from the southwest hence known as the south west monsoon in India. This entire process leads to rainfall in India received during June and July.
The Beginning and Development of the Indian Monsoon.
Rainfall in India begins with the westerly winds that frequently occur throughout the year almost constantly, around the area near India at the Equator. On the other hand, the surface easterlies reach the latitudes near 20° N in February with a strong northerly component. Moving forward they retire north-side with serious changes in the upper-air circulation. During this time one season of monsoon ends and the next one begins.
The high-sun season comes in late March when it reaches the Equator as the wind further moves north. This move takes with it the atmospheric instability, rising and turbulent clouds and rain. Across northern India, the subtropical jet stream towards the west controls the air-flow with the surface winds flowing from the north-east. After March, during April, the high-sun moves northward. This is the time when the summer season is ongoing in the Northern hemisphere. This is also summer time in India and as gradually time progresses the country becomes prone to heating as the cool winds from the north (the central lands in the Asian continent) are obstructed by the Himalayan mountain ranges and other highlands. The three areas around which there is a relative increase in temperature of the troposphere are the southern Bay of Bengal, Plateau of Tibet, and the trunks of various other dry peninsular regions. All together form a very wide and large heat-source region. In the area over the southern Bay of Bengal, the atmospheric pressure is in-between 500-100 millibar pressure level because of the release of condensation heat from the cumulonimbus clouds along the intercontinental convergence zone. Opposite of this condition a heat sink is created over the southern Indian ocean as the cloud-free air cools down any radiation emitted from the surface. Because of the difference between the heat regions, the monsoon winds flow from the heat sink to the heat source. After April, the southwest monsoon gets well-established in May above Sri-Lanka.
In May, the drier surface of Tibet absorbs the heat from the radiation of sun-rays and it is transmitted to the air into the troposphere above it. Following this, an anticyclonic cell rises at 6000 feet above the surface which in turn causes a strong east wind flowing in the troposphere over north India. There is a sudden change of the subtropical jet stream of its course to the northern side of the anticyclonic ridge plus the highlands with brief intervals of southward wind flow. There is a change in the upper-tropospheric circulation above the northern region of India from the jet stream being westerly to easterly coinciding with the turnaround of the vertical temperature and pressure gradients with the range in-between 600-300 millibars. The inverted triangular peninsular region of India gets heated as the sun progresses towards the north. The accelerating heat in combination with the heat being transported by the winds provides the base for the initial monsoonal activity above the Arabian Sea. When the relative humidity of the coastal districts of India rises above 70% which leads to some rain. Although the air was within 1500 meters above the heated land it was kept down by the east wind. This is the period of late May when there are also thunderstorms in the area, even though the rainfall hasn't started yet.
The Period of High Rainfall in India
The jet stream of the east with 150-100 millibars pressure reaches the highest speed to the southern side of an anticyclonic ridge with a deviation of 15° N from China through India. This position of the jet stream controls the situation of the monsoonal rains. An unstable and strong southwestern surface flow provides 80% of the humidity which is the major burst of the monsoon bringing the south-west monsoon. Even though the atmospheric pattern moves equally over the subcontinent, the amount of rainfall in India varies from place to place and year to year.
When the early monsoon wind gets accumulated against the Western Ghats, it leads to most of the group of clouds providing major rainfall in the entire region beginning of summer monsoon in India. As the clouds are pushed against the hills some of the inland air absorbs some of the water. The complex pattern of rainfall in India is distributed variable from the region because of various factors such as topography. The oceanic air flowing below 6000 metres is deflected because of the Coriolis effect. The oncoming air stream gets unstable due to fast convection above the hot land. Because of the thunderstorms and release of the latent heat from the towering clouds, the upper-tropospheric warm air travels northwest from ocean to land. But the main component of air above the 9000 metres altitude is a strong eastward flow.
The monsoon gets well-established in the later part of June-July around the 6000 metres altitude. Moist, cloudy and warm weather is spread all over India bringing varying degrees of rainfall in India due to topographical differences. There are huge differences in the average rainfall of India. Examples include 1300 metres of rainfall in Khasi Hills, an average of 2500 metres in Cherapunji, etc. In the months of July-August, low-pressure waves occur in the monsoonal air which is now travelling from east to west. The easterly jet streams provide bursts of speed strengthening the low-level monsoon airflow. This causes an increase in the rainfall and wider distribution than June.
When the east wind moves north over the southern Himalayas heavy downpour occurs in those regions leaving the central and south region dry. If the wind blows along the south face of the Himalayas it brings dry weather in the north. On the other hand, the southwest monsoon air flowing over the Indus plain cannot hold moisture and hence the area beyond remains dry creating a new heat source.
Retreating Indian Monsoon
As August proceeds the intensity and duration of the sunshine decrease causing the temperature to fall and this results in the decrease in India raining due to the south-west monsoon, starting at the end of the monsoon in north India. In September the dry air from the North circulates in the west of the highlands and above northwestern India. By October there are variable winds all over India covered with northern air. As the surface flow turns to the northeast it causes the winter monsoon in India or the northeast monsoon in October - December in the south and southeastern parts. With the retreat of the moist winds, the monsoon time in India comes to an end.
With most of India now in the sunny, dry, and dusty season, such weather spreads through Punjab in November, Central India, Bengal and Assam in December, Deccan in January and south Deccan in February. Following this, the cycle starts all over again.
The Indian meteorological department is the body responsible for weather forecasting and monitoring and predicts the monsoon periods. It collects data from different weather patterns across the world for more accuracy. For example, for the forecast of early June, a positive correlation of the South American pressure and Indian upper-wind data is accounted for in April. Other such agencies include Skymet India which is a private agency of weather forecasting and rainfall in India prediction.
Thus, Indian monsoon patterns are very complex and as it can be inferred from the article a contribution of many factors making a geographical convergence of moist wind belts to bring rain to the country. Although rainfall in India is continuous from June to December due to different types of monsoon in India, June - September/October are considered to be the major rainy months in India. The Indian meteorological department of the Government of India is the major agency that tries its best to follow, and understand the dynamics of wind patterns and predicts the forecast through a meticulous examination of various factors.