Monsoons are seasonal winds that reverse their direction with the change of season. Indian monsoon, the most unmistakable of the world’s storm frameworks, which basically influences India and its encompassing water bodies. It blows from the upper east during cooler months and turns around heading to blow from the southwest during the hottest months of the year.
• India receives south-west monsoon winds in summer and north-east monsoon winds in winter.
• South-west monsoons are formed due to the extreme low-pressure system formed over the Tibetan plateau.
• North-east monsoons are associated with high -pressure systems over Tibetan and Siberian plateaus.
• Countries like India, Indonesia, Bangladesh, Myanmar, etc. receive most of the annual rainfall during south-west monsoon season whereas South-East China, Japan, etc., during the north-east rainfall season.
About 75 percent of the total annual rainfall is from the Southwest Monsoon winds during the period from June to September.
The rainfall from the monsoon winds is highly variable and quite unreliable.
• The monsoon winds may reach India much before its due date or may be considerably delayed.
• The total amount of rainfall is either more than normal or much less than normal. Some parts of the country always face either the danger of floods due to excessive rainfall or drought and famine conditions due to scanty rainfall.
• The variability of rainfall in amount, time and space creates unstable conditions for agriculture, which hampers the economy.
• The rainfall occurs for a few months in the year, i.e. from June to September (the season of the Southwest Monsoons).
• The rainfall is basically torrential in nature. Even in the rainy season of about four months, the actual rainy days are 40 to 45 days only.
• The heavy downpour occurs with cyclones, which originate in the Arabian Sea and the Bay of Bengal. It results in floods and excessive soil erosion.
• The distribution of rainfall is largely controlled by the relief features.
• The economy and the lifestyle of the people depend largely on the amount and distribution of rainfall, as the whole country is predominantly agrarian.
• The differential heating of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure. The shift of the position of Inter-Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain. It is also known as the monsoon-trough.
• The presence of the high-pressure area, east of Madagascar. The intensity and position of this high-pressure area affect the Indian Monsoon. The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of low pressure over the plateau.
• The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
Normally when the tropical eastern South Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean experiences low pressure. But in EL-NINO years, there is a reversal in the pressure conditions and the eastern Pacific becomes a low-pressure area in comparison to the eastern Indian Ocean. This periodic change in pressure conditions is known as the Southern Oscillation.
The Indian Ocean Dipole (IOD), also known as the Indian Niño, is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.
A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia.
The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
MADDEN-JULIAN Oscillations: The MJO can be defined as an eastward moving ‘pulse’ of clouds, rainfall, winds, and pressure near the equator that typically recurs every 30 to 60 days. When it is over the Indian Ocean amid the Monsoon season, it brings great precipitation over the Indian subcontinent. On the other hand, when it observes a more extended cycle and remains over the Pacific Ocean, MJO brings terrible news for the Indian Monsoon.
• During the month of June, the sun shines vertically over the Tropic of Cancer and the ITCZ shifts northwards.
• The southeast trade winds from the southern hemisphere cross the equator and start moving in the southwest to the northeast direction under the influence of Coriolis force.
• Such winds collect moisture as they travel over the warm Indian Ocean.
• In the month of July, the ITCZ shifts to 20°-25° N latitude and is located in the Indo-Gangetic Plain and the south-west monsoons blow from the Arabian Sea and the Bay of Bengal. The ITCZ in this position is often called the Monsoon Trough.
• The shift in the position of the ITCZ is associated with the phenomenon of the withdrawal of the westerly jet stream from i over the north Indian plains, south of the Himalayas.
• The easterly Jet Stream (Somali Jet) sets in along 15°N latitude only after the western jet stream has left the region. This easterly jet stream is held responsible for the burst of the monsoon in India.
The monsoon winds originating over the Arabian Sea.
The Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent.
The monsoon rains take place only for a few days at a time. If rain fails to occur for one or more weeks, it is known as a break in the monsoon. Reasons for the break are as given below-
1. In northern India, rains are likely to fail if the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ over this region.
2. Over the west coast, the dry spells are associated with days when winds blow parallel to the coast.
• It starts with the beginning of the withdrawal of southwest monsoon and lasts till early January. The monsoons withdraw from the north-west end of the country in September, from the peninsula by October and from the extreme south-eastern tip by December.
• Unlike the sudden burst of the advancing monsoons, the withdrawal is rather gradual and takes about three months. The retreating southwest monsoon season is marked by clear skies and a rise in temperature.
• The land still has moisture. Due to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’. The weather in the retreating monsoon is dry in north India but it is associated with rain in the eastern part of the Peninsula.
• The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula. These tropical cyclones are very destructive.
A bulk of the rainfall of the Coromandel Coast is derived from these depressions and cyclones.
• Unlike the rest of the country, which receives rain in the southwest monsoon season between June and September, the northeast monsoon is crucial for farming and water security in the south.
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