With the advancement in satellite technologies and meteorological instruments, a large and varied data is being generated to develop models and arrive at accurate results about weather and climate. The recent theories related to the origin of monsoon is based on observations of upper air circulation, temperature conditions over Tibetan Plateau, jet streams, Oceanic water circulation, the occurrence of El- Nino and La Nina and Southern Oscillations. These events play a crucial role in Monsoon formation. Their role can be understood as below
Jet streams are narrow bands of high speed winds that generally blow from west to east across the globe. Jet streams are like rivers of wind flowing through the upper layers of the troposphere. Jet streams have a huge influence on climate, as they can push air masses around and affect weather patterns.
M.T.Yin had given this concept stating that the burst of monsoon depends upon the upper air circulation. Two prominent jet streams affect the monsoon winds, the sub-tropical westerly jet stream and equatorial easterly jet stream.
Sub-tropical westerly jet stream: It dominates in winter time in upper troposphere circulation of the northern latitudes and can be located over Indian sub-continent at a height of about 12 km. Himalayan mountain system splits the jet stream into two parts. The jet stream south of Himalaya tends to descend over north- western part of India resulting in atmospheric stability.
Equatorial easterly jet stream: This jet is a prominent feature of the upper air circulation during the Indian monsoon season appearing as a band of strong easterlies extending from south East Asia across the Indian Ocean and Africa to the Atlantic.
Fig4 (a): Direction of Winds in India in winter at the Height of 9-13 km
Fig4 (b) The Direction of Winds at 13 km Altitude in Summer Season
Western disturbance
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- Western disturbance (WD) is an extra-tropical storm which originates in the Mediterranean region. It flows from west to east in the extra-tropical region. Here, Disturbance means an area of “disturbed” or reduced air pressure.
- It bringsrainfall, snowfall and fog in northern India. It is a non-monsoonal precipitation pattern driven by the westerlies. It is characterised by cloudy sky, higher night temperatures and unusual rain.
- It is very important for Indian agriculture, especially for Rabi crops like wheat etc. in north and north-western India. However, Excessive precipitation can cause crop damage, landslides, floods and avalanches. Sometimes, It also bring cold wave conditions and dense fog in Northern India
Formation
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- It originates in the Mediterranean Sea as extra-tropical cyclones. Mediterranean Sea and/or the Atlantic Ocean provide western disturbances its moisture. A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from Polar Regions towards an area of relatively warmer air with high moisture. This generates favourable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extra tropical depression. They gradually travel across the middle-east from Iran, Afghanistan and Pakistan to enter the Indian sub-continent.
Tibetan Plateau plays the role of a heat engine. The height of the plateau lies between 4000 and 5000 m, and it covers an area of approximately 2.5 million sq. km. It is characterized by poor vegetation cover and surrounded by snow clad mountain ranges. Therefore, it gets intensely heated in summer and is around 20C to 30C warmer than the air over the nearby region.
During summer, when there is an apparent movement of sun towards tropic of cancer, temperature over Tibetan Plateau remains high for a sufficiently long duration. This causes air to ascend to the upper troposphere and create a high pressure area in the upper troposphere. Due to this, there is a creation of anti-cyclonic condition and land-ocean pressure difference. This pressure gradient, eventually, leads to movement of air mass to Indian sub-continent as equatorial easterly jet stream. After reaching Mascarene Islands, near Madagascar, it starts descending and come to Indian sub-continent as south west monsoon.
The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. EI-Nino is a complex weather system that generally appears once in every three to seven years. It brings drought, floods and other weather extreme events in different parts of the world.
The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India. EI-Nino is merely an extension of the warm equatorial current which replaces, temporarily, cold Peruvian current or Humbolt current. This current increases the temperature of water off the Peruvian coast by 10°C. This results in the distortion of usual equatorial atmospheric circulation and irregularities in the evaporation of sea water.
As warm water appears off the coast of Peru, it creates a low pressure circulation there. This causes easterly winds to weaken and, therefore, El Nino has been generally known to suppress monsoon rainfall in India, since it is the easterly wind that form water bearing clouds to Indian monsoon rainfall.
ENSO (El Nino Southern Oscillation) refers to the oscillation between the El Nino and the La Nina. ENSO shifts irregularly back and forth between El Nino and La Niña every two to seven years. La Nina is the opposite of El Nino i.e. during La Nina water off the coast of Peru cools by a margin. During La Nina, easterly winds strengthen and there is higher than average rainfall during Indian monsoon period.
It is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia. Due to apparent changes in the thermal and atmospheric conditions of Tropical Indian Ocean, this is also known as ‘Indian Nino’.
It is of two types: positive IOD and negative IOD. It affects Indian summer monsoon rainfall. A positive IOD occurs when the sea surface temperatures are greater than normal in the Arabian Sea and less than normal in the tropical eastern Indian Ocean. When the reverse is the case, a negative IOD is said to have developed.
A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days in Indian sub-continent while negative IOD leads to less rainfall and more monsoon break days (no rainfall).
It is an oceanic-atmospheric phenomenon which affects weather activities across the globe. It brings major fluctuation in tropical weather on weekly to monthly timescales.
The MJO can be defined as a disturbance of clouds, wind and pressure, moving eastward at a speed of 4-8 metres per second, MJO goes around the globe in 30-60 days on average. Sometimes, it can take 90 days. It’s a traversing phenomenon and is most prominent over the Indian and Pacific Oceans.
As it moves, strong MJO activity often splits the planet in to two halves— one in which the MJO is in active phase or enhanced rainfall or convective phase, and the other in which it suppresses rainfall phase.
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- Enhanced rainfall (or convective) phase: In this phase, wind converges at the surface, and ascends throughout the atmosphere and diverges at the top of atmosphere. The rising air motion in the atmosphere tends to increase condensation and, thereby, rainfall.
- Suppressed rainfall phase: In this phase, winds converge at the top of the atmosphere, forcing air to descend and, then, it diverges at the surface. As air descends from high altitudes, its temperature increases and humidity decreases. This leads to decrease in rainfall.
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The journey of MJO goes through eight phases. When it is over the Indian Ocean during the Monsoon season, it brings good rainfall over the Indian subcontinent. On the other hand, when it witnesses a longer cycle and stays over the Pacific Ocean, MJO brings bad news for the Indian Monsoon.
Fig 9: Madden-Julian Oscillation
