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Frequently Asked Questions (Faqs) on Monsoon

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Frequently Asked Questions (Faqs) on Monsoon Frequently asked Questions (FAQs) on Monsoon 1. What is the all India monthly and seasonal rainfall? All India monthly rainfall is the amount of accumulated rainfall received over India for a particular month. For example, All India monthly rainfall of June 2018 is 155.7mm. Similarly All India seasonal rainfall is the amount of accumulated rainfall received over India for a particular season. e.g. All India seasonal rainfall of South- West monsoon (JJAS) of 2018 is 804.1mm. These quantities are not constant; they vary from year to year. 2. What do we mean by long period average (LPA) of rainfall? LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30years, 50-years etc. It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. For example, LPA of south west monsoon rainfall over Kerala for the months June, July, August and September are 556mm, 659mm, 427mm and 252mm respectively. Current LPA of all India south west monsoon rainfall based on the average rainfall over the period 1961 -2010 is 880.6mm. 3. What is large excess, excess, normal, deficient, large deficient rainfall? These are categories of rainfall used to describe realised rainfall averaged over various temporal scales like daily, weekly, monthly etc for spatial scales like districts, states operationally. Accordingly, when the realised rainfall is ≥60%, 20% to 59%, -19% to +19%, -59% to -20%, -99% to -60% of long period average (LPA), the rainfall is categorized as large excess, excess, normal, deficient, large deficient respectively. 4. What is below normal, normal and above normal rainfall for the country as a whole? If ‘m” is the mean and “d” is the standard deviation of a long time series of any climate variable like rainfall, temperature etc. Assuming the time series is normally distributed, 68% of the observations fall within +/- 1 standard deviation (d) from the mean (d). Therefore, when a realised value of the variable falls between m-d to m+d, it is categories as normal. When the realised value is <(m-d), it is categorised as below normal and when the realised value is > (m+d), it is categorised as above normal. In case of monsoon season (June to September) rainfall over India as a whole, the long period average (LPA) is 88 cm and standard deviation is 9cm (about 10% of mean value).Therefore, when the rainfall averaged over the country as a whole is within ±10% from its long period average (LPA) or 90% to 110% of LPA, the rainfall is said to be “normal” and when the rainfall is <90% (>110%) of LPA, the rainfall is said to be “below (above) normal”. 5. What is the role of monsoon trough? Monsoon Trough is an elongated low-pressure area which extends from heat low over Pakistan to Head Bay of Bengal. This is one of semi-permanent feature of monsoon circulation. Monsoon trough may be a characteristic of east west orientation of Himalayan ranges and north south orientation of Khasi-Jaintia Hills. Generally eastern side of monsoon trough oscillates, sometimes southwards and sometimes northwards. Southward migration results in active/vigorous monsoon over major part of India. In contrast, the northward migration of this trough leads to break monsoon condition over major part of India and heavy rains along foothills of Himalayas and sometimes floods in Brahmaputra river. 6. What is heat low? What are its impacts on monsoon rainfall? During the northward march of sun in northern hemisphere, the continent surrounding the Arabian Sea begin to receive large amounts of heat; not only in the form of radiation from sun, but also flux of heat from the earth’s surface into atmosphere (160 Watts/m2 for month of June over the arid zones of northwest India, Pakistan and middle eastern countries). As a result of this large input of power, trough of low pressure forms over this region. It is a semi-permanent feature of monsoon over India. The heat low is very shallow (extending up to 850 hPa (1.5 KM) level and there exist a well-marked ridge above heat low. In spite of occurrence of cloudiness, the precipitation is very small. Intense heat low (pressure departure is below normal) acts as suction devise for moist air along the monsoon trough and to some extend related to good monsoon over India. During weak heat low (pressure departure is above normal) monsoon rainfall over India is greatly affected and results in deficient or scanty rainfall over vast area of country (eg. In 1987, central pressure over heat low area was mostly above normal, which proved to be drought year). Satellite measured estimates of longwave radiation indicates that tropical /subtropical deserts are heat sinks. 7. What is monsoon low, how does it influence monsoon? An area with pressure at the centre lowest one, closed in shape with winds blowing around in anti-clockwise direction in Northern Hemisphere is Low Pressure Area (LPA). The LPA is associated with a whirling motion of air, convergence and upward motion of air. In the low usually clouds and rainfall are present. LPA which seen during monsoon is monsoon low. The monsoon lows may be intensified into monsoon depressions. The monsoon lows and depressions are the principal rain bearing systems of the south west monsoon period over India. Substantial amounts of rainfall are generated by the westward passage of monsoon depressions forming in the Bay of Bengal. These are low pressure areas having wind speeds between 17 and 33 knots in their circulation. On the average, 2 depressions form in each of the monsoon months (June- September). However, year to year variation in their number is quite large. Those that form in early June are responsible for the advance of the southwest monsoon, and are not strictly monsoon depressions. In July and August they usually form north of 18°N in the northwest Bay, and the site of genesis shifts in September southward in the Central Bay. 8. What is Tibetan High? How is it related with monsoon rainfall? Tibetan High is a warm anticyclone (in this wind are changing in a clock-wise direction in the Northern Hemisphere and it will have always outflow of winds) located over Tibetan Plateau (centre latitude at 28ºN, longitude 98ºE) in the middle/upper troposphere during monsoon period. It is marked at 300 hPa level with centre 30ºN, 90ºE and extends 70ºE-110ºE.The outflow of winds from Tibetan High as the easterly flow concentrates into jet stream centred near about the latitude of Chennai at 150 hPa in July. The jet stream runs from the east coast of Vietnam to the west coast of Africa. Thusthe location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall. Shifting its position east or west causes variation of monsoon activity over India. The Tibetan 'High' may sometimes shift much to the west of its usual position. In such a situation, the monsoon may extend further westward into Pakistan and in extreme cases into north Iran, though such a westward position of the Tibetan 'High' would be against its having origin in the heating effect of the Tibetan Plateau. 9. What is Mascarene high? How does it influence monsoon rainfall? Mascarene High is a high-pressure area that is found around Mascarene Islands (in south Indian Ocean) during monsoon period. This is responsible for cross- equatorial flow through south Arabian Sea and it acts as southern hemispheric linkage. The variation in the intensity of High Pressure causes monsoon surges across equatorial flow. These surges are responsible for heavy rains along the west coast. 10. What is Somali jet? Somali jet is low level (1 to 1.5 km asl) inter hemispheric cross equatorial flow of air, attains Jet speed at the west end of monsoon regime along the east coast of Africa. This Jet originates near Mauritius and northern part of Madagascar in the southern Hemisphere. This jet reaches Kenya coast (at about 3ºS) covers the plains of Kenya, Ethiopia and to Somali Coast at about 9ºN) During May, it moves further into eastern Africa, then into Arabian sea and reaches west coast of India in June. It attains maximum strength in July. Short period (8-10 days) fluctuations are observed in Low Level Jet stream. Its strengthening gives rise to strong monsoon over peninsular India. 11. What is Tropical easterly Jet? How does it influence the rainfall? South of the sub-tropical ridge over Asia, the easterly flow concentrates into jet stream centred near about the latitude of Chennai at 150 hPa in July. This is Tropical easterly jet. The jet stream runs from the east coast of Vietnam to the west coast of Africa. Over Africa, the location is at 10° N. Normally, the jet is at an accelerating stage from the South China Sea to south India and decelerates thereafter. The location of the Easterly Jetstream seems to influence the pattern of monsoon rainfall. TEJ weakens to less than 50 knots over India in September. During Break monsoon conditions TEJ moves northwards up to latitude 20ºN. 12. What is the difference between monsoon depression and depression forming in pre-monsoon season and post-monsoon season? The depressions which form in the monsoon season are called the monsoon depressions. These are low pressure areas with two or three closed isobars (at 2 hPa interval), which cause most of monsoon rains. These can be of Bay origin, Land origin or Arabian Sea origin. Their shape is roughly elliptical and its horizontal extension is about 1000’s of Kms of surface.
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