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MONSOON’S L.P.A.
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- What is it: The India Meteorological Department (IMD) is the government’s principal agency for weather forecasting and rainfall monitoring. If it forecasts that the country would receive normal rainfall during a south-west monsoon season (June to Sept), it is basically calling the monsoon as ‘normal’ or ‘deficient’ based on how it fares against its benchmark Long Period Average (LPA).
- What is the LPA: It is the average rainfall received by the country as a whole during the south-west monsoon, for a 50-year period. The current LPA is 88 cm, based on the average rainfall over years 1961 and 2010. This acts as a benchmark against which the rainfall in any monsoon season is measured.
- When deficient and when excess: The country is said to have received deficient rainfall if the actual rainfall falls below 90 per cent of LPA. We say the country received excess rainfall if the rainfall is greater than 110 per cent of LPA.
- When normal: It is deemed ‘normal’ when the actual rainfall received falls between 96 and 104 per cent of LPA.
- Data sets: The IMD’s rainfall data is based on actual rainfall recorded in 2,412 locations, across its 3,500 rain-gauge stations. Based on daily rainfall data received in these stations, monsoon statistics are prepared for the administrative zones such as districts, States and for the whole country. The statistics is compiled for the 36 meteorological subdivisions and for the four broad regions – South peninsula, North West India, Central India and North and North-East India — before being aggregated for the whole country.
- Why important: The LPA uses a 50-year average because annual rainfall can be highly variable. A 50-year average smooths out the day-to-day, month-to-month variations, while also accounting for freak weather events like the El Nino and La Nina.
- Aberrations: Once in every three or four years, Indian monsoons have witnessed aberrations such as severe drought, flooding and storms owing to El Nino — the abnormal warming of waters in the Pacific Ocean. Example : The IMD forecast normal rainfall for the year 2013 but the actual rainfall received was 106 per cent of LPA, which is above normal rainfall.
- Is normal always prosperous: No, not at all. The actual distribution of those 88 cm of rainfall over India’s key growing regions and over the critical sowing months (called spatial and temporal distribution), is important.
- EL NINO - During El Nino, trade winds weaken. Warm water is pushed back east, toward the west coast of the Americas. El Nino means Little Boy, or Christ Child in Spanish. South American fishermen first noticed periods of unusually warm water in the Pacific Ocean in the 1600s. The full name they used was El Nino de Navidad, because El Nino typically peaks around December. El Nino can affect our weather significantly. The warmer waters cause the Pacific jet stream to move south of its neutral position. With this shift, areas in the northern U.S. and Canada are dryer and warmer than usual. But in the U.S. Gulf Coast and Southeast, these periods are wetter than usual and have increased flooding.
- LA NINA - La Nina means Little Girl in Spanish. La Nina is also sometimes called El Viejo, anti-El Nino, or simply "a cold event." La Nina has the opposite effect of El Nino. During La Nina events, trade winds are even stronger than usual, pushing more warm water toward Asia. Off the west coast of the Americas, upwelling increases, bringing cold, nutrient-rich water to the surface. These cold waters in the Pacific push the jet stream northward. This tends to lead to drought in the southern U.S. and heavy rains and flooding in the Pacific Northwest and Canada. During a La Nina year, winter temperatures are warmer than normal in the South and cooler than normal in the North. La Nina can also lead to a more severe hurricane season. During La Nina, waters off the Pacific coast are colder and contain more nutrients than usual. This environment supports more marine life and attracts more cold-water species, like squid and salmon, to places like the California coast.
- IOD - The Indian Ocean Dipole (IOD) 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. A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days while negative IOD leads to less rainfall and more monsoon break days (no rainfall). Hence in light of this IOD would be the saviour factor which is likely to be in the neutral or positive phase during the monsoon.
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