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Tuesday, May 11, 2021

IMD predicts third straight year of normal monsoon

If the forecast is accurate, this will be the third consecutive year of normal monsoon in the country. The four-month southwest monsoon season brings India about 70 per cent of its annual rainfall. The IMD releases its LRF twice every year, in April and June.

Written by Anjali Marar | Pune |
Updated: April 17, 2021 7:18:23 am
IMD predicts third straight year of normal monsoonThe LPA for the southwest monsoon season, calculated over the 50-year period from 1961 to 2010, is 880 mm of rain.

The monsoon will be normal this year, the India Meteorological Department (IMD) said in its first stage Long Range Forecast (LRF) on Friday.

If the forecast is accurate, this will be the third consecutive year of normal monsoon in the country. The four-month southwest monsoon season brings India about 70 per cent of its annual rainfall. The IMD releases its LRF twice every year, in April and June.

Quantitative rainfall during June to September would be 98 per cent of the Long Period Average (LPA), which is in the ‘normal rainfall’ category, IMD Director General Dr Mrutyunjay Mohapatra said in an online briefing.

The LPA for the southwest monsoon season, calculated over the 50-year period from 1961 to 2010, is 880 mm of rain.

While the forecast is normal for the country as a whole, rainfall is expected to be below normal over Jammu and Kashmir, Ladakh, Delhi, east Uttar Pradesh and adjoining Bihar, Jharkhand, north areas of Odisha and Chhattisgarh, and West Bengal, the IMD said.

“There is good news that the monsoon this year will be normal. It will be good for our agriculture,” M Rajeevan, Secretary, Ministry of Earth Sciences, said. Neutral ENSO (El Niño Southern Oscillation) conditions are currently prevailing over the Pacific Ocean, the LRF said. “During the 2021 monsoon season, we foresee no development of El Niño conditions,” Rajeevan said.

ENSO is one of many largescale features that influence the Indian summer monsoon. El Niño refers to the abnormal warming of surface waters of the equatorial eastern Pacific Ocean. It has been observed that southwest monsoon rainfall remains subdued in India in El Niño years.

A condition opposite to El Niño is La Niña, which sees unusually cold ocean temperatures in the equatorial Pacific, and has been seen to favour the Indian summer monsoon.

Closer home occurs a phenomenon known as the Indian Ocean Dipole, which is similar to the ENSO condition that creates the El Niño and La Niña events in the Pacific. Sea surface temperatures in the Indian Ocean get both warmer and cooler than normal, and this deviation influences regional atmospheric and weather patterns, including the Indian monsoon.

“At present, neutral ENSO conditions prevail but latest forecasts indicate that during the monsoon season, the IOD could enter its negative phase,” Mohapatra said.

The ‘dipole’ means the Indian Ocean experiences both warm and cold conditions at the same time. One of the poles is located in the Arabian Sea; the other is in the Indian Ocean, south of Indonesia. The IOD is said to be positive when the western pole is warmer than the eastern one, and negative when it is cooler. Positive IOD events are often associated with El Niño; a negative dipole with La Niña.

Given the demand from both the government and public for a spatial distribution forecast, the Met department will issue forecasts for each of the four months of the monsoon season. Also, starting this season, the department will use a ‘multi-model ensemble’ involving inputs from multiple weather models, and a “robust statistical algorithm” in order to reduce the margin of error in forecasting.

There will also be greater emphasis on monitoring rainfall over the monsoon core zone spanning Madhya Pradesh, Maharashtra, Chhattisgarh, Jharkhand, Odisha, Rajasthan, Gujarat, and parts of Bihar and Karnataka.

“It has been observed that the all-India rainfall variability coincides with the variability recorded along the monsoon core zone by 85 to 88 per cent. Starting this season, IMD will issue a separate forecast for this region which will be done in the second stage LRF,” Rajeevan said.

India’s agriculture being primarily rain-fed, the southwest monsoon is one of the key drivers of India’s economy. The most rain falls in July (289 mm) and August (261 mm).The four-month period is when reservoirs are replenished, and except for some areas in the far north and parts of Tamil Nadu, the monsoon is the main source of drinking water in the country.

Timely onset over Kerala in early June, steady progress along the length and breadth of the country by mid-July, and a good spatio-temporal rainfall distribution mainly during July and August, all are crucial for the country.

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