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Explained: India’s southwest monsoon forecast by IMD

The southwest monsoon covers most of the country and brings 70 per cent of India's annual rainfall. A forecast from the IMD, therefore, is important for governments, businesses and the general public due to its importance for the Indian economy.

In its first long-range forecast for the season, the Indian Meteorological Department (IMD) Monday predicted a “near normal” southwest monsoon and projected that rainfall was expected to be 96 per cent of Long Period Average (LPA). The weather department also downplayed fears that prevailing El Nino conditions would affect India’s rainfall.

Addressing the press, M Rajeevan, Secretary, Ministry of Earth Sciences, said, “The good news is that 2019 will have a near normal south west monsoon seasonal rainfall between June-September of 96% of the long term average, with a model error of plus or minus 5%.

“There are prevailing conditions of a weak El Nino, but we don’t believe this will affect the monsoon adversely. In any way, there is no established one-on-one correlation between El Nino and the Indian monsoon,” he added.

Why is the IMD’s forecast on the southwest monsoon important?

The onset of the southwest monsoon in Kerala begins around June 1, but the Andaman and Nicobar Islands start receiving rainfall from the third week of May. The southwest monsoon covers most of the country and brings 70 per cent of India’s annual rainfall. A forecast from the IMD, therefore, is important for governments, businesses and the general public due to its importance for the Indian economy.

What does a ‘near normal’ southwest monsoon mean?

The IMD’s forecast is for the entire season and for the whole country; the department has not yet predicted region-wise or month-wise distribution of rainfall. It has five categories of rainfall — normal or near normal, below normal, above normal, deficient and excess.

Near normal is when per cent departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA.

What is Long Period Average (LPA)?

‘Long period average‘ (LPA) is the average rainfall recorded from June to September, calculated during the 50-year period. It is the benchmark while forecasting the quantitative rainfall for the monsoon season every year. India receives an average of 89 cm of rainfall during the entire season — this is referred to as normal rainfall.

Why were there fears that El Nino would affect the Indian monsoon?

El Niño occurs when surface temperatures in the equatorial Pacific Ocean see an unusual rise. Warmer temperatures suppress monsoon rainfall, while the opposite phenomenon of La Niña has been found to help bring good rainfall. There is recent research that shows a rise in El Nino conditions due to climate change. El Nino has a bearing on India’s rainfall.

During the press conference Monday, IMD’s Director General K J Ramesh quelled fears that El Nino would affect the monsoon. “During the monsoon months, we expect El Nino to weaken and it should, therefore, not have adverse impact on rainfall. There, however, could be some impact on the monsoon from El Nino in the earlier weeks of June, so the rains could be sluggish at this point,’’ he said.

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