New Delhi: With the onset and withdrawal of the monsoon getting delayed in certain parts of the country due to change in patterns and variability of rainfall, the India Meteorological Department (IMD) has revised the onset and withdrawal dates. The IMD, though, clarified that the key onset time of monsoon in the country, marked by its arrival in Kerala, remains unchanged.
The national weather forecaster has 3,500 rain gauge stations in its network and analysing rainfall data between 1961-2019, they have revised these dates, IMD scientists said on Wednesday. For instance, in cities like Jalandhar, the monsoon onset date was July 13 but based on analysis of data the onset date has been revised to June 28, indicating it is reaching a large part of the country earlier.
For revising onset dates, the IMD created a daily gridded dataset of 1°x1°. This meant the Met department had information on rainfall for every 100-110 km. Along with this, data from 1961-2019 was analysed. For revision of the date of monsoon withdrawal, data between 1971 and 2019 was analysed.
The revision in the dates will also be beneficial for the agriculture sector, power sector, water resources management and pre-monsoon municipal works, said IMD scientists.
IMD Director-General Mrutyunjay Mohapatra said this revision was long due because the existing onset and withdrawal dates were based on dated rainfall data and due to variability noticed in rainfall across the country.
“In some areas of the central and eastern India, monsoon arrival is late by 3-7 days and at the same time, it is reaching parts of Rajasthan earlier than. More importantly, the monsoon is also covering the entire country a week in advance, our analysis shows,” he said.
As per IMD’s data, the monsoon rainfall usual covers entire geographical spread of the country by July 14. However, off late it has been covering the country almost seven days earlier. The delay of onset in central India also indicates the break that is typically seen in the monsoon rainfall once it covers the western coast up to Ratnagiri in Maharashtra and then reaches eastern Maharashtra. For such areas, the IMD has pushed the onset data ahead. So in Bhopal, the usual onset data is now June 22 instead of June 15.
M Rajeevan Nair, secretary at the Ministry of Earth Sciences, said, “We have seen in our analysis there is a delay in the first low-pressure system that brings rainfall in the central and east India region. It could be due to change in circulation over the Bay of Bengal. There are a lot of changes in rainfall pattern too, so it was necessary to revise onset and withdrawal dates in keeping with the times.”
Mohapatra said the monsoon withdrawal from the entire country was getting delayed. “Usually, from September 1 the monsoon withdrawal in the country begins from west Rajasthan. However, there are delays in withdrawal even though it is eventually keeping its date with the usual withdrawal date of October 15,” he said. The cessation of rainfall for five continuous days, anticyclone conditions and reduction in moisture are the factors observed by IMD to announce withdrawal of monsoon.
Last year, the country witnessed the most delayed monsoon withdrawal as it began only on October 9 from west Rajasthan. Before that, the most delayed monsoon withdrawal was recorded in 1961.
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