Explained: What Does IMD's Forecast Of 'New Normal' Mean To India's Monsoon Season?

The India Meteorological Department (IMD), introduced a revised definition of the new rainfall normal while projecting a southwest monsoon for this year as well.

The agency claimed that India would receive 99 percent of the long-term average (LPA) rainfall, which was modified from 89 cm to 88 cm in 2018, and then amended to 87 cm in the annual review in 2022.

IMD Revises All-India Baseline Rainfall

On the basis of new statistics from its system of rain gauges around the country, the weather agency cut the standard for defining normal southwest monsoon rainfall to 868.6 mm from 880.6 mm previously.

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The revised all-India rainfall average was generated using rainfall data collected for a 50-year period from 1971 to 2020 which may be applied to assess rainfall across the region.

The IMD publishes weather predictions and assessments in regards to variations from the normal, that is a 50-year long period average of rainfall. Every ten years, the 'normal' rainfall, also known as the long period average (LPA), is adjusted.

The LPA's previous revision was postponed and completed only in 2018. Until then, the LPA of 1951-2001, which was 89 cm, has been regarded as a baseline for measuring rainfall. According to IMD Director General Mrutyunjay Mohapatra, the LPA definition must be revised every 10 years, as per global standards.

"We take into account a variety of factors. Climate variability, for example, is a variable that changes over time. Secondly, as the amount of rain gauge stations grows over time, we get more information that is more evenly dispersed. As a result, it (the projection) becomes more reasonable, as well as catering to the needs of smaller areas and specific locales," he explained.

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The last revision occurred in 2019 after a seven-year wait considering the time it needed to collect and analyze the data, he explained.

The progressive drop in average rainfall was linked to the inherent multi-decadal epochal variation of dry and wet periods of all Indian rainfall, he said.

Long Period Average

Rainfall statistics are collected from around 2,400 sites and 3,500 rain gauge units by the IMD. Since yearly rainfall varies substantially not only from area to area and month to month but also from year to year inside an area or period, an LPA is required to level out tendencies and provide a fairly reliable forecast.

A 50-year LPA accounts for huge shifts in either way produced by odd years of extremely high or low rainfall (based on situations like El Nino or La Nina), as well as drought periods and the highly prevalent extreme weather episodes induced by global warming.

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The LPA of rainfall, according to the IMD, "is the amount of rain observed over a certain area for a specified frame (such as month or season) averaged over a long time such as 30 years, 50 years, or more." The IMD's estimate of normal rainfall was predicated on the LPA from 1971 to 2020 when the country got an average of 87cm of rain across the region. The LPA has previously been measured by the IMD as 88 cm for the period 1961-2010 and 89 cm for the period 1951-2000.

While this numerical standard relates to the rainfall totals observed across the nation from June to September, the intensity of rainfall that occurs each year differs by region and month.

As a result, in addition to the national figure, the IMD keeps LPAs for each meteorological zone of the state, which varies from roughly 61 cm in the dry Northwest to far high than 143 cm in the wetter East and Northeast.

The IMD presents an instance of the LPA of the southwest monsoon rains over Kerala in its monsoon FAQ page: 556 mm, 659 mm, 427 mm, and 252 mm for June, July, August, and September, respectively. When subdivided month by month for the rest of the region, the LPA results for something like the season are 16.36 cm for June, 28.92 cm for July, 26.13 cm for August, and 17.34 cm for September.

All-India quantitative rainfall reduced?

The average monsoon rainfall decreased between 1961 and 2010 (880.6 mm) and from 1971 to 2020 (710.6 mm) (868.6 mm). The yearly rainfall in the country has also declined, from 1176.9 mm in 1961 to 1160.1 mm in 2010. (based on 1971-2020).

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"However, the rainfall pattern has not changed significantly either geographically or seasonally, with the exception of a slight drop during the southwest monsoon season," Mohapatra said.

Rainfall fluctuations are inherent in such a huge country as India, and they are frequently non-uniform. The amount of dry periods and extreme rain occurrences over short time periods has been on increase across Uttarakhand, according to the Meteorological Authority.

Rainfall in the Northeast region of the country has also been decreasing since the 1950s, especially over the last 3 decades. "Normal rainfall in Arunachal Pradesh, Nagaland, Manipur, Mizoram, and Tripura between 1971 and 2020 was lower than it was between 1961 and 2010," Mohapatra added.

"On regional levels, the altering of rainfall distribution and decrease in rainfall is attributable to global warming, which is being noticed in Northeast India," Guhathakurta added.

The typically dried and arid west-central India provinces of Kutch-Saurashtra, Rajasthan, and adjacent areas of Madhya Pradesh are recording a larger proportion of wet days than average due to the shifting of rainfall. According to the IMD director, these places received more rain from 1971 to 2020 than they did from 1961 to 2010.

Challenges associated with "New normal" rainfall expectations

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According to a government weather scientist, altering weather patterns might not only have serious consequences for the nation's economy, but will also make things more difficult for farmers who rely on timely rainfall to cultivate their major produce during the summer monsoon season.

Weather events and irregular rainfall are also caused by global changes in weather trends. For example, according to scientists, the disappearance of sea ice in the summertime is causing delayed rainfall extremes in India.

According to moneycontrol, the country's senior meteorologist claimed that the irregular spread of rainfall and increased incidence of extreme weather episodes from the last few southwest monsoons plainly indicate climate change's impact. Farmers who struggled to sow their crops in the exceptionally dry June of 2021 suffered serious damage to standing crops that were ready for harvest as a result of the unseasonal torrential rains.

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