Rain Check: After A Scorching Summer, Monsoon Likely to Bring 'Normal' Rainfall over India, Says IMD

After witnessing one of the most intense heatwaves in recent years, India could see normal seasonal rainfall during the June to September period brought by the southwest monsoon. The monsoon is extremely crucial for the agrarian economy of India, as it provides 75% of the total rainfall received by the country annually.

According to India Meteorological Department (IMD), the seasonal rainfall is most likely to be 99% of the long period average (LPA) of 87 cm for the country as a whole. In terms of probability, there is a 40% chance that it will be normal, 26% that it will be below normal, 14% says deficient and 15% above normal. There is a very low five per cent chance of it being in excess. The prediction entails a model error of +/- 5%.

IMD considers rainfall between 96 to 104% of LPA to be normal. Last year, though the rainfall was normal over the country as forecasted, it was very uneven during the four months. While July saw below normal rains, the rainfall deficit widened hugely in August, to be followed by excess rains in September.

The IMD will issue a month-wise forecast later in May, but in terms of the geographical distribution, many parts of the northeastern states, some parts of the north-western states of Punjab, and Haryana, and extreme southern states could see below-normal rains. On the other hand, the monsoon is expected to augur well for northern parts of Peninsular India, adjoining Central India, and foothills of the Himalayas, and some parts of Northwest India with normal to above-normal rains.

According to the weather department, the prevailing intense heatwave conditions may not have a direct impact on the monsoon, as such.

New ‘normal’

Owing to climate variations over the years, IMD has also announced the “new All-India normal” for monsoon. The long-period average (LPA) of rainfall will now be calculated from 1971 to 2020, instead of 1961-2010, due to which the overall rainfall expected during monsoon has dropped from 880.6 mm to 868.6 mm. The new all-India annual rainfall normal has also plummeted from 1176.9 mm in 1961-2010 to 1160.1 mm in 2071-2020. Thus, there is a decrease of 12 mm of rainfall during the southwest monsoon season and 16.8 mm of annual rainfall for the country as a whole from 1961-2010 to 1971-2020.

According to M Mohapatra, director general of meteorology, IMD, New Delhi, this is mainly due to natural decadal variations (epochal behaviour) exhibited by the monsoon, when some decades are drier, while others are wet. “The decadal average of southwest monsoon rainfall for 2011-20 was minus 3.8% from the long-term mean. This will normalise in the current decade (2021-30), and by the start of the next decade in 2031, monsoon will enter a wet epoch with more rains,” he explained.

Elaborating further, the IMD DG said, it is necessary to revise the ‘normal’ every ten years, considering the climate variability, decadal variations, distribution of rains, as well as the expanding rain gauge stations. “We now have as many as 4,132 rain gauge stations across the country, covering as many as 703 districts. So, there is more data, which has enabled us to make a better estimate for rainfall in less time,” he shared.

‘Mixed’ ocean signals

The southwest monsoon is strongly governed by the conditions of the Pacific Ocean and the Indian Ocean and their continuously changing interactions with the land and atmosphere. According to IMD, this year, the monsoon will see an interplay of La Nina – an ocean phenomenon that favours good rains, and negative Indian Ocean Dipole (IOD) which is unfavourable for monsoon rains. The current forecast suggests La Nina conditions are likely to prevail over the equatorial Pacific Ocean during the initial part of the monsoon season, with low sea surface temperatures, and then it may turn neutral.

Rainfall on decline over Northeast

The weather department’s analysis of rainfall over the states for the last 30 years (up to 2018) also suggests that the seasonal rains are on the decline in the north-eastern states, of Arunachal Pradesh, Nagaland, Manipur Mizoram, and Tripura, and the northern state of Uttar Pradesh, while it shows an upward trend for Rajasthan, Saurashtra-Kutch and parts of Central India, including Madhya Pradesh. Meanwhile, due to the climate variability, the monsoon rains are also showing vast intra-seasonal variations. There are long dry spells, interspersed with isolated spells of intense rainfall. The number of days with heavy rainfall is on the rise.

IMD will issue its second-long range forecast in May-end, with monthly forecasts for June, July, August, and September. The onset date of the monsoon will be announced in mid-May.

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