Rains likely to revive this week in Delhi: IMD

ByJayashree Nandi, Hindustan Times, New Delhi
Aug 29, 2021 12:28 AM IST

Monsoon entered a ‘break’ or a ‘partial break’ phase thrice this season, mainly between June 29 to July 11

Following a ‘partial break’ in monsoon rains over northwest, central and west India for 3-4 days there is likely to be gradual revival of rains mainly over central India, India Meteorological Department said on Saturday. Its revival over west and northwest India may take a few more days, scientists said.

Delhi received considerable rain on Saturday after remaining relatively dry for around a week (Sanhit Khanna /Ht photo)
Delhi received considerable rain on Saturday after remaining relatively dry for around a week (Sanhit Khanna /Ht photo)

Monsoon entered a ‘break’ or a ‘partial break’ phase thrice this season, mainly between June 29 to July 11. It was very weak in the first two weeks of August, leading to a significant deficit in rains across the country. While it revived over northwest India on August 19, it weakened again from August 24 onwards.

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Even though the weather body has said that the partial breaks cannot be compared to complete “break” monsoon conditions that have occurred in the past, the monsoon rain distribution appeared to be extremely skewed this year and is likely to impact agriculture in several parts of the country.

“There have been one or two breaks in monsoon during many years in the past. But this time these are not complete breaks in monsoon over the plains. These partial breaks are a result of weakening of monsoon. The first reason behind this is that very few low-pressure systems have formed over the Bay of Bengal during the monsoon season which could bring abundant rain in its track. The second reason is that the Madden-Julian Oscillation, or MJO location, is unfavourable and is inhibiting convective activity. The cross-equatorial flow had also weakened, so we saw a partial break in monsoon rain over northwest India, central India and the west coast. These cannot be compared with break monsoon phases and hence, we cannot say if this has happened for the first time,” said M Mohapatra, director general, IMD.

The MJO location and amplitude strongly modulates the intensity of tropical convection and features like low pressure systems over the north Indian Ocean. MJO is an eastward moving disturbance of clouds, rainfall, winds and pressure that traverses the planet in the tropics and returns to its initial starting point in 30 to 60 days on an average.

Overall, there has been a 10% deficiency in monsoon rain over the country since June 1, 12% deficiency over northwest India, 10% over east and northeast India, 13% deficiency over central India and 3% excess over peninsular India. The weak monsoon phases this time has also led to significantly dry conditions over certain parts of the country.

The standard precipitation index (SPI) for many parts of Gujarat and West Rajasthan is in “severely dry” to “moderately dry” categories. Some districts in Odisha, Kerala, Manipur and Arunachal Pradesh are also “extremely dry” to “severely dry” in August. The SPI is a record used for monitoring drought; it is negative for drought and positive for wet conditions.

A low pressure area over northwest adjoining Bay of Bengal with an associated cyclonic circulation extending up to 5.8 km above mean sea level persists. “It is likely to move over west and northwest region across central and west India during next 4-5 days,” the Met’s forecast on Saturday said.

The western end of the monsoon trough is likely to shift further southwards during next 48 hours and run to south of its normal position. “Entire monsoon trough is likely to run to the south of its normal position from August 30 for subsequent two days and shift northwards thereafter,” it said.

The eastern end is running near its normal position and passing through Bahraich, Patna, Giridih, Digha and thence southeastwards to North Bay of Bengal. It is very likely to remain near normal/south of its normal position during the next five days.

An off-shore trough at mean sea level is running from Karnataka coast to Kerala coast.

Due to these meteorological conditions, widespread rainfall activity with isolated heavy rain is likely over Odisha and Andhra Pradesh till Monday, and in Telangana, Chhattisgarh, Jharkhand, Vidarbha, Madhya Pradesh and Gujarat region till Tuesday. Light to moderate scattered rainfall activity is likely over the rest of northwest India till Tuesday.

“There is a cyclonic circulation over northwest Bay of Bengal and a low-pressure area is likely to develop over south Odisha and north Andhra Pradesh coasts which is likely to travel in the west-northwest ward direction. An offshore trough is already bringing widespread rains to Kerala and Karnataka. The impact of the low-pressure system will be felt over northwest India after August 29,” said RK Jenamani, senior scientist at national weather forecasting centre.

“The low-pressure system which is likely to form may even bring rains to Gujarat and Rajasthan after a very long time. These regions are very parched and the crop in these regions is already destroyed. There may be scattered rain over Delhi, Punjab and Haryana around August 31,” said Mahesh Palawat, vice president, climate change and meteorology, Skymet Weather.

During the past 50 years, scientists have been noticing a weakening of the South Asian monsoon. While this year’s high frequency of weak monsoon phases cannot be directly attributed to long-term weakening of monsoon over India because of lack of scientific analysis and attribution, the Intergovernmental Panel on Climate Change’s latest report on the physical science of the climate crisis released earlier in the month said it is very likely that anthropogenic aerosols weakened the regional monsoon circulations over South Asia, East Asia and West Africa during the second half of the 20th century, thereby offsetting the expected strengthening of monsoon rains in response to global warming.

Anthropogenic aerosols are nothing but air pollution particles which scatter solar radiation. They decrease evaporation and have a cooling effect on surface temperatures. The aerosols are masking the effect of global warming on the rise in monsoon rain and surface temperatures.

“In general, it is understood that increasing greenhouse gas emissions will lead to rise in warming and increase in water vapour, and consequently increase in rainfall. But that hasn’t happened over South Asia. We have seen in the past 50 years that monsoon mean rainfall over central India has reduced by about 6% to 7%. But due to global warming, incidents of heavy and extremely heavy rain episodes have increased. Many studies indicate that aerosol cover has countered the effect of global warming on monsoon,” explained R Krishnan, director, Centre for Climate Change Research at Indian Institute of Tropical Meteorology and IPCC author.

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