Erratic rain patterns mark Delhi’s monsoon this year

Delhi has recorded 33% excess rain since June 1, according to weather experts. But this did not happen because of a wholesome monsoon in the last three months but because of an abrupt spike in rain in July and September, and in the latter case, a week before monsoon is normally expected to start receding. Experts said this could be due to climate change.

According to India Meteorological Department (IMD) data, Delhi’s Safdarjung observatory recorded 47% rain deficiency in June, 140% excess in July, 13% deficiency in August, and 193% excess till September 11. This monsoon, the observatory, which provides representative data for the city, logged seven heavy rainfall days (precipitation of 64.4mm and above) — the most since 1901 — on July 19, 27, and 29, August 20, September 1, 2 and 11.

Weather forecasters said that this year, while there have been fewer rainy days, the intensity of the showers was more. August saw just six rainy days but September has seen eight already.

“Seven heavy rain days in Delhi (Safdarjung) this monsoon are the highest ever recorded, beating the earlier record of six heavy rain days in 1964,” said RK Jenamani, senior scientist at national weather forecasting centre, IMD

This skewed pattern of rain over Delhi is mainly because there were three phases of subdued monsoon in July and August.

Breaks have punctuated monsoon in city this year

Monsoon entered a ‘break’ or a ‘partial break’ phase between June 29 and July 11; it was very weak in the first two weeks of August also, leading to a significant rain deficit across the country. While it revived over northwest India on August 19, it weakened again from August 24 onwards and started reviving very gradually from August 29. These partial breaks were interspersed with only a few days of very heavy rain in Delhi, leading to traffic disruptions, urban flooding and impact on other utilities.

Not only were there fewer rainy days this year, Delhi also recorded intense spells of rain in only a few hours — an indicator of climate change. For example, between 5.30 am and 8.30 am on Saturday, Safdarjung station recorded 81.3mm rain while Palam recorded 98mm.

“The monsoon trough is in the vicinity of Delhi, there is also a low-pressure area over eastern parts of Rajasthan. Moisture laden winds are also penetrating up to mid-tropospheric levels (8000-10,000 feet), which is aiding the formation of convective clouds. This spell is likely to continue for the next 24 to 36 hours then reduce over Delhi and its neighbourhood,” said Mahesh Palawat, vice president, Skymet Weather, a private weather forecasting company.

He also talked about another low-pressure area forming over north Odisha, which is likely to intensify into a depression, triggering another spell of rain over central India, impacting the entire region from Odisha to southeast Rajasthan and Gujarat. “Standing crop is likely to be impacted. Pulses are particularly vulnerable to damage. However the heavy rain spell will fill reservoirs in Gujarat and Rajasthan where there was a very long dry spell in August, which will be useful for the Rabi crop,” Palawat said.

Undeniable role of climate crisis: Experts

Active monsoon conditions coupled with the climate crisis are leading to these intense spells of rain and several dry days. “Certainly, climate change has a role to play in the extreme rainfall we are seeing during a short duration of three to six hours as was the case in Delhi on Saturday morning. With air temperature increasing, the moisture-carrying capacity of the atmosphere has also increased. This mainly leads to the formation of convective clouds that bring thunderstorms, lightning, extremely heavy rain, etc.” Palawat explained.

He said that earlier, such convective clouds were commonly seen during the pre-monsoon season while during monsoon, altostratus clouds or thin sheets of clouds were commonly formed. “We don’t see them forming much anymore. Altostratus clouds used to bring continuous slow rain for 2-3 days during monsoon,” Palawat added.

M Rajeevan, former secretary, ministry of earth sciences, said there was a long phase of subdued rain in August due to global warming. “Rainfall was also subdued over southeast Asia. Models didn’t perform well and this deficit was not anticipated. But we have been underlining that longer dry periods with intense rain spells are becoming the norm. There is intense convective activity. Convective clouds can produce a lot of rain. Cities like Delhi should prepare for this kind of rainfall pattern,” Rajeevan said.

Negative Indian Ocean Dipole over the tropical Indian Ocean — a phenomenon where the western Indian Ocean becomes colder than the eastern one and is unfavourable for monsoon — prevailed throughout August, contributing to deficient rainfall, IMD had said earlier this month. There were also fewer typhoons in the west Pacific and so there was an absence of westward movement of their remnants into the Bay of Bengal. This led to fewer low-pressure systems forming over the Bay of Bengal.

Monsoon withdrawal to be delayed in 2021: Met

“There were some large-scale features that we are still trying to understand. August was an anomaly for many parts of South Asia,” added Rajeevan. IMD had also acknowledged last week that the multi-model ensemble forecasting system couldn’t pick up the deficit in August rain.

“Now there is heavy and widespread rain in many parts of the country because the monsoon is active and monsoon circulation is very strong. We can expect some episodes of heavy rain during such phases of monsoon. It’s also true that with climate change and more water vapour availability, severe localised convective activity takes place, bringing intense rain in a short duration similar to what you are seeing in Delhi,” explained R Krishnan, executive director, Centre for Climate Change Research.

“Usually in Delhi, August is the rainiest month, but our recordings show that in July and September we saw excess rain. In August, because of two long break phases, there was a deficit. In just 11 days, the rainfall recordings in Delhi have already reached record levels. This usually doesn’t happen. For instance, in the last two years in September there has been less than 100mm rainfall,” the official said.

Monsoon is normally expected to start withdrawing from northwest India on September 17, but is likely to be delayed this year, IMD officials added.