Between March 1 and May 6 this year, 59 per cent districts have received ‘large excess’ rainfall and 9 per cent ‘excess’ rainfall. If this weather activity persists for some more days, it may affect the onset process of southwest monsoon
Three more weeks of the pre-monsoon season — March to May — are still left. But, rainfall data of the India Meteorological Department (IMD) shows large parts of the country have already received way beyond their normal share of the pre-monsoon showers.
Of the total 36 meteorological subdivisions in the country, 18 have received ‘large excess’ rainfall between March 1 and May 7 this year. These include Uttar Pradesh, Haryana, Delhi & Chandigarh, Jharkhand, Bihar, East Rajasthan, etc (see map: Subdivision wise cumulative rainfall; March 1-May 7, 2020).
Another six subdivisions — Himachal Pradesh, Karnataka, Telangana, Konkan, Goa, and Madhya Maharashtra — have received ‘excess’ rainfall in the same period. ‘Large excess’ is defined as rainfall 60 per cent beyond the ‘normal’ (calculated for 1951-2000) for that subdivision; and ‘excess’ as 20 per cent to 59 per cent of the ‘normal’ rainfall.
In terms of total districts (666) in the country, this comes to 59 per cent districts with ‘large excess’ rainfall and 9 per cent ‘excess’ rainfall. Gaon Connection has been reporting on how heavy rainfall and hailstorms have destroyed crops in several states.
“This pre-monsoon season we had more rainfall over northern states. But, normally these weather activities should reduce by May. We should watch for some more days. If this activity persists, it could affect at least the onset process of the monsoon,” M Rajeevan, secretary, Union ministry of earth sciences told Gaon Connection.
“Unless these systems move now north to Himalayas, monsoon circulation cannot move northwards from southern Indian Ocean … It may affect the monsoon advance. We will see and come out the onset forecast by May 15,” he added.
Map: Subdivision wise cumulative rainfall; March 1-May 7, 2020
Source: India Meteorological Department
Last month, on April 15, the IMD issued its first stage long range forecast of the 2020 southwest monsoon season rainfall and predicted the monsoon season (June to September) rainfall “to be 100% of the Long Period Average (LPA) with a model error of ± 5%.” The LPA of the season rainfall over the country as a whole for the period 1961-2010 is 88 centimetres.
The same day, the official weather agency also declared new normal dates of onset/progress and withdrawal of the monsoon over India. The monsoon onset date over Kerala, however, still remains June 1.
A closer look at the IMD’s latest pre-monsoon data shows how a large number of meteorological subdivisions in the country have received way beyond their normal share of pre-monsoon showers this year.
For instance, between March 1 and May 7, East and West Uttar Pradesh subdivisions have registered a rainfall departure of 383 per cent and 302 per cent, respectively. All the 24 districts of Jharkhand and 21 districts of Haryana have received ‘large excess’ rainfall.
As against a normal of 23.2 millimetre (mm), Haryana, Chandigarh & Delhi have received 89.1 mm rainfall. East Rajasthan has received 34mm rainfall against a normal of 8.5mm, thus registering a rainfall departure of 301 per cent.
Although these departures are huge, Rajeevan said “We should remember that this large per cent departure (of the order of 200 or more) is due to smaller values of rainfall normal (denominator).” Interestingly, our seasonal forecast models were suggesting above normal rainfall over northern parts. We shall do a detailed study on the causes of excess rainfall, he added.
But, what is the reason behind such large-scale excess rainfall in the country during the pre-monsoon season? “It looks to be a combined effect of increased number of Western Disturbances and neutral El Niño conditions,” said Sridhar Balasubramanian, associate professor with the department of mechanical engineering, Indian Institute of Technology Bombay.
A western disturbance (WD) is an extra-tropical storm, which originates in the Mediterranean region and carries moisture from the Mediterranean Sea and the Caspian Sea, and travels in an easterly direction and causes winter precipitation in northern India.
Rajeevan, too, said the excess rainfall in this pre-monsoon season could be related to more number of western disturbances moving across northern parts of the country. “This year we found that the frequency is more and also the weather systems are extended more to south. Since they [western disturbances] move slowly across northern plains, we get plenty of rainfall, but not intense precipitation,” he explained.
According to Balasubramanian, the year 2018 had a similar trend in the pre-monsoon season. “That year, because ENSO [El Niño–Southern Oscillation] was neutral (inactive), hence the trade winds could bring in lot of moisture to our Indian Ocean basin. But, in 2018, the problem was ENSO developed late in July and hence we ended in a deficit,” he said.
But, 2020 monsoon season is going to be different from 2018, he claimed. Because the ENSO is inactive and its index is gradually dropping, and there are no signs of ENSO developing. If anything, weak La Niña conditions may prevail. “The Eastern Pacific is showing good cooling trend, which means our southwest monsoon has a very good chance of recording slightly above normal rains,” he said.
While the IMD will conduct a detailed study on excess rainfall this pre-monsoon season, Rajeevan assured this weather activity should not affect our monsoon seasonal rainfall, which can be controlled only by large scale phenomenon like the El Niño or Indian Ocean anomalies.
Now all the eyes skyward!