PUNE: The country has seen 1,204 instances of very heavy to extremely heavy rainfall in the first 18 days of August alone, the highest for any monsoon month in the last five years for which such data was made available. This is twice the number recorded in August 2018.

The India Meteorological Department (IMD) calculated the very heavy to extremely heavy rainfall frequency during 24-hour periods for its 3,500 stations spread across India. It found that August 2019 has so far seen the maximum instances when stations received more than 120mm and 210mm rain.

Real time rainfall data is monitored for all 3,500 stations. IMD officials said “very heavy” rain means showers in the range of 115.6mm-204.4 mm, while “extremely heavy” rainfall event is when a station receives 204.5mm or more in 24 hours. India’s monsoon performance this year has so far been “normal”, but huge swathes of the country have witnessed high rainfall that caused flooding and devastation.

According to the data, 914 very heavy to extremely heavy rainfall events were recorded in July this year, the highest since 2015.

Arvind Kumar Srivastava, head of the Climate Research Division, Climate Research and Services, IMD Pune, told TOI that the 2019 monsoon got active after June 22-23, when heavy rainfall wiped out the high monsoon deficit in various districts within a space of a few days.

“Although there haven’t been many deep weather systems during the monsoon till now, the pressure gradient over the west coast was fairly steep. This was mainly because of a significant and consistent positive north-south temperature gradient, the dynamical forcing needed for strong pressure gradient to maintain copious monsoon flow. This, coupled with circulations over the Bay of Bengal, may have caused widespread rain over major parts of peninsular India, west coast and central Maharashtra ,” Srivastava said.

“There were also easterly and westerly interactions over northern/northwestern parts recently. This led to very heavy/extremely rainfall events in situ. Our data showed that in comparison to August 2018, the number of very heavy to extremely heavy rainfall instances registered a two-fold rise in August 2019. In comparison to August 2017, the increase was almost two-and-a-half times in August 2019,” he said.

“Several researches have shown that the occurrence of extreme rainfall events over India during the southwest monsoon season shows spatial variability with preferred regions of occurrence over the entire west coast of India and parts of central India and northeast India. The frequency of extreme rainfall shows a significant increasing trend over the Indian monsoon region during the southwest monsoon season over the 1951–2005 period,” Srivastava said.

Former IMD director general K J Ramesh said, “We are in a global warming period. Extremes of every season are on a rise. The frequency of monsoon lows (low pressure systems) is also increasing over the last decade. A low pressure system is enough to give 300-400mm rain along its trajectory. Low to moderate intensity rainfall events are on the decrease, while heavy to very heavy and extremely heavy rainfall events are on an increase.”

He said pre-monsoon heating was anomalously high this year because of the record temperatures in May and June. “The monsoon core strength was also very good this season. These factors, including the increase in the frequency of low pressure systems and other rains-related variability factors such as the positive Indian Ocean Dipole (IOD), caused an increase in the very heavy to extremely heavy rainfall events," Ramesh said.

"Heavy rain pockets are developing indiscriminately in different regions. Both heavy rainfall episodes spanning a few hours as well as extremely heavy rainfall periods spanning one to five days, seem to have increased in the recent past,” he added.

