Explained: Why October’s been so rainy
The monsoon is over but several parts of the country are still receiving rainfall. Delhi, Kerala, Madhya Pradesh and Uttarakhand, for example, have received very high rainfall in the last few days, resulting in loss of life and property in some places. Delhi has just had one of its wettest 24-hour periods in several decades.
Scientists say a combination of factors — delayed monsoon and development of low-pressure areas at multiple places — have resulted in these rainfall events at several places.
October rains
Rain in October is not unusual. October is considered a month for transition, during which the southwest monsoon withdraws and gives way to the northeast monsoon that largely affects southern peninsular India, mainly on the eastern side.
Western disturbances, which begin to have significant interference in local weather over the extreme northern parts of India, commonly cause either rain or snowfall. Since late last week, Ladakh, the higher reaches of Kashmir and Uttarakhand have reported the season’s first snowfall.
Last week, two low-pressure systems were active simultaneously, one each over the Arabian Sea and the Bay of Bengal regions. Collectively, these triggered severe weather events over Kerala, Tamil Nadu, Andhra Pradesh, Telangana, Delhi, Madhya Pradesh, Uttarakhand, Odisha and West Bengal.
Delayed monsoon withdrawal
The four-month southwest monsoon season normally withdraws completely by early October. During the withdrawal phase, it causes thunderstorms and localised heavy rainfall.
This year, however, the withdrawal began only on October 6 against a normal of September 17. So far, the monsoon has withdrawn completely from the Western, Northern, Central and Eastern India regions. But it remains active over the southern peninsula. Thus, Kerala, Tamil Nadu, Telangana, Karnataka and Andhra Pradesh have had significant rainfall during the last 10 days.
Until Monday, the monsoon had not withdrawn from Manipur, Mizoram, Tripura, parts of West Bengal and Odisha and entire southern peninsular India.
“As there has been a delay in the southwest monsoon withdrawal, good rainfall has continued over Odisha, the Northeast and south India,” said Mrutyunjay Mohaptra, director general, India Meteorological Department (IMD).
Normally, by mid-October, the monsoon winds reverse their direction of flow from the southwest to the northeast.
“Even though the easterlies are beginning to replace the westerlies, the former is yet strengthen and fully establish. The easterly winds indicate the arrival of the northeast monsoon,” said D Sivanand Pai, head, Climate Research and Services, IMD, Pune.
This year, conditions for the onset of the northeast monsoon are expected to develop around October 25.
Extreme rains
For most days last week, at least two low-pressure systems remained active along the east and west coasts and over central India, bringing rains over large parts of the country.
Delhi received 87.9mm (over a 24-hour period) between Sunday and Monday, making it the fourth wettest October day for the national capital since 1901. The month of October has also been the fourth wettest so far. It has received 94.6 mm rains this month so far, which is next only to the 238.2 mm it received in 1954, the 236.2 mm in 1956, and 186.9 mm in the entire Octobers of 1910.
Likewise, Balasore in Odisha recorded 210mm in a day and it was only the second such occasion in a decade for this month.
While Tamil Nadu normally receives good rainfall between October and December, mainly during the northeast monsoon, Coimbatore (110mm) witnessed its wettest October day in a decade even before the onset of the northeast monsoon.
The Western Ghats, northeast and central India are known as high-rainfall receiving regions. However, in recent years, it has been noted that intense spells during a short time span are increasingly becoming frequent.
“Due to climate change, there is definitely a rising frequency in the extreme weather events round the year. But these specific occurrences of heavy to very heavy rains that we are seeing right now can be attributed to the formation of low-pressure systems,” said Mohaptra.
“Whenever there is a low-pressure system, depending on its strength, it results in heavy to very heavy rainfall activity. In addition, when a low-pressure system interacts with western disturbance, further intense rainfall occurs,” he said.
Extreme rainfall in Kerala
A low-pressure system that formed in the east-central Arabian Sea moved and sustained over Kerala between October 15 and17.
Simultaneously, another low-pressure system prevailed over the north Andhra Pradesh coast and southern Odisha. The interaction between them strengthened the southwest winds which brought extreme rainfall over central and southern Kerala during the last weekend.
At some places in Idukki, Ernakulam, Kollam and Kottayam districts, the 24-hour rainfall was over 200 mm. As many of these districts are hilly and covered with dense forests, the water run-off triggered landslides and mudslides.
Rainy days ahead
The low-pressure system that affected Kerala has weakened now. But a similar system is still active over central India, because of which northern India is likely to received good rainfall this week.
Heavy rainfall events are predicted over Western Uttar Pradesh, Uttarakhand and Himachal Pradesh for Tuesday, with a ‘red’ alert having been issued by IMD for these regions.
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Another low pressure — located over Northern Odisha and Gangetic West Bengal — is active and its interaction with the moist easterly winds from the Bay of Bengal is expected to bring heavy rain over West Bengal, Odisha, Sikkim, and Bihar until Wednesday. The maximum impact in terms of extremely heavy rain (more than 204 mm in 24 hours) is likely over some places in West Bengal and Sikkim on Tuesday.
Further, strong southeasterly winds from the Bay of Bengal are expected to cause very heavy rain over Arunachal Pradesh, Assam and Meghalaya until Wednesday.