October is a month of transition when the southwest monsoon recedes to give way to the northeast monsoon, which largely affects the eastern side of southern peninsular India.
The June-September monsoon season has long past its sell-by date but several parts of India continue to receive heavy rainfall. Kerala, Delhi, Uttarakhand, and Madhya Pradesh received very high rainfall over the past few days, causing loss of life and property. Delhi, for example, witnessed one of the wettest 24-hour periods in decades.
Weather scientists said delayed monsoon and low-pressure areas at multiple places combined to cause rainfall events across several parts of India.
Rainfall during October is not abnormal. October is a month of transition when the southwest monsoon recedes to give way to the northeast monsoon, which largely affects the eastern side of southern peninsular India.
Western disturbances, which significantly interfere with local weather over India’s extreme north, commonly cause either snow or rainfall. Since last week, Ladakh, the higher regions of Uttarakhand and Kashmir have reported the first snowfall of the season.
Last week, two low-pressure systems — one each over the Bay of Bengal and the Arabian Sea — were simultaneously active. These two systems combined to trigger severe weather events in Tamil Nadu, Delhi, Kerala, Andhra Pradesh, Telangana, Odisha, Madhya Pradesh, West Bengal, and Uttarakhand.
Delayed monsoon withdrawal
By early October, the four-month southwest monsoon season completely withdraws. During the withdrawal phase, it causes localised heavy rainfall and thunderstorms.
This year, however, the withdrawal phase began on October 6 against a normal of September 17. The monsoon has withdrawn completely from the western, central, northern, and eastern India so far. But the current remains active over the peninsular south. Thus, Kerala, Karnataka, Tamil Nadu, Andhra Pradesh, and Telangana have witnessed significant rainfall activity over the past 10 days.
Until Monday, the monsoon current was yet to withdraw from Mizoram, Manipur, Tripura, parts of Odisha and West Bengal, and the peninsular south.
India Meteorological Department (IMD) Director General Mrutyunjay Mohaptra told The Indian Express with the delay in the withdrawal of the southwest monsoon current, good rainfall had continued over northeast and southern India, and Odisha.
Normally, by mid-October, the monsoon winds reverse their flow to the northeast from the southwest.
Pune-based Climate Research and Services (IMD) Chief D. Sivanand Pai told The Indian Express despite the easterlies beginning to replace the westerlies, the former was yet to strengthen and fully establish. The arrival of the northeast monsoon is indicated by the easterly winds.
Conditions conducive for the advent of the northeast monsoon are likely to develop around October 25 this year.
At least two low-pressure systems remained active over India’s west and east coasts and over central India for most days last week, bringing rainfall to large parts of the country.
Between Sunday and Monday, Delhi received 87.9 mm rainfall over a 24-hour period — the fourth wettest October day since 1901. October 2021 has already become the National Capital’s fourth wettest. It has received 94.6 mm rainfall so far this month, next only to October 1954 (238.2 mm), 1956 (236.2 mm), and 186.9 mm (1910).
Likewise, Odisha’s Balasore district recorded 210 mm rainfall in a single day for only the second such occurrence in a decade for October.
While it is usual for Tamil Nadu to receive good rainfall between October and December, especially during the northeast monsoon season, Coimbatore (110 mm) experienced the wettest October day in a decade even before the northeast monsoon.
Northeast and central India and the Western Ghats are high-rainfall receiving regions. In recent years, however, intense wet spells during a short span have become increasingly frequent.
Mohapatra said the frequency of extreme weather events throughout the year was rising due to climate change.
But these specific heavy-to-very heavy spells of rain right now can be attributed to low-pressure systems, he told The Indian Express.
A low-pressure system, depending on the strength, causes heavy-to-very heavy rainfall. When the low-pressure system interacts with western disturbance, the intensity of the rainfall increases, he added.
Extreme rainfall in Kerala
A low-pressure system over east-central Arabian Sea moved and sustained over Kerala for three days, between October 15 and17.
Simultaneously, another low-pressure system had formed over the coast off southern Odisha and northern Andhra Pradesh. The two systems interacted and strengthened the southwest winds, which brought extreme rainfall over south and central Kerala over the weekend.
The 24-hour rainfall crossed the 200-mm mark at some places in Ernakulam, Idukki, Kottayam, and Kollam districts. As most of these districts are hilly with thick forest cover, the water run-off triggered mudslides and landslides.
Rainy days ahead
The low-pressure system that hit Kerala has now weakened. However, a similar system remains active over central India and is likely to cause good rainfall in northern India this week.
The IMD has predicted heavy rainfall over Uttarakhand, western Uttar Pradesh, and Himachal Pradesh for Tuesday, and issued a ‘red alert’ for these regions.
Another low pressure is active over northern Odisha and Gangetic West Bengal. Its interaction with the moist easterly winds from the Bay of Bengal could bring heavy rain over Odisha, West Bengal, Sikkim, and Bihar until Wednesday.
Additionally, strong southeasterly winds from the Bay of Bengal are predicted to bring very heavy rainfall over Assam, Arunachal Pradesh, and Meghalaya until Wednesday.