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Tuesday, August 03, 2021

Explained: Why is the southwest monsoon early?

The monsoon hit the Kerala coast two days behind schedule, but has already covered two-thirds of the country. What explains its progress so far, and what can be expected in terms of rainfall and agriculture?

Written by Anjali Marar | Pune |
Updated: June 21, 2021 11:56:00 am
A fisherman casts his net as dark clouds hover in the sky after the southwest monsoon made its footfall in Kerala, in Kochi. (PTI Photo)

Just about 10 days after it broke over the Kerala coast two days behind schedule, the southwest monsoon has progressed rapidly to cover two-thirds of the country.

How far has the monsoon progressed?

On Tuesday, the northern limit of the monsoon (NLM) continued to pass through Diu, Surat, Nandurbar, Bhopal, Nagaon, Hamirpur, Barabanki, Bareilly, Saharanpur, Ambala, and Amritsar, according to the India Meteorological Department’s daily weather report.

Across some areas of south peninsular and central India, the monsoon has arrived 7 to 10 days ahead of its scheduled date. So far, the monsoon has missed Northwest India — Gujarat, Rajasthan, western Madhya Pradesh, Haryana, Punjab and Delhi.

As of Tuesday, the entire country except West Bengal and the Northeast, Jammu and Kashmir and Ladakh, Kerala, and Gujarat had received cumulative rainfall (since the official beginning of the southwest monsoon season on June 1) in excess (20%-59%) or large excess (60% or more) of normal.

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Why is it early this year?

Cyclone Yaas, formed in the Bay of Bengal during the third week of May, helped the monsoon make a timely arrival over the Andaman Sea on May 21.

Despite a two-day delay from its normal onset over Kerala, where it arrived on June 3, the southwest monsoon made fast progress in subsequent days. This was mainly due to strong westerly winds from the Arabian Sea, and also the formation of a low-pressure system over the North Bay of Bengal on June 11 that currently lies over eastern Uttar Pradesh and Bihar.

The monsoon currents strengthened and it advanced into the Northeast, West Bengal, Odisha, Jharkhand, Bihar and parts of Chhattisgarh.

An off-shore trough, prevailing for a week between Maharashtra and Kerala, has helped the monsoon arrive early over Karnataka, Goa, Andhra Pradesh, Telangana, Maharashtra and southern Gujarat.

Is this unusual?

In the last one decade since 2011, the monsoon has covered the entire country in June itself on four occasions — 2020 ( June 1–26) , 2018 ( May 28–June 29), 2015 (June 5–26) and 2013 (June 1–16).

In all the other seven years, arrivals were delayed over major cities or regions. Cyclone Vayu in 2019 and Cyclone Mora in 2017 had delayed the monsoon progress by a few days. But overall, advancement during these seven years was as per normal dates and the monsoon covered the country around July 15 (the normal date, followed until 2019).

In the years when the monsoon has arrived early, its progress has picked up towards the final phase; that is, the North and Northwest India regions have witnessed early arrival.

Source: IMD

Will it continue this pace?

Although the monsoon has made rapid progress along the regions on the west and east coasts, and East, Northeast and some Central India regions, further progress is likely to be slower. This is not expected until around June 25. An advance will take place when there is a fresh pulse to revive the monsoon currents.

“Over Northwest India, the monsoon becomes active only when the monsoon currents — either from the Arabian Sea or the Bay of Bengal — reach the region. As it is not expected to happen soon, the monsoon progress will remain slow,” said Mrutyunjay Mohapatra, director general, IMD.

Also, a stream of mid-latitude westerly winds is approaching Northwest India, which will hinder the monsoon advancement in the immediate coming days.

Does early onset mean more rainfall overall?

The time of monsoon onset over a region has no direct impact on the rainfall quantum received during the season, or in the monsoon’s progress.

For instance, the monsoon took 42 days in 2014 and 22 days in 2015 to cover the entire country. Even with such distinct ranges, India recorded deficient rainfall during both years.

This year, the monsoon is most likely to cover the entire country by the end of this month. Although it is too early to predict the seasonal rainfall, it is possible that June rainfall could end in surplus over the normal of 170 mm. As on June 15, it was 31% above normal.

How does early rainfall impact paddy sowing?

Early rainfall will not directly impact paddy sowing, with seedlings still in the nursery stage in most paddy growing states.

“There is still time for undertaking paddy transplanting over most areas that grow rice. Due to rainfall over coastal Karnataka and Konkan, farmers can undertake paddy transplanting in the third to fourth weeks of June,” said R Balasubramanian from the Agriculture Meteorology Division of IMD, Pune. Transplanting is being currently undertaken in Kerala.

However, with not much rainfall recorded over Madhya Maharashtra (except Kolhapur, Satara & Sangli districts and the ghat areas) and Marathwada (except bordering districts with Vidarbha) farmers may undertake sowing once these sub-divisions get sufficient rainfall, he said.

In Odisha and West Bengal, too, saplings are yet to reach the transplantation stage.

The early monsoon also means a shorter summer. Is this unusual?

Although the IMD considers June 1 as the beginning of the monsoon season over India, the summer in Northwest India is not yet over. In West and Northwest India, day temperatures remain above 40°C. For example, Fatehgarh in eastern UP recorded 42.4°C on Monday.

“Recently, Rajasthan and neighbouring areas of Northwest India reported heatwave-like conditions. Once the low-pressure system weakens in the next two to three days, the temperatures over North and Northwest India – where the monsoon is yet to reach – will increase,” said D Sivananda Pai, head, Climate Research and Services, at IMD, Pune.

Can these patterns be placed within the context of climate change?

After the monsoon onset over Kerala, its progress can either be rapid, consistent or slow, based on ocean-atmospheric conditions. The onset of the monsoon over various parts of the country each year can be ahead of time, in time or late. These variations are generally considered normal, given the complexity of the monsoon.

However, climate experts have linked extreme weather events like intense rainfall over a region within a short time span or prolonged dry spell during these four months as indications of climate change.

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