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Thursday, February 20, 2020

The IMD says India has just finished a satisfactory northeast monsoon. What is this monsoon?

The year that just went by witnessed the rare meteorological coincidence of the northeast (winter) monsoon making its onset on the same day as the southwest monsoon withdrew officially.

By: Explained Desk | New Delhi | Published: January 2, 2020 6:54:49 pm
monsoon india, monsoon in october, september rains, rains weather, rains monsoon, (Express Photo By Ganesh Shirsekar)

The northeast, or winter, monsoon has ended on a high, with an overall surplus rainfall being recorded for the season.

The year that just went by witnessed the rare meteorological coincidence of the northeast (winter) monsoon making its onset on the same day as the southwest monsoon withdrew officially.

The two events rarely happen simultaneously, although the three-month winter monsoon season is supposed to begin almost immediately after the end of the June-September summer monsoon season.

In 2019, the retreat of the southwest (summer) monsoon was delayed by a record margin, while the northeast (winter) monsoon set in on time.

So, what is the northeast (winter) monsoon?

In common parlance, a reference to the “monsoon” usually means the southwest summer monsoon. It is the main monsoon season, which brings widespread rain across the country.

For many parts of India, this is the only time they receive rain. These four months bring about 75 per cent of India’s annual rainfall.

However, for some regions of South India, it is the winter monsoon that is much more important. Though much less heard of, especially in the north of the country, the northeast monsoon is as permanent a feature of the Indian subcontinent’s climate system as the summer monsoon.

The India Meteorological Department (IMD) recognises October to December as the time for the northeast monsoon.

During this period, rainfall is experienced over Tamil Nadu, Kerala, and Andhra Pradesh, along with some parts of Telangana and Karnataka.

So why are these two monsoons named thus?

The northeast monsoon does not have anything to do with India’s Northeast, even though a part of the system does originate from the area above it.

Rather, it derives its name from the direction in which it travels — from the northeast to the southwest. (In fact, all winds are named for the direction from which they blow. Thus the westerlies blow from the west, and the easterlies from the east.)

Similarly, the summer monsoon (at least the Arabian Sea branch of it; there is also a branch that swerves in an anticlockwise direction in the Bay of Bengal before entering the Indian landmass and bringing rain to the eastern, northeastern and northern parts of the country) moves in exactly the opposite direction — from the southwest to the northeast. That is why it is called the southwest monsoon.

When does the northeast monsoon set in?

Although October, November, and December are supposed to comprise the northeast monsoon season, the rains normally set in only around October 20.

The southern peninsular region receives rain in the first half of October as well, but that is attributable to the retreating summer monsoon. The summer monsoon season ends on September 30 but the withdrawal does not happen overnight.

The southward withdrawal takes place over a period of three to four weeks. It usually starts around the second week of September and continues till about the second week of October, bringing rain as it retreats. 2019 was unusual in that the withdrawal was completed in just eight days, beginning on October 9.

Where does it rain during the northeast monsoon season?

The northeast monsoon brings rain to just five of the 36 meteorological divisions in the country — Tamil Nadu (which includes Puducherry), Kerala, Coastal Andhra Pradesh, Rayalaseema and South Interior Karnataka.

As such, this season contributes only 11 per cent to India’s annual rainfall of 1,187 mm, compared to about 75 per cent in the summer monsoon season (the remaining rain comes in other non-monsoon months).

Many other parts of the country, like the Gangetic plains and northern states, also receive some rain in November and December but this is not due to the northeast monsoon.

It is caused mainly by the Western Disturbances, an eastward-moving rain-bearing wind system that originates beyond Afghanistan and Iran, picking up moisture from as far as the Mediterranean Sea, even the Atlantic Ocean. In the higher reaches of Jammu and Kashmir, Himachal Pradesh, and Uttarakhand, the precipitation is often in the form of snow.

The northeast monsoon is particularly important for Tamil Nadu, which receives almost half its annual rainfall (438 mm of the annual 914.4 mm) during this season.

The southwest monsoon contributes just 35 per cent to Tamil Nadu’s annual rainfall (the rest comes in other non-monsoon months). Within the state, some districts get up to 60 per cent of their annual rainfall during this time.

Similarly, Rayalaseema region and Coastal Andhra Pradesh both about 30 per cent, and South Interior Karnataka receives about 20 per cent of its annual rainfall during the northeast monsoon season.

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