Third-longest monsoonal break in this century has ended

Subject :Geography

Section: Physical geography

Context:

The current monsoon break that started on August 7, 2023 has finally ended, according to the regional centre of India Meteorological Department, Pune.

Details:

This current monsoonal break makes it the third-longest for this century after 2002 and 2009.
The core monsoon saw suppressed rainfall, which was enough to pull down the all-India cumulative monsoon rainfall anomaly from 7 per cent to –6 per cent in less than a month.

Monsoon break:

Meteorologically, a monsoonal break occurs when the monsoon trough shifts northward, which enhances rainfall along the Himalayan foothills and parts of eastern India while rainfall is suppressed in the rest of the country.
This happens especially in the core monsoon zone area or the region stretching from Gujarat in the west to West Bengal and Odisha in the east, where agricultural activities are rain-fed.
Climatologically, a monsoonal break is declared when normalized rainfall anomaly index or deviation from the long term rainfall average over the core monsoon zone exceeds -1 threshold and the situation persists for at least three consecutive days.

Mechanism of Break in Monsoon:

Overindulgent solar insolation causes a zone of low pressure to develop over northwest India. This low heat gradually moves eastward with the arrival of the monsoon until it creates an elongated low-pressure zone that runs west to east parallel to the Himalayan mountains.
- The Himalayan foothills are roughly parallel to its axis. This monsoon trough periodically shifts north and south of its usual location.
- The pattern of rainfall over India changes noticeably when it moves north and passes by the foothills of the Himalayas.
- While the rains abruptly stop over the northern Indian plains, they intensify just as quickly over the northeast Indian foothills.
- This is referred to as a monsoon “break.”
Since the majority of India’s major river systems originate in the Himalayan region, it creates a paradoxical situation when people in the plains lament the lack of rain while those living in the northeastern parts of the country are troubled by floods.

A combination of factors were at play behind this decade-high monsoonal break:

The developing El Nino played a role in prolonging the break monsoon and making it intense.
It was supported by the absence of sub-seasonal weather patterns that boost rainfall.

Earlier instances of Monsoon break:

While a monsoon season might feature multiple break spells, to assess how dire the situation is currently, researchers accessed data on longest consecutive break spells since 1951.
- According to the data, in the last 73 years, there have been a total of 10 instances when the break spell has stretched over 10 days.
- The longest consecutive break spell was reported in 1972, when the core monsoon zone did not receive any rainfall for 17 days at a stretch, while in 1966 and 2002, the break spell stretched over 10 days on multiple occasions.

Rainfall anomaly in monsoon season of 2023:

Monsoon 2023 has a six percent deficit when compared to the long term average but ‘normal’ as per IMD’s rainfall categorisation.
A total of 264 out of 717 districts remain deficient to large deficient.
Another anomaly that stands out in 2023 has been the drier western regions, specifically the West Rajasthan and Saurashtra-Kutch meteorological subdivisions receiving large excess rainfall while the usually wetter regions like Kerala, Gangetic West Bengal, Bihar and Jharkhand remaining dry due to deficient rainfall.