Explained | Shifting monsoon patterns

Source: The post is based on the article “Explained | Shifting monsoon patterns” published in The Hindu on 26th September 2022

What is the News?

The India Meteorological Department (IMD) has said that the monsoon has begun to retreat from Rajasthan.

What is the Monsoon withdrawal?

The monsoon is a sea breeze that has consistently landed in the Indian sub-continent for thousands of years. 

It enters mainland India between the last week of May and the first week of June — through June 1 is its official onset date over Kerala. 

The IMD only counts the rainfall between June 1 and September 30 as monsoon rainfall. This doesn’t mean that the monsoon system ceases to pour rain over India from October 1. 

In fact, monsoon-related rain can continue well into the first fortnight of October and only really retreats from India by late October. 

It is then replaced by the retreating, or northeast monsoon in November which is the key source of rainfall for several parts of Tamil Nadu, Andhra Pradesh and north interior Karnataka.

When does the monsoon withdraw?

The monsoon begins its withdrawal from the last State it reaches, which is Rajasthan.

Around September 15, cyclonic systems from the Arabian Sea and the Bay of Bengal that fuels the monsoon from June-September are replaced by an ‘anticyclone’ circulation which means dry, windless conditions start to prevail over western and northern India. 

How has the monsoon been this year?

Monsoon rainfall in India has been surplus by around 7% this year though with extreme inequity.

Central and southern India saw a sharp surge in rainfall. Rains in Central India were surplus by 20% and in southern India by 25%, with the last month seeing several instances of flooding in Kerala, Karnataka and Madhya Pradesh. 

On the other hand, large parts of the U.P, Bihar, and Odisha have seen large deficits. The east and northeast of India have reported a 17% shortfall and the northwest 2%. 

What led to excessive rains in Southern and Central India?

These heavy rains are due to La Nina, the converse phenomenon of El Nino and characterized by cooler than normal sea surface temperatures in the central Pacific.

While El Ninas are linked to reduced rains over India, La Ninas indicate surplus rainfall. 

India is seeing an extended spell of the La Nina, called a ‘triple dip’ La Nina which is a phenomenon lasting across three winter seasons in the Northern Hemisphere. This is only the third time since 1950 that a triple-dip La Nina has been observed.