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Recurring rains in October call for change in monsoon definition

In the first 10 days of October, 10 states received large excess rainfall while eight had excess rainfall 
 

Northwest India has been witnessing widespread heavy rainfall in the last few days, even after India’s national weather agency announced that the southwest monsoon has retreated from most of the region. The ongoing spell of rainfall, however, was perceived by experts and general population to have been caused by weather systems associated with southwest monsoons.

If the trend of such unseasonal heavy rainfall holds in the future, then it may call for changes in the definition of monsoon period in India which currently is from June to September. 

The monsoon has already withdrawn completely from Jammu and Kashmir, Ladakh, Himachal Pradesh, Punjab, Haryana, Delhi and Rajasthan, according to the latest information provided by the India Meteorological Department (IMD). 

It has also withdrawn from most of Gujarat and some parts of Uttarakhand and Uttar Pradesh. This phase of withdrawal happened on October 3, 2022, according to IMD, and has stayed the same ever since. 

Some parts of central, south, northeast and eastern India have also received more rainfall than normal in the first 10 days of October. 

The monsoon has not officially retreated from these regions but the volume of rainfall is unusual when the monsoon winds are getting ready to leave the subcontinent. 

During October 1-7, eight states across the country received large excess rainfall (more than 60 per cent excess) and four received excess rainfall (20 to 59 per cent excess). 

As many as 213 districts (30 per cent) received large excess rainfall in the period, while 75 (11 per cent) received excess rainfall. India as a whole received 33 per cent more rainfall than normal for the period. 

By October 10, the rains spread to many other states, especially in northwest India, where many monsoon type weather systems became active along with a western disturbance. 

Western disturbances are extra tropical storms that originate in the Mediterranean region and bring winter rainfall to northwest and some parts of northeast India. 

There were multiple cyclonic circulations that traversed large parts of the country and many low-pressure troughs over long distances, some of which are still active. 

One of the reasons for the formation of these weather systems can be the continuing La Niña phenomenon in its third year and the warming of the Arctic region.

In the first 10 days of October, 10 states received large excess rainfall while eight had excess rainfall. 

The number of districts with large excess rainfall increased to 292 (41 per cent) during the period, while the number of districts with excess rainfall increased to 90 (13 per cent). 

The country as a whole received 67 per cent more rainfall from October 1-10.

Some states in northwest India, which were reeling under severe deficits, witnessed a complete reversal in a matter of days. For instance, Delhi had a rainfall deficit of 94 per cent on October 7 (measured from October 1-7). This changed to an excess of 684 per cent on October 9 and to 706 per cent on October 10. 

Haryana’s deficit of 81 per cent on October 7 swung to an excess of 555 per cent on October 9 and to 602 per cent on October 10. 

Uttar Pradesh is the only state in northwest India which received consistent heavy rainfall throughout October. 

Rajasthan’s deficit of 58 per cent on October 7 changed to an excess of 435 per cent on October 9 and to 443 per cent on October 10. The monsoon withdrew completely from all these three states by October 3. 

The rainfall happening right now will not be categorised as monsoon rainfall by IMD because the weather agency stops recording monsoon data on September 30. 

Any rainfall after this date is counted as ‘post-monsoon rainfall’, even though the rainfall is mainly happening due to weather systems that form during the monsoon season. This is happening for the third year in a row. 

A similar rainfall stretch into October had also happened in 2021 and the year before that. This calls for an assessment of the definition of monsoon period in India, which is currently from June to September and the inclusion of October rains happening now within that definition. 

“The definition of the monsoon may have to change if the trend continues,” Raghu Murtugudde, a climate scientist at University of Maryland and Indian Institute of Technology, Bombay told Down To Earth. 

“It is not a good idea to simply go by a calendar date anyway but thats historical baggage which has to be dropped now,” he added.

Climate models would better reproduce the changes happening to the monsoon season over India if the initial conditions of El Niño, La Niña, Atlantic Niño and the Arctic warming are accounted for as accurately as possible. 

The short (1-3 days), medium (3-10 days) and extended range (2-4 weeks) forecasts for the monsoon season have to be reliable to deal with these climatic phenomena, according to Murtugudde.

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