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October rainfall: Should the monsoon period in India be redefined?

IMD stops recording monsoon data after September 30, classifies October showers as  ‘post-monsoon rainfall’

Should the monsoon period in India be redefined? Several states received excess rainfall in October that went into several hundreds in percentage.

Delhi had a rainfall deficit of 94 per cent on October 7, 2022, which went up to an excess of 684 per cent on October 9 and 706 per cent on October 10. Haryana’s deficit of 81 per cent on October 7 changed to an excess of 555 per cent on October 9 and 602 per cent on October 10.

Rajasthan’s deficit of 58 per cent on October 7 changed to an excess of 435 per cent on October 9 and 443 per cent on October 10.

In such a situation, how did India as a whole receive only 67 per cent more rainfall from October 1-10? This rainfall begs us to question whether the October rainfall should be counted as part of the monsoon rains.

Why should this be done? This is because India Meteorological Department stops recording monsoon data on September 30. Any rainfall after this date is counted as ‘post-monsoon rainfall’, even though this year it mainly happened due to weather systems that formed during the monsoon season.

This is happening for the third year in a row. This calls for a re-assessment of the definition of the monsoon period in India. On October 14, the southwest monsoon withdrew from northwest India after a fortnight’s delay.

If unseasonal heavy rainfall continues in the future, then the definition of the monsoon period in India might have to change. The monsoon withdrew from Jammu and Kashmir, Ladakh, Himachal Pradesh, Punjab, Haryana, Delhi and Rajasthan, Gujarat, some parts of Uttarakhand and Uttar Pradesh on October 3.

The monsoon has officially retreated from these regions, but the volume of rainfall was unusual when the monsoon winds were getting ready to leave the subcontinent. During October 1-7, eight states across the country received 60 per cent excess rainfall and four received 20-59 per cent excess rainfall.

Two hundred thirteen districts received large excess rainfall in the period, while 75 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 or extra tropical storms that originate in the Mediterranean region and bring winter rainfall to the northwest and some parts of northeast India.

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, the number of districts with large excess rainfall increased to 292 during the period, while the number of districts with excess rainfall increased to 90.

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

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