Anomalous non-existence of western disturbances in the region since mid-November
An anomalous lack of western disturbances over northwest and central India is raising concerns for Rabi crops. The region doesn’t have enough rainfall and moisture, which is crucial for growing wheat during the winter.
Among the northwest Indian states, seasonal rainfall (October 1-December 13) is deficient only in Punjab, at 33 per cent below average, according to data from the India Meteorological Department (IMD) till December 13.
The rest of the states have either normal, excess or even large excess rainfall.
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Incessant rainfall in the region in the first two weeks of October due to several active weather systems caused this paradox. A possible impact of the continuing La Niña phenomenon in the equatorial Pacific Ocean and a warming Arctic region might also have contributed.
The rains that took place after the southwest monsoon season had already withdrawn from most of the region.
There has been very little rainfall in the region after that — next to nothing in some states. This has possibly brought down moisture levels, affecting the growth of Rabi crops. The uncommon rainfall turned into the region’s seasonal showers.
The contrast is most stark in Uttar Pradesh, which has an excess seasonal rainfall of 344 per cent from October 1-December 13. The absolute rainfall in this period for the state is 145.6 millimetres.
Between October 1 and October 16, UP received 145.4 mm of absolute rainfall. This means that in almost two months, October 16-December 13, the state received only 0.2 mm of rainfall.
One of the weather systems that brought extreme rainfall at the beginning of October was a western disturbance — extratropical storm systems that bring winter rainfall to northwest and some parts of central India from the Arctic, Mediterranean and West Asian regions.
The main reason for the lack of rainfall since early October is fewer western disturbances in November and a complete absence in December.
There were three back-to-back western disturbances in November. The first affected northwest India from November 5-8, the second November 9-10 and the third November 13-19.
The three western disturbances together caused some rainfall in Jammu Kashmir, Himachal Pradesh and isolated parts of Rajasthan, Punjab, Haryana and UP in the first two weeks of November, according to a Down To Earth analysis of data from IMD.
Usually, there are 3-5 strong western disturbances in December, according to a research paper published in the meteorological publication Quarterly Journal of the Royal Meteorological Society in November 2017.
A similar lack of rainfall in northwest India also occurred in December 2019. Western disturbances not affecting regions towards the plains in the south were cited as the reason behind it by experts. This time, they have been completely absent.
Read more: India lost 6 people daily to extreme weather events in October
“The northeast monsoon has produced rain in the interior and despite cyclone Mandous, rain is deficit over the east coast of India,” Raghu Murtugudde, a climate scientist at the University of Maryland and visiting professor at the Indian Institute of Technology, Bombay, told DTE.
This is the La Niña pressure pattern acting up and the winds will tend to be from the north, which is unfavourable for western disturbances, he added.
“Cyclone/hurricane activity has been weak overall, despite the La Niña. This would make the cold air go further down into the peninsula, so northwest (India) may end up being warmer than normal,” Murtugudde said.
This could be bad news for agriculture and groundwater. But western disturbances may still occur as we transition into spring, he added.
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