Intraseasonal weather patterns absent in first fortnight of September, indicating high likelihood of rainfall deficit across the country
At least 290 millimetres of rainfall are needed from August 22 to September 30, 2023 for the remaining monsoon to be in the normal category. Photo: iStock The date for monsoon withdrawal has been corrected to September 17.
Below average rainfall is expected over most of India in September. The monsoon 2023 forecast for the month has suggested that 32 out of 36 meteorological subdivisions will report below-average rainfall. Cumulatively, the rainfall is expected to be 20 millimetres below the 1996-2013 average.
The forecast — carried out by Akshay Deoras, climate scientist at the University of Reading, the United Kingdom — took stock of three climate models (UK Meteorological Office, European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction) and combined them to get mean values.
According to Deoras, the impact of El Nino climate pattern is palpable.
El Nino and La Nina are the warm phase and cool phases of a recurring climate pattern across the tropical Pacific Ocean called the El Nino-Southern Oscillation. La Nina ended its three-year run this year and El Nino announced its arrival.
“This is a clear impact of the developing El Nino and the lack of favourable intraseasonal weather patterns that boost monsoon rainfall (rainfall in July 2023). This is allowing the penetration of dry air from the regions to the west of India, such as Pakistan,” Deoras said.
However, Deoras expressed caution, as there are no clear indications if these intraseasonal weather patterns will remain weak or absent throughout the entirety of September.
“There is, however, an indication of this happening (the absence of intraseasonal weather patterns) during the first fortnight of September, so the rainfall deficit across the country appears highly likely in this period,” Deoras said.
The all-India weighted average rainfall for southwest monsoon 2023 is likely going to be below average or 96 percent below the long-period average of 1971 to 2020, said the climate cientist.
Read more: Third-longest monsoonal break in this century has ended
Deoras also highlighted when India could expect the commencement of monsoonal retreat.
“Since the weather over western Rajasthan is expected to remain dry during the first fortnight of September due to dry air intrusions, there is an early commencement of the monsoon’s withdrawal around the normal date of September 17,” Deoras said.
The September monsoonal rainfall comes on the heels of various weather experts predicting August 2023 to be the driest in the last 123 years. As per Down To Earth’s calculation, at least 290 millimetres of rainfall are needed from August 22 to September 30, 2023 for the rainfall in the remaining period to be in the ‘normal’ category.
And what are the chances of the Indian Ocean Dipole (IOD) offsetting the extreme impacts of a developing El Nino?
“We are seeing some signs of the development of a positive phase of the IOD. However, it’s likely going to be too late for the IOD to help overcome the negative impact of El Nini in Septemeber,” Deoras said.
Like El Nino, which unfolds in the tropical waters of the Eastern Pacific, IOD is the difference in sea surface temperature stretching from the Arabian Sea to the eastern region of the Indian Ocean along the southern Indonesian coast.
Read more: More dry days ahead? Monsoon on continuous break for past 11 days
A positive IOD usually manifests itself in above average rainfall across the Indian subcontinent.
While it remains to be seen how the monsoon unfolds for the remaining period, according to Deoras, the three climate models used for the forecast far outperform the other climatic models.
“They have performed really well this season. Of course, they couldn’t forecast rainfall over individual states that well in some cases (floods in north India in July 2023), but they were able to simulate the large-scale rainfall tendency in India, including the subdued rainfall during June to August,” Deoras explained.
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