Rainfall likely to decline in August, say experts

An adverse atmospheric phenomenon, known as the negative Indian Ocean Dipole, is expected to cause a decline in rainfall next month, a special IMD bulletin noted.

Monsoon rainfall over India could decline slightly from next month as an adverse atmospheric phenomenon known as the negative Indian Ocean Dipole is likely to develop from August, according to a special bulletin released by the India Meteorological Department in Pune last week.

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The dipole is the difference between the temperature of the Indian Ocean in the Arabian Sea on the west and the Bay of Bengal in the east. A neutral dipole doesn’t affect the monsoon, but a negative dipole is bad news for rainfall.

“Western Indian Ocean likely to remain slightly cooler than normal and Eastern Indian Ocean likely to remain slightly warmer than the normal for the next couple of seasons,” the weather office said in its latest El Nino Southern Oscillation bulletin. “The probability forecast for IOD also indicates the enhanced probability for negative IOD conditions from the July, August, and September season.”

A negative Indian Ocean Dipole can have a negative impact on the monsoon, according to DS Pai, director of the Institute of Climate Change Studies and former climate scientist at IMD Pune. However, since La Nina conditions persist, “we may not see a major reduction in rainfall, “ Pai said.

There is a 45% probability that La Nina conditions will continue for most of August, September and October, the weather bureau has said. La Nina is the large-scale cooling of surface temperatures in the central and eastern equatorial Pacific Ocean, coupled with changes in the tropical atmospheric circulation, pressure and rainfall. It occurs every two to seven years. In India, La Nina is associated with a strong monsoon and above average rainfall, and colder winters.

The first half of July, the peak monsoon month, saw heavy showers and floods over central and western India, while most of the Gangetic plains and northwest India saw patchy rainfall, which could affect the summer cropping season, HT reported on July 17.

This was due to an unusual pattern of a low-pressure area over Odisha and northern Andhra Pradesh persisting for over a week and the monsoon trough remaining south of its normal position through the past two weeks. The monsoon trough has started shifting northwards from July 17 and the Met department has forecast that rains will increase over northwest India and the Himalayan foothills from July 19.

“With the shifting of the trough, even states facing rainfall deficit like Uttar Pradesh, Bihar, Jharkhand and West Bengal will get rains,” Pai said. “So, there is nothing to worry about immediately about negative the IOD in August, but we should be prepared.”

The impact of a negative diploe will be more pronounced in September, said Mahesh Palawat, vice president of climate change and meteorology at Skymet Weather Services, a private forecaster.

“Unlike previous years, when we got widespread and good rains even in September, the last month of monsoon, this time there may be a significant reduction,” Palawat said. “In August, we expect rains to be near normal as La Nina could help compensate the loss that may be caused by negative IOD, but in September, La Nina conditions are expected to weaken a bit.”

The impact of a negative dipole might not be severe, said M Rajeevan, former secretary at the ministry of earth sciences. “Updated forecasts are indicating a negative IOD event developing. But we saw slightly negative tendencies in June and July also, but as you can see rainfall was above normal in July,” Rajeevan said. “There was a lull in June, but my reading is linked to other atmospheric and circulation conditions.

Rainfall forecasts for August will be issued by the end of July. “If there are any adverse effects expected, they will be reflected in our forecast, which is being prepared,” said M Mohapatra, director general, IMD.

Monsoon rainfall from June to September is likely to be normal at 103% of the long-period average, the weather office had said in its forecast on May 31. If the prediction holds true, it will be the fourth consecutive year that the southwest monsoon will be normal or above normal.

Last year, rainfall was 99% of the average, In 2020, the monsoon was 109% of the average and was considered slightly above normal. In 2019, the monsoon was 110% of the long-period average.