Explained: India’s wettest September in more than 100 years

Also, September 30 is officially the end of India’s four-month monsoon season. But this year, the monsoon has refused to go away. The last time it stayed until October was back in 1961 — when the withdrawal of the monsoon started on October 1.

This year, the India Meteorological Department has said, the monsoon might begin to withdraw only after October 10.

So why did September get so much rain this year? It is well into October, and large parts of Bihar, including the capital Patna, are reeling under floods due to massive rainfall events.

Was it the La Niña?

J Srinivasan, distinguished scientist at the Divecha Centre for Climate Change at IISc in Bengaluru told The Indian Express that the last time September produced so much rain was in 1917, which was a La Niña year.

La Niña, the phenomenon in the equatorial Pacific Ocean in which the sea surface temperatures turn unusually cold, is known to strengthen rainfall over the Indian sub-continent during the monsoon months.

However, there is no La Niña this year. In fact, the year started with a weak El Niño, the opposite phenomenon in the Pacific Ocean that has a negative impact on the Indian monsoon, before the situation turned neutral.

The other possibility: IOD

Given the fact that there was no La Niña that could possibly explain the massive September rain, scientists have been looking at a similar phenomenon much closer home, called the Indian Ocean Dipole (IOD), which could have contributed to enhanced rainfall.

“There was a cooling of the eastern equatorial Indian Ocean, below Sumatra, and that could have some role to play in the kind of rainfall that we have seen this year,” Dr Srinivasan said.

The IOD is a phenomenon similar to the ENSO condition observed in the Pacific Ocean which creates the El Niño and La Niña events. The sea surface temperatures in the Indian Ocean get warmer and cooler than normal, and this deviation influences regional atmospheric and weather patterns, notably the Indian monsoon.

But there is one major difference between ENSO and IOD.

While the Pacific Ocean only has an El Niño or a La Niña condition at a time, the Indian Ocean experiences both warm and cold conditions at the same time – hence, a dipole.

One of these poles is located in the Arabian Sea, while the other is in the Indian Ocean, south of Indonesia.

The Indian Ocean Dipole is said to be ‘positive’ when the western pole is warmer than the eastern one, and ‘negative’ when it is cooler.

The Indian Ocean Dipole and ENSO are not unrelated. So, positive IOD events are often associated with El Niño, and negative IOD events with La Niña. Therefore, when the IOD and ENSO happen at the same time, the Dipole is known to strengthen the impacts of the ENSO condition.

Intertropical Convergence Zone

Many scientists like to describe the monsoon in terms of the movement of the Intertropical Convergence Zone, or ITCZ, a region near the Equator where the trade winds of the northern and southern hemispheres come together.

The intense Sun and the warm waters of the ocean heat up the air in this region, and increase its moisture content. As the air rises, it also cools, and releases the accumulated moisture, thus bringing rainfall.

During the monsoon season, this ITCZ is located over the Indian subcontinent. By September, as the temperature begins to go down, the ITCZ starts moving southwards of the Indian landmass, towards the equator, and further into the southern hemisphere.

This year, this process has not yet started.

“In September this year, the northern hemisphere was much warmer than the southern hemisphere, and that could be one reason why the ITCZ has remained longer than usual over the northern hemisphere,” Srinivasan explains.