The lingering monsoon

This has been officially recognised. Three years ago, the India Meteorological Department (IMD) had revised the expected dates of onset and withdrawal of the monsoon for several regions of the country. The withdrawal dates for North, Northwest and Central India were pushed back by one to two weeks to account for the trends witnessed over the last 50 years.

Thus, the October rainfall over North India of the kind that happened over the last few days should no longer surprise anyone. It should in fact, be increasingly seen as the norm rather than an exception.

“People will have to get used to it. These are not freak events. We are likely to see these happening in the coming years as well,” M Rajeevan, former Secretary, Ministry of Earth Sciences, said about the October rainfall over North and Northwest India.

Delhi and its surrounding areas had received pretty good rainfall in October 2021 as well — so much so that Delhi had its fourth wettest October of the last 120 years. Considering the huge amounts of rain that have fallen in the first 10 days of this month — eight times more than the normal — this October could turn out to be even wetter than last year’s.

A longer rainy season

In any case, rainfall in October — after the traditional date of withdrawal of the southwest monsoon — isn’t entirely unheard of. It has happened in several previous years as well. But the rain in those years was mostly caused by different, often local, atmospheric phenomena.

What is being witnessed in more recent years is a clear prolongation of the monsoon season. As such, the nature of rainfall is very different — it is not a short-duration heavy downpour, but sustained rain over a few days.

The recent spell of rainfall over Western Uttar Pradesh, Uttarakhand, Haryana, Eastern Rajasthan and Delhi were a result of the interaction of monsoon winds, moving east to west, with the western disturbance wind system. Such interactions happen several times during the monsoon season.

Is it climate change?

Like most of the changes being witnessed in global weather patterns, the changing trends in the Indian monsoon are also being driven primarily by climate change. In line with the experience in many other parts of the world, rainfall in India is increasingly taking place in short, intense bursts. Extreme rainfall events are increasing both in intensity and frequency.

The extension of the monsoon season could also be seen as a consequence of global warming, said Dr Rajeevan.

“A robust study to establish the cause and effect relationship is yet to be done, but one possible reason for the spillover of monsoon rainfall to October could be the fact that the oceans (Bay of Bengal and Arabian Sea) are now warmer than earlier,” he said.

“Warmer ocean currents help the formation of monsoon winds. Earlier, rainfall during the monsoon season would bring down the temperature of the ocean. But possibly because of global warming, the oceans continue to remain warm even after the traditional monsoon season is over. The oceans could thus be playing a role in keeping the monsoon alive beyond the traditional period,” Rajeevan said.

Global warming is affecting rainfall patterns in other ways too. A warmer atmosphere has a greater capacity to hold water. When this water is finally released, it often results in a heavier downpour than would be expected otherwise. This accounts in part for the increasing instances of extreme rainfall events.

A challenge for forecasting

The changing patterns and increased instances of erratic monsoon behaviour is creating forecasting complications for the IMD. Once infamous for its unreliable and generic forecasts, the IMD has over the past 10-12 years, invested heavily in setting up observational equipment, upgrading computing resources, and fine-tuning weather forecast models. And while it continues to battle public perception in many cases, the IMD’s forecasts are now not just far more accurate and specific, they are also impact-based and actionable.

The increased variability in weather systems brought about by climate change is threatening to dilute the gains made in recent years. IMD Director General Mrutyunjay Mohapatra acknowledges this challenge.

“If we had not augmented our resources in the last few years, we would have been in a very difficult situation. Today, IMD is able to provide fairly good forecasts. It may not be 100 per cent accurate, but it is at par with any other weather forecasting agency in the world. The current spell of rain in Delhi and other areas was forecast accurately well in advance. But climate change is posing a big challenge. We need to set up more observation stations, collect more data, and do more computing. Capacity upgradation has to be a continuous exercise,” Dr Mohapatra said.

Impact on other sectors

Monsoon rainfall is not just a weather phenomenon. It is a key driver of the Indian economy. A significant part of Indian agriculture still depends on monsoon rainfall for irrigation. The supply of drinking water and the generation of electricity are also linked to the monsoon.

Changes in the period and duration of the monsoon requires follow-up action from these sectors. Not just the preferred time of sowing of crops, but the entire cropping cycle — even the choice of crops — might need to be changed.

There are implications for dam management as well. Most reservoirs in the northern and central parts of the country seek to attain full capacity levels by the end of September because not much rain is expected after that. But if the monsoon consistently spills over into October, as is being predicted, this practice would need to be revised as well.

Government officials said the other sectors are yet to react to the change in monsoon calendar that was announced by the IMD three years ago, but it will not be long before they are forced to respond.