Interplay of regular & abnormal weather systems causing rainfall catastrophes in north India

The catastrophic rainfall in north India, especially in the Himalayan states and Punjab, for the past couple of weeks has occurred because of a concoction of weather systems over the region.

These include the western disturbances originating over north Pakistan and north Afghanistan, the low pressure areas and associated cyclonic circulations from the Bay of Bengal and moisture-laden winds from the Arabian Sea that pushed the low pressure areas northwards.

The increase in frequency of western disturbances during the monsoon period and the moisture influx from the Arabian Sea are relatively new occurrences and need further scientific exploration. Western disturbances are extra tropical storms in the middle to upper layers of the troposphere (the lowest layer of the atmosphere) that usually originate around the Mediterranean region and travel towards India via Iran, Afghanistan and Pakistan.

Over Pakistan and India, they induce cyclonic circulations, a swirling of winds in the middle layers of the troposphere that in turn induce convective storms closer to the surface, causing rainfall. Western disturbances are most common between December and March when four to six of them affect India and cause most of the winter and spring rainfall and snowfall. 

The rainfall excesses in the north Indian states, along with multiple extreme rainfall-induced flash floods, landslides and riverine floods as reported in the news, give clear evidence of both the volume and intensity of rainfall.

Among the north Indian states, Punjab has recorded the highest excess rainfall of 53 per cent from June 1, 2025 through September 4. Delhi is second highest at 52 per cent and Himachal Pradesh is third with 50 per cent more rainfall than the normal for the period. The Union territories of Jammu & Kashmir and Ladakh recorded 46 per cent and 444 per cent excess rainfall. 

The southwest monsoon has been quite active this year over the entire country, especially the northwest, but the rains have been further exacerbated by continuous presence of western disturbances and moisture-laden winds from the Arabian Sea.

In the past few years, the frequency of western disturbances has been increasing during the summer and monsoon months as well. In the current monsoon season, there have been 17 western disturbances that have affected the country, according to data from the India Meteorological Department (IMD) analysed by Down To Earth (DTE). Five of these happened in June, five in July, six in August and one in September.

The most recent western disturbance started affecting northwest India on September 2 and is still active. Five of the systems have been persistent, lasting five to seven days. 

Western disturbances during the southwest monsoon system are extremely rare and have led to catastrophic rainfall events in the past, such as the Uttarakhand cloud burst and flash floods of 2013 that killed more than 5,000 people. “There are rarely two western disturbances in June and zero or one western disturbance in July and August,” Kieran Hunt, Natural Environment Research Council independent research fellow in tropical meteorology and artificial intelligence at the University of Reading, United Kingdom, told DTE. 

This means that there should have been a maximum of four western disturbances during the entire monsoon season compared to the 17 that have already occurred since June 1 this year. There is still a month of southwest monsoon season and the IMD has forecast more than normal rainfall for north India in September. Hunt thinks that the number of western disturbances in the current monsoon season is “unusually high". 

The last two of the western disturbances have also actively interacted with low pressure areas formed in the Bay of Bengal that moved towards north India and induced cyclonic circulations of their own. The low pressure areas or monsoon lows form mostly in the Bay of Bengal and rarely in the Arabian Sea and cause most of the rainfall over eastern, central and western regions of the country. The first of these interactions occurred in the last week of August and the second started occurring in the first week of September. 

The interaction was made possible by the movement of the low pressure areas more northwards than the usual track over central India. The possible push could have come from anomalous moisture-laden winds from the Arabian Sea, according to Raghu Murtugudde, retired professor at the Centre for Climate Studies, Indian Institute of Technology, Bombay and emeritus professor at the University of Maryland. 

“The surface winds are moving northward and the low-pressure systems themselves are steered northward of their usual path because of this push. If you look at the daily rainfall anomaly for September 2, it’s obvious where the convergence is happening. Low pressure areas should bring more rain along their track but the convergence in the northwest is due to the Arabian Sea contribution,” he told DTE.

“Even the cumulative rain so far shows the signature of the northward shift in the low-level jet over the Arabian Sea. Note that the rainfall is generally deficit over the southern Western Ghats and peninsular India vs the northern Western Ghats and the northwest.”

One reason for the increased occurrence of western disturbances during the early monsoon season is the delayed retreat of the subtropical jet stream. The subtropical jet stream is a band of winds in the upper layers of the atmosphere that carries the western disturbances.

During the monsoon season, this jet stream shifts northwards to allow the setting up of the monsoon trough. The monsoon trough is an extended low pressure area around which most of the rainfall occurs during the season.

The delayed retreat of the subtropical jet stream is connected to the rapid warming of the Arctic region and the decrease in the gradient of heating between the equator and the North Pole which is leading to greater instability of the Arctic jet stream. 

The probable reason for the occurrence of western disturbances late into the monsoon season could be due to their place and type of origin, which is different from the usual western disturbances that originate over the Mediterranean and Caspian Sea regions. The systems in the current monsoon season have originated close to north Pakistan and north Afghanistan, according to data from the IMD analysed by DTE and corroborated by Murtugudde. 

“My hypothesis is that there are different flavours of western disturbances,” said Hunt. The first flavour is the classical one that originate in the Mediterranean region and travel over Iran, Afghanistan and Pakistan towards India. The second flavour are known as Potential Vorticity (PV) cut-off lows that are cold core, low pressure systems in the upper layers of the troposphere that get cut off from the Arctic jet stream and move towards the lower latitudes and look similar in shape to the classical western disturbances, according to Hunt. 

“They move slowly, are stronger and have deeper convection and account for 10 per cent of the total western disturbances,” said Hunt. “The evidence of the increased number of western disturbances during the current monsoon season could be due to the increase of PV cut off lows as they do not need the subtropical westerly jet stream to reach India but this needs to be further investigated,” he added.