‘Develop a single index for low-pressure systems'

Low-pressure systems have become a buzzword during the monsoon, fuelling the impression that rainfall across India is increasingly driven by these systems rather than by monsoon winds. But the picture is more complex. The projected increase in Indian summer monsoon rainfall under a warming climate is primarily associated with rising atmospheric water vapour over the country, says Govindasamy Bala, a professor at the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science, Bengaluru.

Akshit Sangomla (AS): What role do low-pressure systems play in determining rainfall during India’s southwest monsoon? Is their frequency increasing, decreasing or remaining broadly stable, and what factors explain the observed trend? 

Govindasamy Bala (GB): During the southwest monsoon, rainfall over India is driven by two main features: orography and low-pressure system (LPS) activity. Orographic rainfall dominates the Western Ghats, the Himalayas and the northeast, while LPS activity determines the rainfall over nearly the rest of the country. Their activity is therefore a key driver of Indian monsoon rainfall.

Interestingly, LPS activity shows great diversity in terms of genesis location, tracks, number, intensity, duration and spatial extent. Because of this diversity, a single measure of LPS such as the number or LPS days cannot be a good indicator of seasonal mean rainfall over India.

 Our modelling research suggests that LPS statistics remain broadly stable under climate change. Instead, the projected increase in Indian summer monsoon rainfall under a warming climate is primarily associated with rising atmospheric water vapour over the country. Future research could therefore focus on developing a composite LPS index that accounts for the systems’ diverse characteristics and provides a more reliable indicator of monsoon rainfall. No such comprehensive index is currently available.

AS: Data with India Meteorological Department (IMD) show that during the monsoon season of 2026, as many as 14 low-pressure systems were formed, which is near normal. But these systems persisted over 77 days, against a climatological normal of 57 low-pressure days. How anomalous is this figure? 

 Govindasamy Bala (GB): There is no surprise that the monsoon LPS count is normal, but LPS days are larger than normal. This is because LPS count and LPS days are not perfect indicators of seasonal mean monsoon rainfall. There are other metrics or measures related to the characteristics of LPS, such as the intensity, size and tracks of the systems, which are equally important. In 2026, even though the LPS days are larger, the size of the systems was smaller, the intensities were weaker and most of the systems were stuck near West Bengal. These stagnant, weaker and smaller size of the LPS systems likely explain the all-India-rainfall deficit in 2026. 

AS: Our analysis of IMD data shows that the number of low-pressure days in the past five years have been significantly above normal—67 in 2022, 65 in 2023, 69 in 2025 and 77 in 2026. At 52, only 2024 recorded lower-than-normal LPS. What could be the possible reasons for this? 

GB: As mentioned above, "LPS days" is just one measure of LPS activity. It is not the best indicator of the performance of the summer monsoon. I am not too much concerned about either the LPS days or LPS count. What if the LPS are intense and spatially cover larger areas in a year when the LPS days are below normal? It is possible to get normal or above-normal rainfall in those years. 

AS: What long-term changes does your research indicate about the number of low-pressure systems and low-pressure days during the monsoon season?

GB: Our research indicates that the number of LPS events and LPS days are nearly unchanged in a warmer world. Hence, the LPS characteristics, such as the count and LPS days, are stable. However, as the amount of water vapour increases in a warmer world, our model simulations indicate increased all-India rainfall even when the number of LPS event or LPS days does not change much.

AS: Research also suggests that the number of depressions per decade has increased since 1979 while the number of low-pressure areas has decreased, indicating a trend towards more intense low-pressure systems. What could be the possible factors influencing this trend?

GB: You are right. Some studies around 2010 found a significant decline in the number of depressions over the Indian subcontinent since 1979. However, a 2014 study by Cohen and Boos [Naftali Y Cohen and William R Boos of Department of Geology and Geophysics, Yale University, USA] used modern reanalysis, satellite data and a better LPS tracking algorithm and found no such decline. As per Cohen and Boos, IMD had missed tracking multiple LPS tracks for decades, and this IMD error has misled the scientific community for years now. 

AS: In your research papers you suggest that the number of low-pressure areas could remain the same under a future warming scenario. How robust is this result and how can we understand this better? What would be the trend for the intensity of low-pressure systems in terms of wind speeds and rainfall under the future warming scenario? 

GB: The more important factor in a warming climate is the increased amount of water vapour over India. The number of LPS events per summer monsoon season or LPS days is not sensitive to climate change. Preliminary results from our recent modelling research confirm this: the number of LPS does not change with climate change. Then, what controls the rainfall over India under climate change?

The mean rainfall change over India is controlled by what is known as the "equivalent potential temperature" which is a function of both temperature and water vapour. Under climate warming, equivalent potential temperature increases over India, and more rainfall is projected. Under colder climates, equivalent potential temperature decreases, and less rainfall over India is projected. The LPS statistics is nearly unchanged under both cold and warmer climates. 

This interview was originally published as part of the cover story “Intimate takeover” in October 16-31, 2026, issue of Down To Earth