Many tropical cyclones originate in the North Indian Ocean and move on more frequently into the Bay of Bengal than into the Arabian sea. Although this region accounts for only a small share of global tropical cyclone activity, cyclones here have a great impact. Due to the high coastal population and low disaster preparedness, the region experiences a disproportionately high number of deaths and widespread damage to property, crops and natural vegetation.
Tropical cyclones in this region usually develop during the pre-monsoon season, from April to June and during the post-monsoon season from October to December.
Surprisingly, during the pre-monsoon seasons of 2025 and 2026, the North Indian Ocean saw no cyclones. Why did cyclones fail to form in these years, even during a season that generally favours their development? Had such gaps in cyclone formation occurred earlier?
A team of researchers from the Indian Institute of Tropical Meteorology, Pune, investigated these questions.
To examine cyclone activity over the North Indian Ocean, the researchers collected records of tropical cyclones between 1980 and 2025 from the India Meteorological Department and the Joint Typhoon Warning Center. They also examined data from the International Best Track Archive for Climate Stewardship, a global database of cyclone tracks.
They found that the Bay of Bengal was more favourable for cyclone formation than the Arabian Sea, during the pre-monsoon season. Completely storm-free pre-monsoon seasons were, however, not unusual. In the 46-year record from 1980 to 2025, there were 12 such years.
The post-monsoon season showed a different pattern. At least one named cyclone formed somewhere over the North Indian Ocean every year in the same 46-year period.
Why did cyclones form in some years but not in others?
The researchers compared the atmospheric and oceanic conditions of the storm-free years with those of ten years in which at least one cyclone formed in each basin. They examined the broader atmospheric conditions that existed throughout the season, rather than the short-term changes associated with individual weather systems. To process the atmospheric and oceanic data, they used a 10-day low-pass Lanczos filter, a mathematical technique . The filter reduced short-term fluctuations such as strong winds and heavy moisture associated with passing weather systems, including cyclones and other storms. This helped the researchers focus on the larger-scale background conditions that could favour or suppress cyclone formation.
To compare sea surface temperature, moisture moving through the atmosphere due to changes in wind speed and direction at different heights, as well as the rotation of air in the lower atmosphere between active and suppressed years, the researchers used atmospheric data from ERA5, a global reanalysis dataset, along with satellite observations and other climate records.
They found that, even in years without a pre-monsoon cyclone, the atmosphere could contain enough warmth and moisture to support storm development. This suggests that warm ocean waters and atmospheric moisture alone cannot explain why cyclones sometimes fail to form.
One important difference was the amount of rotation in the lower atmosphere. The low-level relative vorticity, the tendency of air to rotate around a point, helps clusters of thunderstorms organise themselves into a larger, rotating system. When this rotation is weak, it is harder for these disturbances to develop into cyclones.
The researchers identified low-level relative vorticity as the main factor distinguishing active years from years with no cyclones. The difference in wind speed and direction at different heights also played a role, particularly over the Bay of Bengal.
What are the determining factors behind low-level relative vorticity?
The researchers examined large-scale atmospheric waves and climate patterns. The El Niño and La Niña phases of the Southern Oscillation had no correlation with active and inactive years of cyclone formation in the North Indian Ocean.
The Indian Ocean Dipole, a regional climate pattern caused by temperature differences across opposite sides of the Indian Ocean, also did not show any consistent difference between the active and suppressed years.
The researchers then examined the Madden–Julian Oscillation. This oscillation is a large-scale pattern of clouds, rainfall and wind that moves eastward through the tropics over a period of weeks.
The researchers found a strong link between cyclone formation and the Madden–Julian Oscillation. In contrast, the Madden–Julian Oscillation was generally weak or inactive during suppressed years.
During years of active cyclone development, out of the 8 distinct geographic phases, the Madden–Julian Oscillation remained strong for longer periods directly over the North Indian Ocean and Indonesia/Malaysia. These conditions seemed to help strengthen low-level rotation, and the movement of moisture into the region to create a more favourable environment for storms to develop.
Large-scale atmospheric waves, including the equatorial Rossby waves and Kelvin waves, were also stronger during active years. These findings suggest that the timing and strength of large-scale atmospheric activity can help determine the possibility of cyclone formation.
A better understanding of the Madden–Julian Oscillation, a large-scale pattern of changing weather in the tropics, and the rotation of air near the ocean surface can help weather agencies predict cyclone formation earlier and more accurately. Meteorologists and weather forecasters can now focus on subseasonal tracking of Madden–Julian Oscillation pulses and low-level wind vorticity, leading to vastly improved extended-range weather models.
More accurate cyclone forecasts will help reduce false alarms and panic for coastal residents who often face severe anxiety whenever ocean temperatures rise in April and May. It will also give people in vulnerable areas more time to prepare and protect their lives and property.
Pre-monsoon cyclones pose serious risks to small fishing boats and workers on offshore platforms. Knowing in advance can help fishing communities plan safe deep-sea expeditions or bring boats ashore before cyclones develop. Knowing whether a pre-monsoon season will be active or silent helps farmers time seed sowing, irrigation, and crop harvesting without unexpected storm surges or sudden salt-water inundations.
Scientific Reports 16, 28590 (2026);
DOI: 10.1038/s41598-026-61019-2
Reported by Sanghamitra Deobhanj
Freelance science writer, Cuttack
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