In May 2020, as India battled a nationwide lockdown during the COVID-19 pandemic, a more significant threat emerged silently over the Bay of Bengal. Within just a few days, Cyclone Amphan intensified from a mild depression into one of the strongest storms ever recorded in the region. Packing winds of more than 240 kilometres per hour, it tore across coastal Odisha and West Bengal, flattening homes, flooding villages, and uprooting decades-old mangroves.
A new study by Jayanarayanan Kuttippurath at the Indian Institute of Technology Kharagpur has revealed why Amphan intensified so quickly and what its behaviour tells us about the future of tropical cyclones in a warming world. Published in Tropical Cyclone Research and Review, the paper titled “Rapid intensification of the Super Cyclone Amphan: Environmental drivers and its future projections” provides one of the most detailed investigations of how changing ocean and atmospheric conditions are fuelling stronger storms.
A perfect storm in the Bay of Bengal
The Bay of Bengal is a known cyclone hotspot, accounting for approximately 7% of global tropical storms each year. Its shallow basin and high sea surface temperatures make it an ideal breeding ground for destructive systems. Yet Amphan stood out. Within just 36 hours, it transformed from a severe cyclonic storm into a super cyclone.
The researchers found that the sea surface temperature during Amphan’s formation exceeded 31 degrees Celsius, roughly 2 degrees above normal. This excess heat acted as high-octane fuel. Beneath the surface, “warm-core eddies” rotating pockets of hot water added another layer of energy. Together, they provided the storm with a massive heat reservoir, allowing it to intensify faster than most cyclones in recorded history.
According to the study, the Tropical Cyclone Heat Potential (TCHP) and Upper Ocean Heat Content (OHC) remained unusually high throughout the event. These parameters measure the total energy stored in the upper ocean that can be transferred to the atmosphere. Even as the cyclone churned up colder water from below, the underlying warmth persisted, keeping the system alive and violent.
Why the winds didn’t tear it apart
Cyclone growth often depends on a delicate balance of atmospheric forces. One of the critical factors is vertical wind shear, which refers to the change in wind speed and direction with increasing height. High shear can disrupt a storm’s vertical structure, preventing it from strengthening. During Amphan’s life cycle, however, wind shear in both shallow and deep layers was unusually low, falling below 2 metres per second.
This calm environment allowed Amphan’s vortex to grow vertically and symmetrically, forming a stable eye surrounded by intense convection. The researchers also found that mid-tropospheric relative humidity and potential vorticity played key roles. A well-organised vortex in the mid-troposphere helped the system maintain coherence, while high moisture levels sustained deep convection.
Low wind shear and high ocean heat content acted in unison to supercharge Amphan. It was a perfect combination of conditions that rarely align so precisely.
— Jayanarayanan Kuttippurath
Climate change and the next generation of storms
To understand how future cyclones might behave under warmer conditions, the researchers simulated Amphan using a coupled atmosphere–ocean model, COAWST, which integrates the Weather Research and Forecasting (WRF) model with the Regional Ocean Modelling System (ROMS). They ran multiple scenarios representing different climate pathways: RCP 4.5 (moderate emissions) and RCP 8.5 (high emissions).
The results were sobering. In the RCP 8.5 scenario, future versions of Amphan became about 12 percent stronger, with a deeper central pressure and faster movement toward land. The storm’s translational speed increased from 1.96 to 2.78 metres per second, meaning it would strike coastlines earlier and with less time for evacuation. In both future scenarios, the minimum central pressure dropped by as much as 9 hectopascals, indicating an enhanced intensity.
Even though the ocean surface cooled slightly after the cyclone’s passage, the remaining heat in deeper layers was still enough to feed further intensification. This pattern suggests that future cyclones could draw more energy from the ocean even after surface cooling, leading to shorter but more destructive events.
Oceans as energy engines
Amphan’s rapid intensification underscores how ocean warming is reshaping storm dynamics in the Indian Ocean. The researchers found that the Upper Ocean Heat Content before Amphan’s formation was about 1 to 1.2 × 10¹⁰ joules per square metre, among the highest ever recorded for the Bay of Bengal.
Under future warming, these heat reservoirs will continue to expand. In the RCP 8.5 scenario, sea surface temperature cooling during a cyclone’s passage reached nearly three degrees Celsius, yet the total energy extracted from the ocean was even higher than today. This means more energy is likely to be transferred to the atmosphere, resulting in cyclones with greater rainfall, wind intensity, and storm surge.
Such changes have alarming implications for coastal India and neighbouring Bangladesh, where millions live within a few metres of sea level. Faster-moving storms will reduce warning times, while stronger ones could overwhelm existing disaster management systems.
Lessons for the future
The findings from IIT Kharagpur highlight the importance of integrating climate projections into cyclone forecasting systems. Improved prediction models that account for subsurface ocean heat and eddy structures could enhance the accuracy of early warnings. Additionally, coastal infrastructure and evacuation protocols must adapt to shorter lead times and stronger wind loads.
The research also highlights the importance of high-performance computing and coupled climate models in understanding the complex interactions between the ocean and atmosphere. The team utilized IIT Kharagpur’s Paramshakti supercomputer, supported by India’s Department of Science and Technology, to run their simulations, a resource-intensive process that reflects how modern climate science increasingly relies on big data and computation.
As the Bay of Bengal continues to warm, every extra degree of heat absorbed by the ocean increases the potential for devastation. Understanding these dynamics is crucial not only for India but for the entire Indo-Pacific region, where millions depend on the stability of monsoon systems influenced by these powerful storms.
Reference
Akhila, R. S., Kuttippurath, J., Chakraborty, A., Sunanda, N., & Peter, R. (2025). Rapid intensification of the Super Cyclone Amphan: Environmental drivers and its future projections. Tropical Cyclone Research and Review, 14(1), 27–39. https://doi.org/10.1016/j.tcrr.2025.02.005
