Impacts of the El Niño Southern Oscillation on cyclone frequency and intensity during the post monsoon season in Bangladesh

Bangladesh sits on a vast, low-lying delta where the Ganges and Brahmaputra rivers meet the Bay of Bengal, making it one of the most climate-vulnerable places on Earth. Every year, this region faces the fury of tropical cyclones, massive storm systems that draw their power from warm ocean waters and can bring devastating winds and flooding to coastal communities. To understand when these storms might strike and how strong they could become, scientists look at a global climate pattern called the El Niño-Southern Oscillation, or ENSO. This pattern involves the warming and cooling of the surface waters in the central Pacific Ocean, which sends ripples through the atmosphere that can alter weather patterns thousands of miles away. When the Pacific warms, it is known as El Niño; when it cools, it is La Niña. These shifts do not just change the weather in the Pacific; they can suppress or encourage the formation of cyclones in the Indian Ocean, acting like a distant switch that influences the intensity of storms hitting South Asia. For people living in Bangladesh, knowing whether this distant switch is set to "on" or "off" could mean the difference between a manageable season and a catastrophic one.

A team of researchers from Pabna University of Science and Technology and the Bangladesh Oceanographic Research Institute set out to map the connection between these distant Pacific temperature shifts and the cyclones that batter the Bangladeshi coast. They focused specifically on the post-monsoon season, the period from October to November when the region is most prone to severe storms. By gathering historical records of cyclones dating back centuries and cross-referencing them with modern data on ocean temperatures and atmospheric conditions from 1982 to 2023, the team sought to uncover a reliable pattern. They wanted to know if the state of the Pacific Ocean could predict the strength of the storms in the Bay of Bengal, and if so, whether they could use that knowledge to forecast future risks.

The researchers found a clear, consistent relationship: when the Pacific Ocean is warmer than usual, indicating an El Niño event, the cyclones hitting Bangladesh tend to be weaker and less energetic. Conversely, when the Pacific is cooler, signaling a La Niña event, the storms that form in the Bay of Bengal are often more intense and carry significantly more energy. This negative correlation was not a one-time fluke but held true across four different decades of data. The team calculated that during periods of El Niño, the total energy of the cyclone season drops, while La Niña years are associated with a higher frequency of the most dangerous storms, those classified as very severe or super cyclonic. The data showed that while El Niño years might still produce a fair number of storms, they rarely reach the highest levels of destruction. In contrast, La Niña years, though sometimes having fewer total storms, are far more likely to produce the massive, high-intensity systems that cause the most damage.

To understand how this relationship works over time, the scientists looked at the timing of these events. They discovered that the influence of the Pacific Ocean on the Bay of Bengal is not immediate; it takes about four months for the changes in the Pacific to fully manifest as changes in the local sea temperatures that fuel cyclones. This delay means that the state of the Pacific Ocean in the early part of the year can serve as a warning sign for the severity of the storm season later in the year. The researchers also examined the Indian Ocean Dipole, another climate pattern involving temperature differences between the eastern and western Indian Ocean, and found that the most destructive storms tend to occur when the Pacific is cool and the Indian Ocean is in a neutral state. This combination creates an environment where the winds are less likely to tear a storm apart, allowing it to grow into a massive, powerful system.

Looking ahead, the team used advanced computer models to project what might happen in the coming decades. They simulated the behavior of the Pacific Ocean's temperature index for the next forty years, from 2025 to 2065. These simulations suggest a significant shift: the Pacific is likely to enter a prolonged period of cooling, with temperatures dropping to levels that indicate strong La Niña conditions, particularly between 2030 and 2045. If these projections hold true, the Bay of Bengal could face a future where the conditions are increasingly favorable for the formation of intense cyclones. The models predict that while the year-to-year variability will remain high, with some quiet seasons and some active ones, the overall trend during this period could see a rise in the frequency of high-energy storm seasons.

The implications for Bangladesh are profound. The study suggests that the risk of facing devastating cyclones may increase in the coming decades, not because the storms are becoming more frequent in a simple linear way, but because the climate conditions are shifting to favor the most powerful types of storms. The researchers emphasize that while the total number of storms might not skyrocket, the likelihood of those storms being of the highest, most destructive categories is rising. This insight is crucial for disaster preparedness, as it suggests that coastal communities need to prepare for stronger, more energetic storms rather than just more frequent ones. By understanding that a cooling Pacific often precedes a dangerous storm season in Bangladesh, officials and residents can better anticipate the risks and strengthen their defenses. The work underscores that while the ocean is vast and the climate complex, there are discernible patterns that, when understood, can help vulnerable populations navigate an uncertain future.