Contrasting Changes in Tropical Cyclone Activity over the East Sea of Vietnam during 1983–2024

The ocean west of the Philippines and east of Vietnam is a vast, warm stretch of water known as the East Sea. It sits in one of the most active regions on Earth for tropical cyclones, those massive rotating storms that bring fierce winds and heavy rain. For decades, scientists have watched this area, trying to understand how these storms behave over time. They look at how often storms appear, when they happen during the year, how much total energy they release as they move, and how strong they get at their absolute peak. A common question in recent years has been whether these storms are simply getting stronger and more dangerous everywhere, or if the picture is more complicated. Understanding these patterns is vital for the millions of people living along the coast, as it helps them prepare for the storms that could threaten their homes and safety.

A team of researchers recently took a fresh look at the history of these storms in the East Sea, examining every recorded event from 1983 to 2024. They gathered data on thousands of storm positions and wind speeds, focusing specifically on the storms that passed through this region. Their goal was to see if the storms were changing in a uniform way—getting more frequent, stronger, or more energetic all at once—or if different parts of the storm picture were changing in different directions. What they found was a story of contrasts, where some aspects of the storm climate are shifting noticeably while others remain surprisingly steady.

The researchers discovered that the number of storms hitting the region is not changing in a simple, straight line. When they looked at the total number of storms over the entire year, the change was too small to be certain. However, when they split the year into two parts, a clear pattern emerged. During the main storm season, which runs from May through November, the number of tropical storms has dropped significantly. The data shows a decrease of nearly seven percent every ten years during these months. In contrast, the storms that appear outside of this main season, from December through April, have not shown any significant change in their frequency. This means that while the region is seeing fewer storms during the peak of the season, the off-season activity has remained steady, creating a shift in when the danger is highest rather than just how much danger there is overall.

This shift in timing also affects the total energy released by the storms. Scientists use a measure called accumulated cyclone energy to calculate the total power a storm exerts over its entire life in the region. The study found that this total energy has been trending downward, dropping by about six and a half percent every ten years. This decline is just on the edge of what statisticians consider a firm result, but it aligns with the drop in storm numbers during the main season. The researchers noted that this reduction in total energy does not mean the storms are becoming weaker at their strongest point. When they looked at the peak wind speed of the most intense storms each year, they found almost no change. The strongest storms in the region are still reaching the same high wind speeds as they did decades ago, even though there are fewer of them and they are releasing less total energy overall.

The intensity of these storms tells another part of the story. While the average strength of storms has not shifted, the very strongest ones continue to appear with the same regularity. The data shows that the most powerful typhoons, those with winds reaching up to 170 knots, are still occurring in recent years just as they did in the past. The researchers found that the middle range of storm intensities has stayed relatively stable, but the extreme outliers at the top end have persisted. This suggests that even if the region is experiencing fewer storms and less total energy, the threat from a single, massive super typhoon has not diminished. The danger from the most violent storms remains a constant reality, regardless of the decline in the number of smaller or moderate storms.

Looking deeper into the timeline, the researchers found evidence that the changes did not happen all at once. There appears to have been a turning point in the mid-1990s, around 1996, when the frequency of storms and the total energy they released began to shift. This change point was visible in the data for how often storms occurred and how much energy they carried, but it was not seen in the data for the peak strength of the storms. This indicates that the factors driving the number of storms and their total energy are different from the factors that determine how strong a single storm can get. The storms that form or pass through the East Sea are responding to different environmental signals, leading to a situation where the climate of the region is evolving in a complex, non-uniform way.

The study concludes that the idea of a single, simple trend for tropical cyclones in this region is incorrect. The East Sea is not simply getting more or less active in a straightforward manner. Instead, the region is experiencing a reorganization where the main season is becoming quieter, the total energy is dropping, and the timing of storms is shifting, all while the most extreme storms remain as powerful as ever. This means that relying on just one number to describe the future of storms in this area would be misleading. Fewer storms do not necessarily mean a safer region, because the most dangerous storms are still capable of forming with the same intensity. For those living in Vietnam and the surrounding areas, understanding this nuance is crucial: the risk is changing, but it is not disappearing, and the nature of the threat is becoming more specific to the season and the type of storm.