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Tropical cyclones, commonly known as typhoons or hurricanes, can form in clusters and impact coastal regions back-to-back. For example, Hurricanes Harvey, Irma and Maria hit the U.S. sequentially within one month in 2017. The Federal Emergency Management Agency failed to provide adequate support to hurricane victims in Puerto Rico when Maria struck because most rescue resources and specialized disaster staffers were deployed for the responses to Hurricanes Harvey and Irma.

A new study in Nature Climate Change confirms these hurricane clusters are becoming more frequent in the North Atlantic in recent decades鈥攁 trend projected to continue in the near future.

Tropical cyclone clusters describe the event where two or more tropical cyclones are present simultaneously within the same basin. This phenomenon is not rare, as historically only 40% of tropical cyclones appeared alone. Beyond the combined impacts of individual storms, tropical cyclone clusters can cause disproportionate damage as coastal communities and infrastructures need time to bounce back from the impact of the first storm. Understanding tropical cyclone clusters and their future is thus important for coastal risk management.

Analyzing the historical observations of tropical cyclones, the authors found that during the past few decades, the chances for tropical cyclone clusters decreased in the northwestern Pacific basin, while they increased in the North Atlantic basin.

"We tried to develop a probabilistic framework to understand this trend," said Dazhi Xi, a climatologist at HKU who co-led the study and developed the methodology. "If tropical cyclone clusters are formed by chance, then only storm frequency, storm duration, and storm seasonality can impact the chance.

"So, as a first attempt, we simulated the formation of tropical cyclone clusters by probabilistic modeling, considering only these three mechanisms, and hoped we could find out why tropical cyclone clusters have changed in the past decades."

However, the probabilistic model is only partly successful. For some years, it significantly underestimates the chance of tropical cyclone clusters. It is because some storms coexist with other storms not simply by chance, rather, they have physical linkages.

"The previously failed statistical model will soon become a powerful tool that can distinguish physically-linked tropical cyclone clusters from those by pure chance," said Wen Zhou, a climatologist at Fudan University and the corresponding author of the study.

For those years that the probabilistic model fails, the authors find that synoptic scale waves, a series of train-like atmospheric disturbances, enhance the chance of tropical cyclone cluster formation.

The study further discovered that the La-Nina-like global warming pattern, characterized by slower warming in the eastern Pacific compared to the western Pacific, is the reason behind the observed shifts in tropical cyclone cluster hotspots.

"The warming pattern not only modulates the frequency of tropical cyclones in the North Atlantic and northwestern Pacific basins, but also impacts the strength of the synoptic scale waves, together causing the shift of tropical cyclone cluster hotspot from northwestern Pacific to North Atlantic basin" said Zheng-Hang Fu, a Ph.D. student at Fudan University who co-led the study.

The research establishes a probabilistic baseline model for investigating tropical cyclone cluster events and their underlying physical mechanisms. This framework not only explains the observed shift of tropical cyclone cluster hotspots from the northwestern Pacific to the North Atlantic basin, but also provides a transferable methodology applicable to other ocean basins worldwide.

Importantly, the authors identify the North Atlantic as an emerging hotspot for tropical cyclone clusters in recent decades. This finding calls for heightened attention from Atlantic coastal nations, urging them to develop proactive strategies against these compounding hazards.