The North Atlantic has claimed a new title as the global hotspot for hurricane clusters, with the likelihood of these forming increasing tenfold in the past 46 years. A new study provides the first robust quantification of how the Earth's warming patterns are shifting where these multiple-storm events happen, as scientists warn that the US East Coast should be prepared.
Researchers from Fudan University made these findings after analysis of observational data from the past 46 years alongside high-resolution climate model simulations. The data revealed that the probability of the North Atlantic having more clusters than the former leading hotspot of tropical storm clusters – the western North Pacific – has risen from 1.4% to 14.3%. This has most likely shifted due to changing cool-warm water regions that now provide ideal conditions for multiple storms to emerge around the same time from a shared basin.
As the term suggests, a hurricane cluster is where two or more tropical storms become active in the same ocean region at the same time, often hitting in quick succession and compounding the damage. In 2017, hurricanes Harvey, Irma and Maria hit in quick succession, which crippled emergency and recovery efforts. In 2020, five named storms spun across the Atlantic at once.
For the US, this puts the Gulf Coast and the East Coast, as well as territories Puerto Rico and the US Virgin Islands, at much higher risk of experiencing worse, overwhelming hurricane events. The threat is of particular concern for areas where population density and infrastructure vulnerability are high, since recovery time between storms can be severely reduced.
"We tried to develop a probabilistic framework to understand this trend," said Dazhi Xi, a climatologist at the University of Hong Kong, 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 simulate the formation of tropical cyclone clusters by probabilistic modeling, considering only these three mechanisms, and hoped we could find why tropical cyclone clusters changed in the past decades."
However, for various reasons, this modeling isn't the full picture due to the unpredictability of how some clusters form – for example, by chance rather than being physically linked. So for those years where modeling didn't capture cluster anomalies, extra data like synoptic scale waves – large, moving patterns in the atmosphere that can create the right conditions for tropical storm formation, sometimes helping one storm trigger another – shed new light on just how prone the North Atlantic had become to these cluster events.
"The previously seemed-failed statistical model now soon becomes a powerful tool that can distinguish physical-linked tropical cyclone cluster with those by pure chance," said Wen Zhou, a climatologist at Fudan University and the corresponding author of the study.
Their data also revealed that it's a "La Niña-like" warming pattern – slower warming in the Eastern Pacific versus the Western Pacific – has caused this tenfold rise in North Atlantic cluster risk.
"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," explained Zheng-Hang Fu, a researcher at Fudan University who co-led the study.
While this is probability modeling, it does show a dramatic shift in warming patterns over the last half a century. And the researchers point out that such a rise in statistical likelihood of cluster hurricanes comes with a very real risk to areas facing the North Atlantic.
When hurricanes arrive in rapid succession, emergency services can get overwhelmed, supply chains can be cut off and infrastructure still damaged from a recent storm becomes more vulnerable. Recovery time shrinks to days or hours, and overlapping disasters make coordinated relief – challenging in the aftermath of any hurricane – all that more difficult.
The team's predictions have suggested that the current trend – more cluster hurricanes – will continue through at least the middle of the century, meaning more seasons with concentrated bursts of these damaging storms likely to make landfall. The work, the researchers hope, will help preparedness plans that factor in not just hurricanes spread across the season but several hitting in close succession.
"Our research highlights the importance of TC [tropical cyclone] clusters for hazard assessment, which often assumes independent TC events," the scientists noted. "Future research could explore more sophisticated modelling to explicitly capture dynamic interactions within TC clusters and investigate the landfall phase of TC clusters to support hazard assessment frameworks towards better representation of such temporally compound events."
The research was published in the journal Nature Climate Change.
Source: Fudan University via EurekAlert!
