The research focuses on ocean hot spots where both surface and subsurface waters are warm enough to maintain very high wind speeds even as storms churn the upper ocean. In many basins, strong cyclones bring cooler water to the surface, which limits their intensity, but in these hot spot zones the deep reservoir of warm water prevents that self-limiting effect. As a result, storms that pass over these regions can retain or reach extreme wind intensities for longer periods, increasing their potential impact when they approach land.
Atmospheric scientist I-I Lin of National Taiwan University has examined the most intense cyclones for more than a decade, including Typhoon Haiyan, which struck the Philippines in November 2013 at peak strength and caused thousands of deaths. In 2014, Lin and colleagues published a paper in Geophysical Research Letters arguing that tropical cyclones with sustained winds above 160 knots should be recognized as a new Category 6, since the Saffir-Simpson scale's existing top category groups all storms stronger than 137 knots together despite other categories having wind ranges of about 20 knots.
Under this proposed extension, Category 4 would continue to describe storms with winds of 114 to 137 knots and Category 5 would cover 138 to 159 knots, while Category 6 would encompass cyclones with winds of at least 160 knots. Several well-known storms reach or exceed that threshold, including Hurricane Wilma in 2005, the most intense hurricane on record in the Atlantic basin, as well as Typhoon Haiyan and Typhoon Hagibis, which hit the Tokyo region in 2019 and caused major damage from rain and wind despite weakening before landfall. Hurricane Patricia, which formed off Mexico in the eastern Pacific, reached about 185 knots, a strength Lin notes would qualify as a hypothetical Category 7 were that level to exist.
Lin and co-authors reviewed records of tropical cyclones from 1982 to 2023 to track how often storms attained Category 6-level winds. Between 1982 and 2011, eight tropical cyclones exceeded 160 knots, but from 2013 to 2023 alone, ten storms reached that threshold, meaning 10 of 18 Category 6-intensity cyclones in the past roughly 40 years occurred in the most recent decade. This shift suggests that the environmental conditions required for such extremes have become more frequent in recent years.
The team's ongoing work, highlighted in a presentation at the American Geophysical Union's 2025 Annual Meeting in New Orleans, shows that most of these Category 6-level storms develop within identifiable hot spots in the western Pacific and North Atlantic. The largest of these zones lies in the western Pacific east of the Philippines and Borneo, while another covers parts of the North Atlantic around and east of Cuba, Hispaniola and Florida.
Further analysis indicates that these hot spots have grown in extent over time. In the North Atlantic, the region of deep warm water now stretches eastward beyond the northern coast of South America and westward across much of the Gulf of Mexico, while the western Pacific hot spot has also expanded. This widening means a larger stretch of ocean now provides the thermal structure needed to sustain very high wind speeds once storms form.
The conditions that favor Category 6-strength storms arise when warm water penetrates well below the surface, so that mixing by a cyclone does not pull up enough cooler water to weaken it. In other areas, storm-driven upwelling of cold water typically lowers sea surface temperatures beneath the cyclone and curbs its maximum intensity, but in these hot spots the deep warm layer allows intense winds to persist. Lin cautions that storms forming in hot spots do not automatically become Category 6 events, because atmospheric factors such as wind shear and humidity also must align, describing the hot spots as a necessary but not sufficient condition for these extremes.
To estimate what drives the expansion of these deep warm layers, the researchers separated the influence of long-term global warming from natural climate variability. Their assessment suggests that both play roles, but that human-induced climate change accounts for about 60% to 70% of the increase in hot spot size and the associated rise in Category 6-intensity cyclones. This contribution indicates that continued greenhouse gas emissions are likely to further strengthen and enlarge these regions if warming trends persist.
Lin argues that formally recognizing a Category 6 on operational scales would help coastal communities better prepare for the most destructive storms, particularly where hot spots are growing and these events are becoming more frequent. "We really think there is a need just to provide the public with more important information," Lin said, noting that clearer communication of extreme wind risk could guide infrastructure design and emergency planning.
Lin will present the work in an oral session on tropical cyclones at AGU25, which runs from 15 to 19 December at the Ernest N. Morial Convention Center in New Orleans and brings together more than 20,000 scientists in Earth and space science fields. The presentation, titled "A31A-06 Category '6' Tropical Cyclone Hot Spots in the Warming Climate," is scheduled for Wednesday, 17 December, from 9:34 to 9:45 Central Time in Room 278-279.
Research Report:A31A-06 Category '6' Tropical Cyclone Hot Spots in the Warming Climate
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American Geophysical Union
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