Catastrophic Intensity: Why Is Hurricane Irma Gaining Strength So Quickly?

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Astrid Caldas, climate scientist | September 5, 2017, 3:54 pm EDT

In a world that is increasingly defined by superlatives, let’s start with this just-released statement from the National Hurricane Center: Hurricane Irma is the strongest hurricane in the Atlantic basin outside of the Caribbean Sea and the Gulf of Mexico in their records, and a potentially catastrophic one, tied for second place as the strongest ever in the Atlantic. And it is following on the footsteps of Hurricane Harvey, which gathered strength very fast and dumped record amounts of rain on Texas and Louisiana.

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Harvey underwent a very rapid strengthening from tropical storm to category 4 hurricane (about 48 hours), and gained strength as it approached landfall, as opposed to the usual weakening. It had so much rain associated with it that the National Weather Service had to create new colors for their precipitation maps in order to properly depict the amounts. These extreme rain events are becoming increasingly common, with a number of 500-year rain events (i.e., those with a probability of 0.2% of occurring at any one year) that should be rare happening all over the country in a matter of months (Houston alone has now seen three since 2015).

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We know from studying hurricanes that many factors cause and drive them, but their main fuel is a warm ocean. Warm surface waters produce heat and water vapor. Hurricanes feed on and intensify from both, and the amount of rain they ultimately dump can be increased by both the higher availability of water vapor from the warm water and the fact that a warmer atmosphere can hold more of that available moisture (more on this here). Therefore, a trend of both increased intensity and rainfall associated with hurricanes can be expected, and in fact recent studies are in agreement with that (see here and here).

The waters off the coast of Texas when Harvey intensified from a category 2 to a category 4 hurricane were 2.7-7.2°F above average. The sea surface temperature where Irma was located as of the morning of 9/5 appears to be at least 2.7°F above average, which may have had a role in it turning into a category 5.

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Studies have detected the influence of human-made global warming both on the near-surface amount of water vapor, and in sea surface temperature in the tropical Atlantic ocean, among other areas such as the Western Pacific and South Asia – and it is worth noting that China has recently had a series of powerful typhoons, Hato and Mawar being the latest ones to wreak havoc in that part of the world, among other areas.

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