Scientists are working to understand what caused Hurricane Otis, originally forecast to make landfall on the western coast of Mexico as a strong tropical storm or minimal hurricane, to explode in intensity overnight. Most forecast models missed this change, leaving coastal residents unprepared for 165 mph winds the storm brought to Acapulco in the early hours of Wednesday morning.
When I studied landfalling hurricanes in graduate school in the early 2000s, rapid intensification was very unusual and often associated with eddies of warm water that separated from the Loop Current in the Gulf of Mexico. In 2005 Hurricane Katrina rapidly intensified after moving over one of these warm eddies and at the time it was remarkable.
If you have been paying attention to hurricanes in the past five years, however, you will know that rapid intensification is no longer as remarkable as it once was. Part of the reason for that is that unusually warm sea surface temperatures, and the related quantity upper ocean heat content, are no longer confined to warm water eddies. A large portion of the extra heat energy trapped in the earth system is stored in the oceans. And that means more fuel for strong hurricanes.
A Washington Post article lists eight infamous rapidly intensifying hurricanes in the Atlantic basin that have made landfall since 2017: Harvey, Irma, Maria, Michael, Laura, Ida, Ian, and Idalia.
Meteorologists define rapid intensification as more than 35 mph increase in wind speed in less than twenty four hours. The destructive potential of wind is related to the square of the wind speed, so a big increase in wind speed can mean an even bigger increase in the destructive potential of a storm. Rapid intensification right before landfall is the nightmare scenario. That means that people in the storm’s path who expect a less intense storm are unprepared.
This is what happened to the residents of Acapulco, Mexico Tuesday evening when Tropical Storm Otis, which was forecast to make landfall as a strong tropical storm or minimal hurricane, rapidly intensified into a Category 5 hurricane, the highest of the hurricane classifications on the Saffir Simpson scale, in fewer than 24 hours. The storm’s peak wind speed increased by 110 mph in that period, leaving residents with little time to evacuate or make other preparations for the storm, which made landfall with wind speeds of 165 mph.
We are beginning to hear reports of the situation in Acapulco now and the conditions are dire. Twenty-seven lives lost and widespread destruction.
With so many hurricanes undergoing rapid intensification recently, you might wonder if climate change has anything to do with it.
According to the Yale Climate Connections blog, Otis had some characteristics that made rapid intensification more likely. It was a small hurricane, with hurricane force winds extending only thirty miles from the center of circulation. And the water off the coast of Acapulco was unusually warm, because of the very warm late summer weather we had this year. Colorado State meteorologist Philip Klotzbach told the Washington Post that the warm water off the western coast of Mexico gave Otis an “extra boost.” Conditions in the upper level of the atmosphere also favored rapid intensification, by ventilating the storm from the top.
That question of how climate change will change the intensity and frequency of tropical cyclones was the subject of a lively debate I attended among prominent hurricane scientists at the 2002 American Meteorological Society Hurricanes and Tropical Meteorology Conference, held in Miami, Florida. Warm water is not the only determining factor for hurricane intensity, some argued, other conditions like shear, dry air, and steering flow could all change in different ways in a warming climate, making it hard to know how hurricane intensity and frequency will be affected. The debate went on for so long that it had to be continued at a nearby bar after the conference adjourned.
But the effect of climate change on tropical cyclones has since come into focus. One of the scientists involved the 2002 conversation, Massachusetts Institute of Technology researcher Kerry Emanuel, later reported the results of a March 2017 study in which he found that rapid intensification is more likely to occur in a warming climate.
Emanuel’s research finds that the potential rate of intensification is a function of the maximum potential intensity of a given hurricane, a value that is related to the sea surface temperature. That means that a higher sea surface temperature supports not just a stronger hurricane, but a faster rate of intensification. And since sea surface temperatures are projected to increase because of climate change, that would mean that intensification rates of tropical cyclones would also increase.
Emanuel told the Washington Post that “Exactly how the planet’s record warmth may have contributed to Otis’s rapid development will take months if not years to untangle. It’s possible record global heat in July, August, and September helped contribute to warmth deep into the water column.”
Sea surface temperatures in the area where Hurricane Otis grew into a Category 5 storm were 88°F, nearly 2°F warmer than climatological averages.
Research points to increases in both hurricane intensity and the rate of hurricane intensification in a warming climate, something coastal cities like Houston should be concerned about. Large coastal cities like Houston require lots of advanced notice to prepare effectively. Evacuations can be dangerous if they are not carried out in an orderly fashion, which we learned during Hurricane Rita in 2005.
As Texans, we know how disruptive even minor hurricanes can be. Dangerous rapid intensification is just one more reason to mitigate climate change by transitioning away from burning fossil fuels. Scientists warn that the dangers of climate change will not increase to a new normal and then stay there, but that conditions will become more dangerous and unpredictable as the global temperature increases. As research and observations show, rapidly intensifying hurricanes are part of the dangers of a warming world.