Boulder researchers link Hurricane Harvey’s intensity, climate change

Daily Camera (Boulder, CO)

May 11--Not for the first time, a prominent Boulder climate change scientist has linked the intensity of an historic storm with global warming, reporting that record warmth in the Gulf of Mexico "supercharged" 2017's Hurricane Harvey, the costliest tropical cyclone on record.

That assertion is made in a new study led by Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research. Trenberth previously drew a connection between global warming and the intensity of the devastating rains associated with the floods of 2013 on Colorado's Front Range.

"It is important to point out that hurricanes are natural, but now they are being supercharged by climate change," Trenberth, who is traveling this week, said in an email. "Of course, many appear as not unusual, but (hurricanes) Harvey, Irma and Maria were exceptional, and fit the mold expected with climate change."

Trenberth said that for the first time, a particular hurricane -- Harvey, in August 2017 -- was isolated in the Gulf of Mexico in such a way that it was possible to show that the volume of rain corresponds to the amount of water evaporated from an unusually warm ocean.

As climate change continues to heat the oceans, more supercharged storms such as Harvey can be expected, he said.

Trenberth and his partners in the new study were able to accomplish a detailed study of how Hurricane Harvey fed off the heat stored in the 930-mile-wide ocean basin.

Scientists compared temperatures in the upper 525 feet of the Gulf before and after the storm using data gathered by Argo, a network of autonomous floats that measure temperature as they move up and down in the water.

In order to measure rainfall over land, scientists utilized a new NASA-based international satellite mission, known as Global Precipitation Measurement.

According to a news release, as hurricanes move over the ocean, their winds "strafe" its surface, making it easier for water to evaporate. The warmer the temperatures are in the upper depths of the ocean and at its surface, the more energy is available.

Then, as hurricanes move across the ocean, evaporating water all the while, a cold wake is left in its path. With Hurricane Harvey, researchers found that the surface was not very cold. This was because so much heat was available in the upper levels of the ocean that as surface temperatures cooled from the storm, heat from below welled up, reheating the surface and continuing to feed the storm, increasing the evaporation from the surface by a factor of 10 or more.

Trenberth and his colleagues found that the near-surface ocean temperature before Harvey's passage was upward of 86 degrees, and after it passed, it was still around 83 degrees. Sea surface temperatures of 79 degrees are usually needed for a hurricane to keep growing.

Even after Harvey made landfall, its arms reached out over the ocean, enabling it to keep drawing strength -- and moisture -- from the still-warm Gulf.

Trenberth said the implication is that warmer oceans increase both the risk of greater hurricane intensity and duration.

"While we often think of hurricanes as atmospheric phenomena, it's clear that the oceans play a critical role and will shape future storms as climate changes," he said.

Trenberth stopped shot of saying that the series of devastating storms of 2017 would likely be reprised in the immediate years ahead.

"The paper does highlight the large natural variability, and so far this year does not look like a repeat, at least in the Atlantic," he said. "The big years seem to occur about every five years or so, in part because of things like El Niño and competition with the Pacific."

Trenberth is concerned that essential adaptation to the natural hazards of climate change is not happening in many vulnerable areas -- with major consequences. Harvey, for example, has been blamed for at least $125 billion in damages in Houston and beyond. In fact, he said, "Under this administration, we are going backwards.

"The Trump administration is doing away with regulations for no good reason other than ideological," he added. "Now, many of us do not like some regulations and some are senseless. But one should examine them and see whether they make sense. A lot could be said, here."

Other co-authors on the paper are Yongxin Zhang and John Fasullo, also of NCAR; Lijing Cheng, of the Chinese Academy of Sciences, and Peter Jacobs, of George Mason University.

The study appears in the journal Earth's Future, which is a publication of the American Geophysical Union. Funding came from the U.S. Department of Energy and NCAR's sponsor, the National Science Foundation.

Charlie Brennan: 303-473-1327, [email protected] or twitter.com/chasbrennan

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