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What happened in 1996 that made heavier rainstorms more common?

  • This chart shows that the number of  "extreme precipitation" events in the Northeast U.S. was largely unchanged for many decades until 1996, but since then have been more common. Exactly why is unclear. Jonathan Winter et al—Dartmouth College


Monday, May 29, 2017

About 20 years ago, umbrellas suddenly became a lot more useful in New Hampshire.

Why do I make that odd statement? Because of a new study looking at “extreme precipitation events” – more than 2 inches of water falling in 24 hours as rain, snow or hail – in the Northeast.

The analysis of a century of rainfall data from 5,867 weather stations found, to everybody’s surprise, that such storms had been happening through nine-tenths of the 20th century, and then suddenly started happening more often.

“It’s not a linearly increasing trend. Rather, things are pretty flat until 1995, then there’s a shift. ... We’re getting about 53 percent more extreme precipitation since 1995,” said Jonathan Winter, an assistant professor in geography at Dartmouth College and one of the study’s co-authors.

Researchers did the study as a follow-up to a 2014 National Climate Assessment that said the Northeast had a much bigger increase in extreme precipitation than any other part of the country.

“That kind of jumps out,” Winter said. “We were interested in seeing whether that was actually true and whether we can pin it down so we know where to start looking for mechanisms.”

So they gathered more detailed data than was used in the national study and found that the jump mostly happened in the spring and fall. Heavy rainfall from 1996 to 2014 was about 83 percent higher in those seasons than it had been from 1901 to 1995.

This indicates that a likely culprit is change in the frequency or movement of nor’easters and tropical cyclones.

But why would that have changed in 1995? (No jokes about Bill Clinton’s first term, please.) That’s where they’re trying to figure out now.

“Right now, we’re looking into the past, days that had extreme events, and seeing the properties of the atmosphere when it happens. ... We’re less concerned about the strength of the storms and more interested in what kind of conditions are sitting around during these events,” Winter said.

For example, he said, they’re seeing if atmospheric conditions “may be steering (tropic storms) back into the Northeast, when they would have normally trucked off after North Carolina” and whether there are jet stream changes altering Nor’easter frequency.

It will be neat if they find something in the atmosphere that shifted suddenly in the mid-1990s, but “neat” isn’t the point. What they really want to do is improve computerized climate models so people can better predict what will happen in the future and better understand how human emissions are altering the climate.

“The third part of the study is to do modeling and run hypotheticals – take greenhouse gases out of the atmosphere – and see if we can get the same jump in extreme jumps without gases ... then if we put greenhouse gases back in, see if we still get the jump,” he said.

This sort of computer modeling is the target of much scoffing – “You can prove anything with a computer model” is the usual put-down, at least when beliefs are challenged – but it’s a valuable tool when trying to understand something as mind-bogglingly complicated as the climate.

In the meantime, I’m going to keep my umbrella close at hand.

The study was co-authored by Huanping Huang and Erich C. Osterberg in the Department of Earth Sciences at Dartmouth, as well as folks from Columbia University and the University of Vermont. It was published online in the American Meteorological Society’s Journal of Hydrometeorology.

(David Brooks can be reached at 369-3313, dbrooks@cmonitor.com, or on Twitter @GraniteGeek.)