Dartmouth Researchers Look For Causes Of Rise In 'Extreme Precipitation'

Aug 16, 2018

In 1996, the Northeastern U.S saw a sudden increase in the number of "extreme precipitation" events. And ever since then, that number has stayed elevated. So a group of researchers at Dartmouth College set out to figure out why.

Jonathan Winter, an assistant professor of geography at Dartmouth who co-authored the study, spoke to VPR's Henry Epp about the researchers' findings, which were published earlier this summer in the Journal of Geophysical Research: Atmospheres.

Listen to the conversation with Winter above; below find excerpts of Winter's responses during the interview.

The extreme precipitation experience:

"The primary driver of this shift has been hurricanes and tropical storms, so they've caused almost half of the increase in extreme precipitation that occured in 1996. And then we've also seen increases in extreme rain and snow from other types of events, and these include intense downpours along cold fronts, and these are mostly during severe thunderstorms — they're responsible for 25 percent. And then extratropical storms, which are storms that don't form in the tropics — an example of this is nor'easters — and they're responsible for 15 percent."

Causes, part 1:

"Behind the cause of the increase in hurricanes, we think, is warmer Atlantic Ocean temperatures and more water vapor in the atmosphere. So with warmer ocean temperatures, you're able to fuel the growth and intensity of hurricanes and tropical storms. So since 1996, we've had elevated ocean temperatures in the Atlantic. And then warmer air can hold more water vapor. So basically you can think of it as having a bigger bucket so when you actually do get the rain event, you can empty that bigger bucket and cause a bigger event."

Causes, part 2:

"When we look at the cold fronts and the extratropical cyclones, like nor'easters, we find that they're associated with a wavier jet stream. So the jet stream is basically what brings us our weather — it comes from west to east — and the wavier it is, a few things can happen: So you can have basically cold air coming down from the north and be able to form kind of larger and more intense cold fronts. And then actually as that jet stream gets wavier, it also sets the conditions for the formation of things like nor'easters, so extratropical cyclones."

Where climate change fits in:

"I think it would be fair to say that climate change is behind some of it, but not all of it ... We know that there's climate variability and we've been in a relatively warm phase of the Atlantic Ocean's temperatures, so we know that part of that is probably just natural. And then you can think about basically that we've just added kind of a manmade signal on top of that."

What happened in 1996:

"We do know that in 1996 we transitioned from a relatively cool phase of the Atlantic to a relatively warm phase. We know that that's part of what's driving the increase in extreme precipitation, but we also know that the last time the Atlantic Ocean was warm, we didn't have this increase in extreme precipitation. So that's still an outstanding question."

What next?

"We do basically expect this pattern to continue, but the big question is when this kind of warm phase of the Atlantic ends and we go back into our cold phase, what exactly is going to happen. That's kind of the next question we're going to tackle, is how much of this is caused by that shift in ocean temperatures, caused by climate variability, and how much of it is really driven by greenhouse gas emissions and climate change.

Advice for community leaders and planners:

"This shift, for now, is here to stay, so when we go about sizing infrastructure or other types of decisions that could put people at risk for being flooded, they should design with this kind of a reference frame — so this most recent period, 1996 to present — versus previous time periods when we've had less extreme precipitation."