The Rise and Fall of Rainfall in a Warming World

by Bill Chameides | November 4th, 2010
posted by Erica Rowell (Editor)

Yes, Betty Sue, it really is getting drier; and you’re right, too, Uncle Bob, it’s also getting wetter.

Don’t put it down to nostalgia and poor recall when the old-timers down at the feed store relate their hard-luck stories about how their crops just can’t keep up with the summer rain’s feast or famine that seems so much worse these days than back in the day. A new study in the Journal of Climate by my colleague Wenhong Li of Duke University’s Nicholas School of the Environment and co-authors confirms such observations — that shifting weather patterns over the last 60 years are contributing to more variability in summer rainfall over the Southeast. And that shift seems to be driven by climate change.

Rainfall is expected to become more variable with climate change, leading to more and heavier rains but also to longer droughts. Evidence of this pattern has been trickling in from around the country (see examples here, here and here) as well as from the Southeast. 

Li and her colleagues dug into this evidence to try to get at the root causes.

Extremes on the Rise

The first step in the Li et al study was to confirm that the Southeast has in fact experienced a change in rainfall patterns — and that they did.

The authors found that the frequency of extreme rainfall variability more than doubled during the back half of the study period from 1978 to 2007 as compared to the earlier 30-year period. The change perhaps seems a little less astounding when it is expressed in terms of the number of extreme years in each period: there were four extreme summers in the first 30 years and 11 in the latter. Still the change in rainfall patterns is pretty substantial. What could be the cause? The authors’ hypothesis focused on the so-called Bermuda High.

What’s Up With the High?

The Bermuda High, more formally known as the North Atlantic Subtropical High, is a high-pressure system that forms each summer and hangs around just off the east coast of the Atlantic in the general area of its Bermuda namesake. As the high changes in intensity and position, it exerts a major influence on a lot of the summer weather (hurricane tracking, etc.) that comes our way in the Southeast.

For example, the high’s influence on rainfall in the Southeast gets stronger the closer the high is to the North American continent — when it jags to the west, it exerts more influence; a jag east means less influence. Similarly, the Bermuda High’s influence grows as it becomes more intense. As it  shifts westward, it also influences the amount of rain we get in the Southeast by migrating northward or southward — suppressing rainfall when it moves to the north and enhancing it when it shifts to the south.

So maybe, Li et al reasoned, the increase in extremes in rainfall in the Southeast has been caused by changes in the Bermuda High. And it was a good conjecture on their part.

Over the 60-year study period, from 1948-2007, the authors found that the high has grown stronger — increasing in intensity by about 0.9 geopotential meters (a measure of strength) on average per decade. At the same time that the high has grown stronger, it has grown larger and moved its western ridge west, closer to the continental United States by about 1.2 degrees longitude per decade.

But Is There a Climate Connection?

So, rainfall is getting more extreme and it appears to be due to a change in the Bermuda High. But what’s behind these trends, and do they reflect natural variations or anthropogenic changes?

To answer these questions, Li et al used an array of climate models to see what kinds of processes could explain the change in the Bermuda High. Natural processes that could lead to this sort of long-term multidecadal trend that the authors considered were the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation. But they found little to no correlation between these oscillations and the shift and intensification of the high. The one thing they did find correlated with the changes observed over the 30-year period was global warming.

The implications: the increase in rainfall extremes in the Southeast are probably caused by a shift in the Bermuda High which has, in turn, been driven by climate change. And Li et al’s model projections through the 21st century predict that the high will continue to intensify and lead to more variable weather.

So, the next time you see an old-timer harking back to the good old days, remember to commiserate — the new normal may be abnormal.

filed under: climate change, faculty, global warming, North America, rainfall, weather
and: Atlantic Multidecadal Oscillation, Bermuda High, extreme weather, North Atlantic Subtropical High, Pacific Decadal Oscillation, United States, Wenhong Li