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Posted at 12:45 PM ET, 11/ 1/2010

Southeast rainfall more variable as climate warms

By Andrew Freedman

* Why was last year so snowy? | Freeze watch tonight: Full Forecast *

cwg_Freedman.jpgIn recent years the southeastern U.S. has had a string of summers with unusual amounts of rainfall. There was the withering drought in 2007, during which Georgia Governor Sonny Perdue famously held a prayer service for rain. This eventually worked -- albeit too well. Late last summer the drought-stricken region was hard hit by record flooding.

A new analysis of six decades of weather and climate data finds that such extreme summers are becoming more common in the region, due to shifting atmospheric steering currents that appear to be related to manmade global warming.

The study, published in the early online edition of the Journal of Climate, investigates a key driver of summertime weather in the Southeast -- the Bermuda High, referred to in the study as the "North Atlantic Summertime High" or "NASH." This High Pressure cell is typically centered in the vicinity of Bermuda, and often acts as a heat pump along the eastern seaboard, drawing warm and humid air up the coast throughout the summer. (You have the Bermuda High to thank for many of our summer heat waves in Washington).

A North Atlantic Summertime or Bermuda High that pumped oppressive heat and humidity this past summer into the Southeast U.S. and D.C. metro region. Courtesy College of Dupage.

By analyzing precipitation, wind, and humidity observations, along with data of the height of pressure surfaces aloft, the researchers found the western extent of the High pressure ridge has crept closer to the continental U.S. in recent decades, at the rate of 1.22 longitudinal degrees per decade. During that same period, the High has also grown stronger, and rainfall has grown much more variable from one summer to the next.

The study finds that during the period from 1948 to 2007, the frequency of summers with "strongly anomalous precipitation" has "more than doubled." The increased rainfall variability can have major consequences for natural systems and human society, particularly agricultural production.

"With the westward extension of the NASH, the north-south migration of its western ridge has had a greater impact on the summer precipitation over the SE [Southeast] US," the study states.

Here's the study's somewhat technical explanation of why the location of the western ridge is so important for determining regional rainfall:

"When the western ridge moves to the north, rainfall tends to decrease in the SE US because this region is located within [the] western part of the NASH and the weather pattern is dominated by subsidence [sinking air]. Likewise, when the NASH shifts southward, rainfall tends to increase in the SE US because this region is located to the northwest of the NASH where winds along the western edge of the NASH transport ample warm and moist air from Gulf of Mexico."

(By the way, such technical details aren't necessarily vital to understanding the gist of the study, but I include them because they will enable you to sound uber smart the next time you're in, I don't know, South Carolina during the summer and it's really dry out. Now you know to say, "Boy I sure hate it when the NASH shifts to the north like this.")

The study includes an attribution analysis that employs 23 global climate models to determine whether manmade global warming lurks behind the trends, or whether natural factors are the primary player in causing changes in the NASH. The conclusion? Manmade climate change is helping to strengthen and shift the western edge of the High, but natural factors are playing a role as well, particularly in moving the High north or south from year-to-year.

"Our attribution analysis suggests that global warming seems to be contributing to the changes of the NASH," the study states. The computer models showed even greater precipitation variability may be in store for the Southeast as greenhouse gas concentrations continue to increase, and the climate continues to warm during this century.

"Our analysis of the... models suggest that the NASH system will likely intensify, expand and move further westward in the 21st century with the increase of CO2 [carbon dioxide], indicating increased likelihoods of both extreme rainfall events and droughts over the SE US in the future," the study states.

"This is not a natural variation like El Nino," said lead author Wenhong Li of Duke University's Nicholas School of the Environment in a press release. "We thoroughly investigated possible natural causes, including the Atlantic Multivariate Oscillation (AMO) and Pacific Decadal Oscillation (PDO), which may affect highs, but found no links."

This study is consistent with some of the arguments put forward by senior Weather Channel senior meteorologist Stu Ostro, who has been noting changes in weather extremes, such as increasing air pressures aloft -- or rising heights, in weather-geek speak -- that he posits may be due to global warming.

The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.

By Andrew Freedman  | November 1, 2010; 12:45 PM ET
Categories:  Climate Change, Freedman, Science  
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It is amazing how the climate was completely static for 5 billion years until the Industrial Revolution started.

Posted by: DankWin | November 1, 2010 1:30 PM | Report abuse

From the image caption: "A North Atlantic Summertime or Bermuda High that pumped oppressive heat and humidity this past summer into the Southeast U.S. and D.C. metro region". Our biggest anomalies this past summer came from the continental high pressure. That's why the whole continent with the exception of the west coast was much warmer than normal. We shattered the high temperature records with dry heat, not Bermuda high moist heat.

"1948 to 2007" Ignores the 1930's and early 40's which had even greater anomalies than this past summer.

"the researchers found the western extent of the High pressure ridge has crept closer to the continental U.S. in recent decades, at the rate of 1.22 longitudinal degrees per decade." Their next claim, that the high has grown stronger, makes sense. Is the center further west or is the western extent further west because it is stronger? If the center of the high is further west, then why is that?

It's too bad they didn't study natural variations of the 1930's because those years were a better analog to this past summer.

Posted by: eric654 | November 1, 2010 1:42 PM | Report abuse

Eric564 - the study discussed here didn't analyze this past summer. The graphic is of a Bermuda High that did contribute to an extreme heat event in the Mid-Atlantic area this summer.

The study found that the western extent is further west, a trend that is related to the increase in the High's intensity rather than a shift in where the High is centered.

Posted by: afreedma | November 1, 2010 1:58 PM | Report abuse

East Coast and Southeast droughts and rainfall variation are nothing new. Some of the worst droughts on record, outside of the Dust Bowl days of the 1930s, were back in the early-mid-1960's. As a kid (I followed weather even back then) I can remember dry summer months in this area when we were getting around a half-inch or so...sometimes even less. Some of those dry summers, as in 1965, were followed by huge winter blizzards, as in January of 1966.

Posted by: MMCarhelp | November 1, 2010 5:52 PM | Report abuse

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