Joseph Romm: Global warming is driving increased frequency of extreme wet or dry summer weather in southeast, so droughts and deluges are likely to get worse

Global warming is driving increased frequency of extreme wet or dry summer weather in southeast, so droughts and deluges are likely to get worse

by Joseph Romm, Climate Progress, October 28, 2010

A new study by a Duke University-led team of climate scientists suggests thatglobal warming is the main cause of a significant intensification in the North Atlantic Subtropical High (NASH) that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States.

The NASH, commonly referred to as the Bermuda High, is an area of high pressure that forms each summer near Bermuda, where its powerful surface center helps steer Atlantic hurricanes and plays a major role in shaping weather in the eastern United States, Western Europe and northwestern Africa.

That’s from the Duke University news release for a new study in the Journal of Climate.
In a September 2009 post, “Hell and High Water hits Georgia,” I noted that, “as climate scientists have predicted for a long time, wild climate swings are becoming the norm, in this case with once-in-a-century drought followed by once-in-a-century flooding.”  And in fact, the flooding was more like a once in 500 year event.
Now a team of scientists has quantified the rise in extreme wet and dry summer weather — and finds global warming is likely the main cause.  The release continues:

By analyzing six decades of U.S. and European weather and climate data, the Duke-led team found that the center of the NASH intensified by 0.9 geopotential meters a decade on average from 1948 to 2007.  (Geopotential meters are used to measure how high above sea level a pressure system extends; the greater the height, the greater the intensity. 

The team’s analysis found that as the NASH intensified, its area enlarged, bringing the high’s weather-making western ridge closer to the continental United States by 1.22 longitudinal degrees a decade. 

“This is not a natural variation like El Nino,” says lead author Wenhong Li, assistant professor of earth and ocean sciences at Duke University’s Nicholas School of the Environment.   “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. 

“Our analysis strongly suggests that the changes in the NASH are mainly due to anthropogenic warming,” she says. 

An early online edition of the study, published in the Journal of Climate, is available at the American Meteorological Society’s website at http://journals.ametsoc.org/ doi/ pdf/ 10.1175/ 2010JCLI3829.1.

The study has a firewall.  For the text alone, you can get around it here.

As the NASH intensified and migrated westward, Li’s team’s analysis found that its meridional variation, or north-south movement, also was enhanced from 1978 to 2007, a period when the frequency of extreme summer rainfall variability in the Southeast more than doubled over the previous 30 years.  

From 1978 to 2007, 11 summers – defined in this study as the months of June, July and August – had total seasonal precipitation anomalies greater than one standard deviation from the mean.  Six of the summers were abnormally wet, while five were abnormally dry. 

To forecast future trends in the NASH’s intensity, the team used climate models developed for use by the Intergovernmental Panel on Climate Change’s Fourth Assessment Report in 2007. The models – known as  Coupled Model Intercomparison Project Phase 3 (CMIP3) models – predict the NASH will continue to intensify and expand as concentrations of carbon dioxide and other greenhouse gases increase in Earth’s atmosphere in coming decades. 

“This intensification will further increase the likelihood of extreme summer precipitation variability – periods of drought or deluge – in southeastern states in coming decades,” Li says. 

If the NASH ‘s western ridge’s meridional movement jogs a little to the north as it expands, the likelihood increases for more extreme dry weather in the Southeast that summer, she explains. If the NASH wobbles a little to the south, extreme wet weather becomes more likely… 

In addition to long-term rainfall data and the CMIP3 models, the team used atmospheric reanalysis data from the U.S. National Center for Environmental Prediction/National Center for Atmospheric Research and the European Centre for Medium-Range Weather Forecasts to conduct the study.

For a meteorologist’s take on the amazing 2009 Georgia flooding, also using the NCEP reanalysis data, see “Weather Channel expert on Georgia’s record-smashing global-warming-type deluge.”

The peer-reviewed analysis concludes that “the NASH system will likely intensify, expand and move further westward in the 21st century with the increase of CO2, indicating increased likelihoods of both extreme rainfall events and droughts over the SE US in the future.”

The scary part, as I’ve said many times, is that we’ve only warmed about a degree Fahrenheit in the past half-century.  We are on track to warm nearly 10 times that this century (see M.I.T. doubles its 2095 warming projection to 10 °F — with 866 ppm and Arctic warming of 20 °F ). 

And that’s just business as usual. 

The plausible worst-case scenario is beyond comprehension:

• UK Met Office: Catastrophic climate change, 13-18 °F over most of U.S. and 27 °F in the Arctic, could happen in 50 years, but “we do have time to stop it if we cut greenhouse gas emissions soon.”

Drought and deluge — Hell and High Water — we ain’t seen nothing yet!

Link:  http://climateprogress.org/2010/10/28/global-warming-extreme-wet-dry-summer-weather-in-southeast-droughts-and-deluges/