Variable southeast summer rainfall linked to climate change

October 27, 2010

DURHAM, N.C. - A doubling of abnormally wet or dry summer weather in the southeastern United States in recent decades has come from an intensification of the summertime North Atlantic Subtropical High (NASH), or "Bermuda High."

And that intensification appears to be coming from global warming, according to a new analysis by a Duke University-led team of climate scientists.

The NASH 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.

By analyzing six decades of U.S. and European weather and climate data, the 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 grew, 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.

As the NASH intensified and migrated westward, Li's team's analysis found that its 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, six summers were abnormally wet, while five were abnormally dry. Those 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.

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 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.
-end-
Li's coauthors are Laifong Li of Duke University; Rong Fu of the University of Texas at Austin; Yi Deng of the Georgia Institute of Technology; and Hui Wang of the National Atmospheric and Oceanic Administration's Climate Prediction Center in Camp Spring, Md., and Wyle Information Systems in McLean, Va.

In addition to long-term rainfall data and the CMIP3 models, the team conducted the study on 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.

Funding for the study came from Duke University and the Nicholas School Office of the Dean.

Duke University

Related Climate Change Articles from Brightsurf:

Are climate scientists being too cautious when linking extreme weather to climate change?
Climate science has focused on avoiding false alarms when linking extreme events to climate change.

Mysterious climate change
New research findings underline the crucial role that sea ice throughout the Southern Ocean played for atmospheric CO2 in times of rapid climate change in the past.

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.

Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.

Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.

Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.

A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.

Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).

Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.

Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.

Read More: Climate Change News and Climate Change Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.