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Texas heat wave of 2011 largely caused by drought, ocean temperatures, says NOAA-led study

Texas heat wave of 2011 largely caused by drought, ocean temperatures, says NOAA-led study

Thursday, November 15, 2012

Contact: Monica Allen, 301-734-1123

Drought and heat are no strangers to Texas, but the scorching summer of 2011, about 5 degrees Fahrenheit hotter than average, smashed previous heat records in the state. La Niña-related sea-surface temperature conditions were the most important factor leading to the drying out in Texas and the U.S. Southern Plains last year, according to a new study by NOAA scientists and colleagues. And last year's heat, which mostly resulted from such lack of rainfall, was rare but within the range of natural temperature swings, the researchers found.

The researchers found that human-induced climate change, however, increased the chances of breaking state heat records by making the temperatures somewhat warmer than would otherwise have been the case.

"Our study shows that even in in the absence of human-caused climate change, heat waves over Texas can be substantially hotter than we’ve observed so far in the historical record," said lead author Martin Hoerling, Ph.D., a research meteorologist at NOAA’s Earth System Research Laboratory in Boulder, Colo.

The new paper, published online in the Journal of Climate this month, also concluded that the 2011 heat wave was well anticipated by a suite of NOAA forecast systems including official seasonal predictions.

Sun over the horizon

Sun over the horizon

The summer of 2011 was hotter and drier than usual in Texas. A new NOAA study shows that the extreme heat was caused mainly by nine months of drought in Texas and the rest of the Southern Plains leading up to last summer. Image: public domain

Natural factors, primarily La Niña, drove weather patterns that increased the risk of drought and extreme heat in Texas, and were likely responsible for about 4 of the 5 degrees Fahrenheit that exceeded the state average last summer. Extreme drought beginning in the fall of 2010 and continuing into the summer played a crucial role in the extreme summertime heat, the researchers found.

Drought makes extreme heat more likely, as a region with dry soil can warm more than one with wet soil. Prior hot and dry periods over Texas during the last century often occurred during La Niña years. The study also found that climate change contributed about an additional 1 degree to the summertime warmth. Climate change did not appreciably affect rainfall patterns.

To dissect the effects of many weather and climate factors on last summer's Texas heat wave, Hoerling and his colleagues drew on diverse tools: weather records, historical simulations of global weather patterns from sophisticated climate and weather forecast models set with the particular conditions leading up to summer 2011. His team also analyzed results from the Coupled Model Intercomparison Project, an international climate modeling program that integrates many different global climate models and model runs.

Importantly, the team found a high level of predictability of the particular severe conditions that materialized over Texas in 2011, a finding that may help improve future forecasting of extreme heat events.

Climate change will likely nudge Texas toward more extremely hot summers in the future, said Randall Dole, Ph.D., a co-author on the paper and also a research meteorologist at NOAA's Earth System Research Laboratory. "By later in this century, even in the absence of severe drought such extreme heat will become increasingly common due to human-caused climate change."

Co-authors of the new paper, "Anatomy of an Extreme Event," are Arun Kumar (NOAA), John Nielsen-Gammon (Texas A&M and state climatologist), Jon Eischeid (CIRES), Judith Perlwitz (CIRES), Xiao-Wei Quan (CIRES), Tao Zhang (CIRES), Philip Pegion (CIRES) and Mingyue Chen (NOAA). Read the study onliine here [opens a pdf, subscription may be required].

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