31 January 2003

NOAA Scientists Link International
Droughts and Ocean Temperatures

Weather patterns could be consequence of greenhouse gases

Droughts in the United States, Southern Europe and Southwest Asia in 1998-2002 were caused by the "perfect ocean for drought," according to a study published in the magazine Science by researchers from the National Oceanic and Atmospheric Administration (NOAA).

Cold sea surface temperatures in the eastern tropical Pacific and warm sea surface temperatures in the western tropical Pacific and Indian Ocean worked together to create an abnormal pattern in tropical rainfall, according to a January 31 NOAA press release. Climate scientists Martin Hoerling and Arun Kumar came to these conclusions after entering actual sea surface temperatures in climate simulation computer programs.

The scientists said, "What is suggested by the atmospheric modeling results of 1998-2002 is an increased risk for severe and synchronized drying of the mid-latitudes in the future, if these oceanic conditions continue to occur."

The unusual warming in the Pacific and Indian Oceans was attributed in part to increased greenhouse gases, according to the press release. NOAA officials say that further study should be conducted to learn more about ocean temperatures and climate change.

Following is the text of the press release:

National Oceanic and Atmospheric Administration

January 31, 2003

NOAA Scientists Attribute Recent Droughts to Ocean Influence

NOAA researchers studying the 1998-2002 droughts that spread across the United States, Southern Europe and Southwest Asia, believe they were linked by a common thread -- ocean conditions.

The findings are published in the Jan. 31 issue of Science. Lead author Martin Hoerling, a scientist at the NOAA Climate Diagnostics Center in Boulder, Colo., and colleague Arun Kumar, from the NOAA Climate Prediction Center in Camp Springs, Md., say cold sea surface temperatures in the eastern tropical Pacific and warm sea surface temperatures in the western tropical Pacific and Indian Oceans worked together synergistically to cause wide-spread drying in the mid latitudes. According to Hoerling, it was the "perfect ocean for drought."

During 1998-2002, the prolonged below normal precipitation and above normal temperatures caused the United States to experience drought in both the Southwest and Western states and along the Eastern seaboard. These droughts extended across southern Europe and Southwest Asia. "During the four-year period, as little as 50 percent of the average rainfall fell in these regions," said Hoerling. According to Hoerling, this was an abrupt change for the United States from what had been ranked as the wettest decade since at least the1890s.

Using climate simulations, the scientists assessed how the ocean conditions over the four-year period influenced climate. "We used the true monthly varying sea surface temperatures and then, using high-speed computers, ran several climate models more than 50 times and averaged their responses," Kumar said. "By running them multiple times, we could identify the common, reproducible element of the atmosphere's sensitivity to the ocean."

What the researchers found was that the tropical oceans had a substantial effect on the atmosphere. "There were unprecedented warm sea surface conditions in the western tropical Pacific, while at the same time, we had three-plus consecutive years of cold La Niña conditions in the eastern tropical Pacific," Hoerling said. "Usually, the La Niña conditions would have cooled the whole ocean."

According to Hoerling, "The warmth in the west Pacific during 1998-2002 simply has no precedent in at least the past 150 years." The researchers say that the combination of the warm and cold oceans shifted the tropical rainfall patterns into the far west equatorial Pacific.

What caused the remarkable conditions that occurred in the 1998-2002 period? The researchers say that while the cold sea surface temperatures were unusual, they were not unprecedented, but the warmth of the tropical Indian Ocean and the west Pacific Ocean was unsurpassed during the 20th Century. "Climate attribution studies find that this warming (roughly 1 degree Celsius since 1950) is beyond that expected of natural variability and is partly due to the ocean's response to increased greenhouse gases," they said.

The scientists added, "What is suggested by the atmospheric modeling results of 1998-2002 is an increased risk for severe and synchronized drying of the mid latitudes in the future, if these oceanic conditions continue to occur."

Randall Dole, director of the NOAA Climate Diagnostics Center, says "The study provides compelling evidence for the crucial role that the tropical oceans played in producing widespread severe and sustained drought over the period 1998-2002."

Dole says that while the study's primary focus was not to analyze the causes of the warming of the tropical oceans, the study does suggest that these droughts may be partly related to climate change and that further work needs to be done to completely understand the unprecedented warming of the western Pacific.

NOAA is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and providing environmental stewardship of the nation’s coastal and marine resources.

NOAA is part of the Department of Commerce.

end text