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USA drought and the ocean's influence.January 31, 2003 — 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.

Relevant Web Sites
NOAA Climate Diagnostics Center

NOAA Climate Prediction Center

NOAA Drought Information Center

Media Contacts:
Barbara McGehan, NOAA Research, (303) 497-6288 or Carmeyia Gillis, NOAA Climate Prediction Center, (301) 713-8000 ext. 7163