Warming Oceans Linked to Four Year Drought

By Cat Lazaroff

CAMP SPRINGS, Maryland, February 4, 2003 (ENS) - Droughts that spread across the United States, southern Europe and southwest Asia over the past four years may have been linked by a common thread: ocean conditions created by a warming climate. A new study suggests that cold sea surface temperatures in the eastern tropical Pacific and warm sea surface temperatures in the western tropical Pacific and Indian Oceans worked together to cause widespread drying.

Researchers at the National Oceanic and Atmospheric Administration (NOAA) studying the 1998-2002 droughts discovered the link when they took a closer look at ocean conditions during the same time period. According to lead author Martin Hoerling, a scientist at the NOAA Climate Diagnostics Center in Boulder, Colorado, it was the "perfect ocean for drought."


Drought leaves behind cracked mud near Mobile, Alabama. (Photo courtesy U.S. Army Corps of Engineers, Mobile District Visual Information)
Hoerling and his colleague Arun Kumar, from the NOAA Climate Prediction Center in Camp Springs, published their findings in the January 31 issue of the journal "Science."

During 1998-2002, a prolonged period of below normal rain and snowfall, 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 also 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. He explained that this was an abrupt change for the United States from what had been ranked as the wettest decade since at least the 1890s.

Using climate simulations, the scientists assessed how the ocean conditions over the four year period influenced climate.

refugee camp

Iranian Red Crescent refugee camp on the Afghanistan/Iran border is plagued by drought and swept by strong winds. (Photo courtesy IFRC)
"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 Niņa conditions in the eastern tropical Pacific," Hoerling said. "Usually, the La Niņa conditions would have cooled the whole ocean."

However, he added, the warmth of the western Pacific during 1998-2002 "simply has no precedent in at least the past 150 years."


The Big Sandy River in Wyoming is at record low flow conditions following several years of drought. (Photo by J. Wheeler, courtesy U.S. Geological Survey)
The researchers say that the combination of the warm and cold oceans shifted the tropical rainfall patterns into the far west equatorial Pacific, leaving the mid-latitudes high and dry.

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.


Drought shrinks a pond on an Oklahoma farm. (Photo courtesy NOAA)
"Climate attribution studies find that this warming (roughly one 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 wrote. "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, said 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 said 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.


Some parts of the United States, particularly the Pacific Northwest and the northeastern states, are still facing drought conditions, according to NOAA's 2003 predictions. (Graphic courtesy NOAA)
"It is an open question whether such tropical oceanic forcings will become more prevalent during the 21st century," the researchers wrote.

While current models of the interactions between oceans and the atmosphere do not offer much confidence regarding predictions of future droughts, the researchers conclude that the modeling suggests that much of the Earth could continue to face severe, simultaneous drought if tropical sea surface temperatures continue to rise or to become more variable.