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The Perfect Ocean for Drought

Global droughts during 1998-2002 attributed to Ocean Influence

by Barbara McGehan

"The warmth in the west Pacific during 1998-2002 simply has no precedent in at least the past 150 years."

Drought became heartbreakingly real last year for many Colorado farmers and ranchers as they watched their cattle starve and their pastures turn to dust. For four years now, due to prolonged below normal precipitation and above normal temperatures, farmers, cattlemen, and regular suburbanites, in the Southeast, Southwest, and Western United States, have struggled to raise crops, feed their livestock and water their grass and plants.

But the drought that has held the U.S. in it's vise-like grip has not been an isolated event. Across the world, in southern Europe and Southwest Asia, drought conditions have also ruined crops and caused devastating hardship. "During the 4-year period from 1998 to 2002, as little as 50 percent of the average rainfall fell in these regions," said Martin Hoerling, a research meteorologist with NOAA's Climate Diagnostics Center in Boulder, Colo. According to Hoerling, in the U.S., the 1990's had been ranked among the wettest decades since at least the 1890's

Surface temperature and precipitation deviations (from normal) that were observed and annually averaged during the 4-year period from June 1998 to May 2002. The largest warm and dry departures are highlighted in red. (larger image)

After looking at this data, Hoerling became curious and decided to investigate whether there was a common cause for these droughts. During the four year period from 1998-2002, cooler waters had been in effect in the eastern Pacific, part of the phenomenon known as La Niña.

While scientists have known for some time that La Niña can cause drought in the United States, the evidence for it's affecting other parts of the world has not been as obvious. But Hoerling and colleague Arun Kuman from NOAA's Climate Prediction Center in Camp Springs, Md., suspected there may be a common cause for the world drought, namely, ocean temperatures. The researchers published their findings recently in the Journal Science.

Sea surface temperature (SST) deviations or anomalies for the period from June 1998 to May 2002. The inserts illustrate the monthly time series of Sea Surface Temperature deviations for the warm pool region of the tropical Indian and west Pacific (left) and the cold tongue region of the equatorial east Pacific (right). (larger image)

What they found was 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 synergistically to cause wide-spread drying in the midlatitudes. According to Hoerling, it was the "perfect ocean for drought."

Three different climate models were run more than 50 times. This figure is the simulated, annually averaged surface temperature (left) and precipitation (right) differences for the 4-year period. The largest warm and dry departures from normal are highlighted in red. (larger image)

Using climate simulations, the scientists assessed how the ocean conditions over the 4-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."

They soon discovered 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 3-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."

Hoerling says "The warmth in the west Pacific dring 1998-2002 simply has no precedent in at least the past 150 years." The researchers believe 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? While the cold sea surface temperatures were unusual, they were not unprecedented. But, says Hoerling, 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." And, say the scientists, this may be a harbinger of future droughts. "What is suggested by the atmospheric modeling results of 1998-2002 is an increased risk for severe and synchronized drying of the midlatitudes in the future, if these oceanic conditions continue to occur."

Hoerling says that while we've swung back to El Niño conditions recently with it's increased precipitation, it's only a matter of time before the situation changes and there's another cooling in the east Pacific because that's a naturally fluctuating phenomenon. "In the meantime, the west Pacific and Indian Ocean is likely to remain warm and, thus, it may again put us at risk for a substantial drought."

For additional information on the Climate Diagnostics Center, consult their web site at www.cdc.noaa.gov and for more information on the Climate Prediction Center, look at www.cpc.ncep.noaa.gov/

[3/24/03]

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