Skewed climate. The tropical Pacific's odd temperatures shifted both real and simulated worlds to the dry (red-orange) side.

Ocean Oddities Cast Powerful Dry Spell

It's the grandest connection between ocean and recent climate since El Niño's reign was recognized. Researchers are blaming droughts in Afghanistan and the U.S. Southeast on the distant tropical Pacific. Odd doings in that part of the ocean seem to be responsible for 4 years of dryness spread across North America, southern Europe, and central-southwest Asia.

In the 31 January issue of Science, climate dynamicists Martin Hoerling of the National Oceanic and Atmospheric Administration's (NOAA's) Climate Diagnostics Center in Boulder, Colorado, and Arun Kumar of NOAA's Climate Prediction Center in Camp Springs, Maryland, compare climate-model simulations with the pattern of Northern Hemisphere drought. The comparison shows how two parts of the tropical Pacific ganged up on the mid-latitudes to create persistent drought in widely separated, seemingly unrelated regions.

Hoerling and Kumar ran three models independently developed by three institutions for 51 runs. In each run, Hoerling and Kumar inserted the actual sea-surface temperatures observed from 1998 to 2002 and saw how the models' atmospheres responded. Averaged together, the simulations showed a striking similarity to the real droughts.

The researchers narrowed the ocean's influence to the tropical Pacific by running a fourth model, altering temperatures of only certain parts of the ocean from their long-term averages. Including just the hot spots--in those years the western Pacific and eastern Indian Ocean--produced some parts of the mid-latitude dryness. El Niño's opposite number, La Niña, provided the period's most abnormally cool area, in the central and eastern tropical Pacific. When they included that chill in the model, it reinforced some parts of the warmth-induced dryness and produced other parts of the pattern.

"It's very convincing," says modeler Siegfried Schubert of NASA's Goddard Space Flight Center in Greenbelt, Maryland. Although the most recent drought seems to be easing, global warming may trigger similar trouble in the future, warns meteorologist Mathew Barlow of Atmospheric and Environmental Research Inc. of Lexington, Massachusetts. If the tropical Pacific warms in the west, drought in parts of central-southwest Asia could become commonplace, for example, Barlow says.

Related site
Hoerling and Kumar's Science paper