March 19, 2004

Dust bowl caused by ocean highs and lows

Sea temperatures linked to 1930s US drought.

by Mark Peplow
Nature News

RELATED STORIES Are greenhouse gases drying Africa's dust bowl? US citizens choking on African dust may have themselves to blame. RESOURCE Nature: The Story of a Drought RELATED SITES Dust Bowl History National Drought Mitigation Centre Print this article Register for updates when new articles are posted Discuss this article

The devastating US drought of the 1930s was caused by unusual sea surface temperatures, according to new climate research. The work could help predict future dry spells, and demonstrates that tiny changes in ocean temperature can have a massive impact on the land.

The researchers fed sea surface temperature data from the tropical Atlantic and Pacific oceans into a computer model, which accurately predicted the drought that affected three-quarters of the United States between 1931 and 1939.

The Pacific was on average a few tenths of a degree cooler than normal during the thirties, while the Atlantic was slightly warmer. This combination was apparently enough to turn the Southern Plains, including parts of Colorado, Texas, Kansas, Oklahoma and New Mexico, into a giant 'dust bowl'.

"The Dust Bowl is unique in the last 100 years, and that is because of the unusual combination of Pacific and Atlantic effects," says Siegfried Schubert of the NASA Goddard Space Flight Center in Greenbelt, Maryland, who led the research team.

More than 2 million farmers were forced from their land as the soil dried out. At the time, it was estimated that almost a billion tons of topsoil blew off the Southern Plains in 1935.

Once the soil had dried out there was less evaporation, which meant even less rain. This formed a positive feedback loop that probably doubled the effect of the drought, says Schubert. They report their findings in this week's Science1.

Researchers know that sea surface temperatures can have a profound effect on precipitation. "Just look at El Niño - changes in the Pacific can cause tornados over Florida and droughts in Africa," says Jonathan Bamber, a climatologist at the University of Bristol. But it is very surprising that these small temperature variations could cause such a huge drought, says Schubert.

Megadroughts

The Great Plains get most of their rain in spring and summer. The westerly 'trade' winds that flow around the equator normally carry moisture westwards from the Gulf of Mexico over the United States, keeping the Plains well watered.

But a warmer Atlantic heats the air just above the sea surface, making it less dense and allowing it to rise, explains Bamber. This opens the door for cooler, high-pressure air to sweep eastwards off continental America. So the wet air over the Gulf flows eastwards instead.

The same trade winds usually sweep westwards over the continent and the Pacific, picking up moisture from that ocean as they go, before doubling back and carrying their watery cargo back towards the United States in the upper atmosphere. A cooler Pacific disrupts this cycle, so that the air returning to the United States is dryer, flows at a lower altitude, and is less likely to bring rain to the Plains.

Schubert believes his team's computer model should work just as well for the Sahel region, on the southern edge of the Sahara Desert, where changes in Atlantic and Indian Ocean sea surface temperatures are creating droughts.

At the moment, the model can give about six months to a year advance warning of a drought. Including deep-ocean temperature measurements in the calculations could improve its predictive power, says Schubert.

Modern farming practices, such as improved irrigation, would moderate the effects of any future droughts. "But the basic weather that caused the Dust Bowl will almost certainly happen again," Schubert says. "Over the last 500 years, we've had these kinds of megadroughts about once or twice every century. Historically, these events are very likely."

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