Why did the mid-section of the United States suffer two severe droughts in the 20th Century — the infamous "Dust Bowl" of the 1930s and the Southern Plains drought of the 1950s?  Most meteorologists pondering this question have been looking at sea surface temperatures and expecting to find a common thread.  New research, however, contradicts this thinking, finding that the two events spring from different causes.

The research may sound like an academic exercise to farmers who experienced the hardships of living through these economic disasters.  This image of a devastated Dust Bowl cornfield, courtesy of the National Oceanic and Atmospheric Administration, is a case in point.  But in fact investigations into the mechanisms behind abrupt changes in rainfall patterns are at the cutting edge of some of the most consequential work in regional climate science.

Large and agriculturally important areas of the world — not least the United States — have been subject to droughts that were longer lasting and more severe than those in the instrument record of the last few centuries, research shows, and anything that helps forewarn of such episodes could be critically valuable.

New work by meteorologist Martin Hoerling and colleagues at NOAA's Earth System Research Lab in Boulder, CO, shows that the 1950s drought in the southern plains was linked — as expected — to cool ocean surface temperatures in the eastern equatorial Pacific — a series of La Niña episodes.

But the 1930s drought centered in the northern plains was linked to random variability in the atmosphere — a result that conflicts with earlier work.  Hoerling said his more detailed analysis did not support "the prior speculation that the Dust Bowl drought had early warning indicators in the ocean temperatures."  The new study, published in the journal Geophysical Research Letters, supports other work that pointed to "dust aerosol feedbacks" between low soil moisture and farming practices in the 1930s that left the soil exposed to windstorms.

The new research discourages the hope that a network of ocean surface temperature monitors could provide an early warning system for all major droughts.  Recognizing ocean temperature changes would come in handy for forecasting many droughts in the US and elsewhere, to be sure, but drought caused by "random atmospheric variability" is inherently unpredictable. 

The study also points up the complexity of climate on a regional scale.  Like a lot of things in science, the closer you are able to look, the more detail you are able to see, the more complicated things seem to be.