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Climate Team's Model Correctly Predicted Ongoing Drought in U.S.

Record Dryness Is the Worst in Decades

UMass Lowell Image
Parched, cracked earth is one of the telltale signs of below-normal precipitation.

By Edwin L. Aguirre

Food and gasoline prices are expected to rise significantly this summer as the record-breaking drought affecting much of the central United States shows no sign of letting up.

Farmers in the Midwest are struggling to save their crops, especially corn, soybean and wheat, which are succumbing to the searing heat and dryness. Corn is used mainly for producing ethanol additive in gasoline and as livestock feed.

“This drought is definitely more than the seasonal cycle — that is, it’s much more dry than usual for this time of year in many areas,” says climate science Assoc. Prof. Mathew Barlow of the Environmental, Earth and Atmospheric Sciences Department.

According to the U.S. Drought Monitor map, as of July 24 more than 53 percent of the country was experiencing moderate drought, 38 percent severe, 17 percent extreme and 2 percent exceptional. And the conditions are likely to spread, intensify and persist into the fall. 

“The cause for the drought is currently a subject of much debate, whether it’s due to natural variability or global climate change brought about by human activity, or both,” says Barlow. “Nobody knows for sure, but the anomaly is consistent with expectations for global warming.”

The Siberian Connection

Barlow is part of a team of researchers led by Judah Cohen, a climate modeler at the Lexington-based consulting firm Atmospheric and Environmental Research Inc. that developed a sophisticated seasonal forecast model which correctly predicted this year’s warm winter and hot summer. Their findings were published in January in the journal “Environmental Research Letters.”

Weather in the eastern United States and Europe is influenced mainly by a climate phenomenon known as the North Atlantic Oscillation or Arctic Oscillation (N/AO). Researchers found that when there is less snow in Siberia in October, the N/AO index that following winter tends to be high (positive) and there is a decrease in the frequency of severe weather in the eastern United States, Europe and East Asia. This is because the jet stream in the upper atmosphere, which divides the cold air mass to the north and the warm air mass to the south, can flow freely from west to east, quickly carrying along with it any weather systems. Residents at mid-northern latitudes therefore enjoy long episodes of mild winter weather as they remain cut off from the frigid, arctic air.

On the other hand, when there is above-normal snowfall in Siberia in October, the opposite happens — in winter the N/AO index becomes low (negative) and the jet stream’s flow gets “blocked” at high latitudes. The jet stream is deflected from its normal path as it meanders north and south around the blockage. As a result, the air masses get mixed, with warm, humid air flowing northward to the Arctic and the cold, dry air flowing southward into the mid-latitudes. 

“The Eastern U.S., Europe and East Asia, which lie in the mid-latitudes, experience more severe winter weather, as Arctic air masses repeatedly drive southward,” notes Cohen in his blog. “Furthermore, the mixing of warm and cold air masses fuels storms, and with more abundant cold air, the precipitation from these storms often falls as snow.”

This year’s drought was probably set in motion when a strongly positive N/AO deflected major winter storms away from the United States. When spring arrived, there was very little snow to melt and moisten the ground, and thus very little water to evaporate and create rain. The dryness continued to steadily intensify, along with a decline in rainfall caused by an above-average persistence of a high-pressure system over the afflicted area. The summer heat wave caused further evaporation of groundwater, lakes, reservoirs, rivers and streams.

However, Barlow cautions that the real picture might be more complicated than that.

“These are contributing factors in this drought, probably not the primary cause, as much of it seems to be due to an overall decrease in precipitation rather than a switch from snow to rain,” he says.