Predicting Droughts With Greater Certainty

In the 1930s, a drought that lasted almost ten years wrought havoc on the Midwest region of North America. The enormous dust storms accompanying it gave the “Dust Bowl” drought its name. This drought had devastating socio-economic consequences for America. John Steinbeck immortalised the tragic story of farmers already impoverished by the economic crisis of the time in his novel “The Grapes of Wrath”. And the legendary “Route 66”, along which the farmers fled towards California, was made famous in part by the Dust Bowl.  

Digitalised historical data improve model

Scientists have been studying the Dust Bowl phenomenon for decades, and until now the mechanisms that caused this exceptionally long period of drought have not been fully understood, as little information has been available on the atmospheric circulation. Stefan Brönnimann, Professor at the Institute for Atmospheric and Climate Science at ETH Zurich, and his team have now used historical data to reconstruct and analyse the three-dimensional circulation during the Dust Bowl drought. At the time of the drought, wind and temperature readings were already being taken using balloons and aircraft, initially at altitudes of three to eight kilometres, and later at much higher altitudes. These data have now been digitalised as part of a US project and a project undertaken by the Swiss National Science Foundation. Based on these data, Brönnimann’s team used statistical methods to reconstruct the upper air circulation at an altitude of up to 15 kilometres.  

Based on computer models, researchers have up to now conjectured that unusual sea surface temperatures in the Pacific and Atlantic Oceans would have altered the wind systems, thereby triggering the drought. At the same time, the dying vegetation, the parched soil and the dust created by these conditions could have further intensified the drought. However, according to Brönnimann, observations to date have offered insufficient confirmation of these hypotheses based on simulated models.  

Exceptionally cold Pacific

In their study, the scientists focused on three known circulation patterns which characterise the basic wind conditions of the region and the wider area. Using the new data, they were able to show that a specific wind flow, the Great Plains Low-Level Jet, was shallower at the time of the Dust Bowl. This air current usually carries moist air from the tropical Atlantic far into the region, which covers approximately two million square kilometres. In addition, the Jet did not penetrate as far north as usual, as it was deflected too early towards the east.  

The researchers believe this was caused by a high-pressure system that built up over the Plains and was associated with an abnormal upper air flow extending from the Pacific across North America to the Atlantic. “Overall, these features are clearly consistent with the flow conditions that climate models predict as the effect of a cold Pacific coinciding with a warm Atlantic”, explains Brönnimann. Because the temperatures of the tropical oceans can to a certain degree be predicted, the scientists see here the possibility of predicting periods of drought as well. However, the study also shows up some remaining shortcomings in the models: for the most part, they would not correctly depict the spatial shift of the Low-Level Jet, and in many models the drought is located too far to the south.