Predicting droughts with greater certainty
Using new data and reconstructions of the “Dust Bowl” drought in America during the 1930s, climatologists at the ETH have shown for the first time a three-dimensional picture of the atmospheric circulation that led to the drought. This will enable climate models to be evaluated and further improved. The scientists hope this work will make it possible to predict future periods of drought more accurately.
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.
The new study by Brönnimann and his team has been published in the renowned professional journal “Geophysical Research Letters”, and selected by the American Geophysical Union as a “journal highlight”.
References
Brönnimann S et al. Exceptional atmospheric circulation during the ‘Dust Bowl’. Geophysical Research Letters. 2009; 36: L08802. doi:10.1029/2009GL037612
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