An Analysis of Climatological Conditions Favorable for Large-Scale Drought
Jonathan J. Rutz
March 2, 2009
METEO 6030:
Earth Climate System
Image at http://www.nasa.gov/centers/goddard/images/content/95248main_theb1365.jpg
(1) Brönnimann et al. (2009), Exceptional atmospheric circulation during the
“Dust Bowl.” Geophysical Research Letters, 36, L08802, DOI:
10.1029/2009GL037612.
 (2) Hoerling et al. (2003), The Perfect Ocean for Drought. Science 299, 691,
DOI: 10.1126/science.1079053
 (3) Schubert et al. (2004), On the Cause of the 1930s Dust Bowl. Science 303,
1855, DOI: 10.1126/science.1095048
 (4) Seager et al. (2007), Would Advance Knowledge of 1930s SSTs Have
Allowed Prediction of the Dust Bowl Drought? Journal of Climate, DOI:
10.1175/2007JCLI2134.1


Background and Motivation
 Socio-economic & climatological significance

Geopotential Height Primer

Dust Bowl – impacts, seasonality, and timescale
Anomalous SSTs – effects on general circulation and
Precipitation

 Pacific & Atlantic contributions

Conclusions

Concerns

Likely exacerbated due to previous years featuring
anomalously abundant precipitation (Seager et al.)
 Favorable growing conditions led to rapid transition from
natural grassland to less-resistant cropland

Widespread drought persistent throughout the 1930s
(Brönnimann et al.)
 1936 generally regarded as the ‘worst’ year
 Coincided with the Great Depression

An estimated three million left their farms (Seager et al.)
 Total out-migration to other states in excess of half a million
▪ Numbers equaled only recently by aftermath of Hurricane Katrina
Data from Extreme Weather, Burt, C. (2004)

Thickness of a fluid
layer is proportional to
its temperature
 Warm air has a lower
density than cool air 
expansion
 An area of high pressure
at upper levels ( > 500
mb) usually implies a
warm atmosphere
beneath it
Images at http://apollo.lsc.vsc.edu/~wintelsw/MET2110/notes/lesson06.thickness



Major precipitation
deficits ( > 10 mm/mon.)
across U.S. central plains
Widespread negative
SST anomalies across
northeastern Pacific
Widespread positive SST
anomalies across the
North Atlantic
 Coinciding with very
anomalously high pressure
Sea-level Pressure (hPa): Climatology (black), Anomalies (green)
Image from Brönnimann et al. – for the period AMJJA (1931-1939)

Most pronounced
precipitation deficits
occurred during JJA and
SON
 Suggestive of primarily
warm-season phenomena
to explain the observed
anomalies
Image from Schubert et al. – model ensemble, seasonal variations in precipitation (1932-1938)


Atmosphere alone is capable of producing widespread
temperature and precipitation anomalies based on
internal dynamics (Hoerling et al.)
Beyond one month, the atmosphere contains little
‘memory’ (Hoerling et al.)
 In other words, it can’t easily maintain it’s own patterns beyond
this time period

Implies another agent of forcing  the global oceans
 The data reveals some very well-defined and pronounced SST
anomalies during the Dust Bowl period
Strongly positive
Atlantic temperature
anomalies (esp. north)
Negative (La Niña)
ENSO signal in
Eastern Pacific
Neutral SST
conditions across the
Indian Ocean
Image from Schubert et al. – differences from 1902-1999 climatology in °C

Generally positive GPH
anomalies across central
United States (region of
the ‘Dust Bowl’)
 Multiple levels
 Multiple seasons
Images from Brönnimann et al. – reconstructed 200 and 500 hPa GPH anomalies (1931-1939)

Instability is primarily a
product of two factors
Temperature
New Temperature
 (1) Temperature lapse rate
(dT/dz) of the environment
▪ Higher dT/dz  more likely to be
unstable
▪ Lower dT/dz  less likely to be
unstable
▪ Positive upper level height
anomalies lead to warmer
temperatures aloft,
implying a lower dT/dz
Dewpoint
Strongly positive
Atlantic temperature
anomalies (esp. north)
Negative (La Niña)
ENSO signal in
Eastern Pacific
Neutral SST
conditions across the
Indian Ocean
Image from Schubert et al. – differences from 1902-1999 climatology in °C

Formed by superposition of
two mechanisms
 Horizontal temperature gradients
and the thermal wind relationship
 Anti-cyclonic flow around subtropical
‘Bermuda’ high
Images at http://www.meted.ucar.edu/dlac2/mod2/media/graphics/print/s2p6_data.jpg, http://weather.ou.edu/~cmwalsh/wx/850mb_LLJ.png

Weakening of the lowlevel jet – two primary
aspects:
 Strongest at or above
height of maximum LLJ
strength
 More pronounced on the
eastern periphery of the
climatological LLJ
(according to authors)
Image from Brönnimann et al. – AMJJA nocturnal meridional wind profiles (1931-1939)

Instability is primarily a
product of two factors
 (2) Near-surface moisture
▪ More moisture  more unstable
▪ Less moisture  less unstable
▪ Shift in position of ‘Bermuda’ high
and resultant effects on low-level
jet act to provide less moisture to
U.S. central plains region
Images at http://svs.gsfc.nasa.gov/vis/a010000/a010000/a010033/

Instability is primarily a product
of two factors
Temperature
New Temperature
 (1) Temperature lapse rate
(dT/dz) of the environment
▪ Higher dT/dz  more likely to be
unstable
▪ Lower dT/dz  less likely to be
unstable
▪ Positive upper level height anomalies
lead to warmer temperatures aloft,
implying a lower dT/dz
 (2) Near-surface moisture
▪ More moisture  more unstable
▪ Less moisture  less unstable
▪ Shift in position of ‘Bermuda’ high
and resultant effects on low-level jet
act to provide less moisture
New Dewpoint
Dewpoint

Extreme drought conditions over the central U.S. plains
associated with the Dust Bowl were driven by persistent
SST anomalies in the Pacific and Atlantic
 Could be predicted given accurate SST forecasts
▪ Global circulations models (GCMs) struggle with this

Dual importance of Pacific/Atlantic
 Cold tropical Pacific (La Niña) alters flow of Pacific jet, favoring
positive height anomalies over central U.S.
 Warm Atlantic shifts position of dominant high pressure in such
a way that GPLLJ is weakened, reducing moisture feed

Meteorological observations from the Dust Bowl period
are sparse at best
 Many of the graphics shown are based on reconstructions
 In some cases, these reconstructions have differing
methodologies behind them


Most recent drought of similar scope (1998-2002) was
characterized by primarily winter/spring anomalies
(Hoerling et al.)
Given the rapid rate of climate change, in my opinion, it
may soon become difficult to refer to past climatologies
(e.g. 1970-2000, etc…)