1. On the Causes of the 1930s Dust Bowl Siegfried Schubert, Max Suarez, Philip Pegion, Randal Koster and Julio Bacmeister Global Modeling and Assimilation Office Earth Sciences Directorate 28th Annual Climate Diagnostics and Prediction Workshop Reno, Nevada 20-24 October 2003

2. The Dust Bowl I.What is the role of SSTs? - different ocean basins - extratropical versus tropical II. Are land/atmosphere feedbacks important? III.How is it different from other Great Plains droughts of the 20th century? IV. Can we predict the next one?

3. AGCM: NSIPP-1 (NASA S-I Prediction Project) Climatology and Skill (Bacmeister et al. 2000, Pegion et al. 2000, Schubert et al. 2002) Global grid point dynamical core, 4rth Order (Suarez and Takacs 1995) Relaxed Arakawa-Schubert Convection (Moorthi and Suarez 1992) Shortwave/Longwave Radiation (Chou et al. 1994, 1999) Mosaic interactive land model (Koster and Suarez 1992, 1996) 1 st Order PBL Turbulence Closure (Louis et al. 1982) AGCM runs with Specified SST (C20C project) HadISST and sea ice dataset (1902-1999) 14 ensemble members - same SST, different ICs Model resolution: 3 degree latitude by 3.75 degree longitude (34 levels)

4. The Dust Bowl

15. Other Droughts

16. Corr=0.57 ?

18. PPP ENSO Trend Low Pass EOF 1 (Trend)Low Pass EOF 2 (PPP) Residual EOF 1 (ENSO)PC Time Series

19. Correlation between PPP and Great Plains Precipitation (low frequency -> 6 years and longer) Individual ensemble member precipitation Min = 0.31 max = 0.70, Ensemble mean precipitation (= 0.74) Observed precipitation (= 0.41)

20. 1970s Dust Bowl?

21. Borchert, John R. “The Dust Bowl of the 1970s”, AAAG, vol 61, March 1971 ABSTRACT: Major droughts in the Grassland region of the central United States have occurred rhythmically during the period of the instrumental record. The time for the next one may be near. ….

22. “By mid February of 1977 the midwestern US was experiencing one of the most severe droughts in its recorded history … dramatic dust storms blew up in eastern Colorado and western Kansas and Oklahoma …drought was actually most severe in Minnesota and Dakotas …” “By the fall of 1977 the general drought picture …. had changed significantly” Symposium on North American Droughts - February 21 1977 (Introduction by Norman Rosenberg, AAAS Selected Symposium 15)

25. Conclusions based AGCM Results Tropical SST anomalies and land/atmosphere feedbacks caused the 1930s dust bowl All three tropical ocean basins played a role –Cool Pacific and Indian (remote response - upper level flow) –Warm Atlantic (local response - upper and low level flow) –Extra-tropical SSTs acted to extend the drought to the east The SST forcing of the Dust Bowl differs from that of other major droughts (1910s and 1950s where the Pan Pacific Pattern of SST is the main driver) Predicting future Dust Bowls/droughts: need better coupled models with improved simulation of decadal ocean variability, and reduced uncertainty in land/atmosphere coupling; also need better understanding of role of ENSO and global warming