1. Effects of trends in anthropogenic aerosols on drought risk in the Central United States Dan H. Cusworth Eric M. Leibensperger, Loretta J. Mickley Corn crop in Florence, Missouri, summer 2012

2. About 30-50% of variability in U.S. precipitation can be explained by patterns in sea surface temperatures. … SSTS predict a 1970s drought in the Central US that did not occur. Schubert et al., 2004; Seager and Hoerling, 2014 R = 0.57 Precipitation anomaly Missing drought Dust Bowl Timeseries of precipitation anomalies in Great Plains. “peculiar model behavior”

3. We previously modeled the regional climate impacts of U.S. anthropogenic aerosols during 1970-1990. Modeled aerosol loading in the 1970s and 1980s: Cooled eastern US by as much as 1 o C Increased cloud cover by 3% Increased soil moisture availability Δ Soil Moisture availability (%) NASA GISS climate model, Leibensperger et al., 2012a,b wetdry Cooling of -6 Wm -2

4. Did 1970s aerosol loading reduce the vulnerability of the Central US to drought? Local Effect: Reduction in AOD increases surface solar radiation, drying soils and decreasing humidity. Pre- 1990 Clean Air Act Post- 1990 drywet

5. We examine observations and reanalysis data together with a 1-D radiative transfer model to extend the Leibensperger (2012a,b) results. Focus for now is on Illinois. Preliminary findings Dan H. Cusworth Illinois, July 2012

6. The largest increases in net SW radiation (+20 W/m 2 ) realized in summer months during peak insolation hours. Δ Net surface SW radiation from [1988-1998] to [2002-2012] 40 0 -40 W/m 2 Month of Year 12AM 12PM 12AM SummerWinter 0 Day 24hrs Day 24hrs 20 W/m 2 20th Century Reanalysis data show an increase in surface solar radiation in recent decades over the Central US (105-85W, 37-48N). J J A S O ND M A M F Cusworth, ms in progress

7. SURFRAD shows similar increase in surface solar radiation during daytime hours in Bondville, Illinois. Net surface SW radiation Anomalies (Wm -2 ) We find surface solar radiation increases +13 Wm -2 since 1995. AOD at 1000 nm -0.002 a -1 p < 0.05 1995 1998 2001 2004200720102013 +0.64 Wm -2 a -1 +40 0 -40 Anomalies AOD 2013 1997 At the same time, AOD decreases significantly. Cusworth, ms in progress

8. Illinois rural sites show decreasing JJA soil moisture even though precipitation increases. Soil temps increase over the same time period. Data show consistent picture of aerosol trends driving decline in soil moisture. 10cm 30cm 50cm p < 0.01 -0.6% a -1 Moisture anomalies Soil Moisture at three depths (%) +0.06°C a -1 p < 0.05 Temp anomalies ( o C) 8-in Soil temperature anomalies p < 0.11 +1.0e-3cm a -1 Precipitation anomalies Precip (mm) Cusworth, ms in progress

9. We apply 1-D radiative transfer model to understand observed change in surface solar radiation over Bondville, Illinois. We find warming from aerosol trend is +14.3 Wm -2. Simulation with observed AOD zero AOD 31-day smoothing Annual average SWdn (Wm- 2 ) JJA net shortwave radiation Simulation with observed AOD (p < 0.11) SWdn anomalies (Wm -2 ) JJA net shortwave radiation anomalies zero AOD Preliminary results: no clouds, noon Cusworth, ms in progress

10. Questions arising from preliminary analysis. 1.Did aerosol loading during the 1970s reduce the vulnerability of the Central US to drought? 2.Has the recent decline in anthropogenic aerosols increased drought risk in the Central US and elsewhere? We plan to use new data from the NASA Soil Moisture Active-Passive mission (SMAP) to better understand sensitivity of soil moisture to meteorology and aerosol loading.

12. ENSO Correlation 0 1 -0.5 Seager and Hoerling, 2014 Precipitation EOFs SST patterns can explain 30-50% the variability in North American precipitation. First 3 EOFs for U.S. precipitation correlate with patterns of observed SSTs -35% 35% dry wet

13. Observed SSTs 1970s 1970s predicted drought 1970s observed precipitation wetdry Pattern of cool SSTs in tropical Pacific predicts drought in model of Seager and Hoerling (2014). mm month -1 oCoC