1. Intensification of Summer Rainfall Variability in the Southeastern United States in Recent Decades Hui Wang 1,2, Wenhong Li 1, and Rong Fu 1,3 1 Georgia Institute of Technology 2 NOAA Climate Prediction Center 3 University of Texas at Austin NOAA’s 33rd Climate Diagnostics and Prediction Workshop and CLIVAR Drought Workshop October 20–24, 2008, Lincoln, Nebraska

2. Highlights: Intensified SE rainfall variability in recent 3 decades. Consistent with the shift of rainfall PDF. Closely tired to the Atlantic SST variability. Certain predictability based on tropical SST.

3. Lake Lanier – main water source for Atlanta Southeast drought 2007 Southeast drought – Worst in 100 – Worst in 100 years Background: Summer droughts repeatedly struck the Southeast in recent decades. Questions: Has the rainfall variability been intensified in recent decades? What might cause the intensification?

4. Previous works: Stahle & Cleaveland 1992: decade-long spring rainfall extremes have been a prominent feature of SE US over the past 1000 yrs. Mo & Schemn 2008: model simulations show seasonally varying ENSO forcing on SE US; Seager et al. 2008: model simulations show weak connection with SSTA, thus low predictability

5. Data Precipitation CPC U.S. Unified Precipitation (1948–1998, 0.25 o x 0.25 o ) CPC realtime U.S. Daily Precipitation Analysis (1999–2007) SST NOAA Extended Reconstructed SST (ERSST v3, 2 o x 2 o ) Wind and height NCEP/NCAR Reanalysis (850 and 200 hPa, 2.5 o x 2.5 o ) Period JJA 1948–2007, 60 years

6. Data: 1984–2007 CPC JJA Rainfall & Variability Southeast: Large mean rainfall High variability Part 1

7. Normalized JJA Precipitation Anomaly Year 1 stdv=0.64 mm/day 1st 30 years 1948–1977 2nd 30 years 1978–2007 Wet and Dry (> 1 Stdv) 411 Contribution to Precipitation Variance 32%68% Change in SE Rainfall Variability

8. SE SPI (3-month) SE SPI (9-month) Standardized Precipitation Index: 4 : 11 Moderately to extremely wet/dry 5 : 14 1 : 3 1 : 7 Moderately to extremely wet/dry Severely to extremely wet/dry 1st 30 yrs 2nd 30 yrs

9. 60 yrs1st 30 yrs2nd 30 yrs ClimatologyWet (5)Wet (5)Dry (5) Shift of Rainfall PDF 2 nd 1 st clm 1 st 2 nd Frequency Intensity 2nd 30 years: Decrease of frequency and intensity in dry summers. Increase of frequency and intensity in wet summers. 518121511 Difference at 11% significance level Difference at 5% significance level Bootstrap test

10. Midwest floods Warm Season Rainfall Southeast and Midwest: out of phase

11. Composite: Drought-related Large-scale Circulation Regression vs. SE precip index JJA 1948–2007 Contour: u200 Shading: z200 Shading: prcp Vector: wind 850 Contour: div 925 LLJ

12. 1st 30 yrs 2nd 30 yrs Change in Zonal Wind Variability at the Jet- Steam Level

13. ENSO ATL Zonal Mode Warming Trend Co-variability of SE Rainfall with SST Homogeneous correlation map PCF SST SVD Mode 1 ATL SST SVD Mode 2 ATL SST SVD Mode 1 Part 2

14. 200-hPa Zonal Wind Variance: 1st 30 yrs vs. 2nd 30 yrs 1st 30yrs2nd 30yrs ENSO SST ATL Zonal Mode Warming Trend

15. SST US precip R = 0.58 R = 0.60 R = 0.63 ENSO 1st 30yrs 2nd 30yrs SST51%49% US P35%65% Zonal mode 1st 30yrs 2nd 30yrs SST39%61% US P37%63% Warm- ing 1st 30yrs 2nd 30yrs SST35%65% US P39%61% Variance ENSO ATL zonal Mode Warming SVD Time Series

16. Total (R=0.92) ATL ZM (R=0.87) Warming (0.63) ENSO (R=0.39) ENSO dominant Reconstruction of SE Precipitation Index Multilinear regression based on SVD precipitation time series. Correlation with OBS

17. Empirical Forecast System 1. Project SST (OBS or FCST) onto the SVD SST pattern SST projection coefficient 2. Based on SVD SST – US precipitation relationship, precipitation projection coefficient 3. Based on SVD precipitation pattern, forecast of precipitation anomaly Part 3 Cross-Validation Anomaly Correlation: Hindcast vs. OBS Hindcasts: JJA 1948–2007 rainfall based on observed SST ENSO 2 ATL modes

18. “Forecast” of Precipitation Anomaly for JJA 2008 Based on Observed SST “Forecast”Observation Dry Wet

19. Summary SE summer precipitation exhibits higher interannual variability in recent 30 years. The shift of rainfall variability is linked to tropical Atlantic SST variability, suggesting certain predictability of SE summer rainfall based on tropical SST.