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Intensification of Summer Rainfall Variability in the Southeastern United States in Recent Decades
Hui Wang1,2, Wenhong Li1, and Rong Fu1,3
1Georgia Institute of Technology2NOAA Climate Prediction Center
3University of Texas at Austin
NOAA’s 33rd Climate Diagnostics and Prediction Workshop and CLIVAR Drought Workshop
October 20–24, 2008, Lincoln, Nebraska
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.
Lake Lanier – main water source for Atlanta
2007 Southeast droughtSoutheast drought – – Worst in 100Worst 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?
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
DataPrecipitation CPC U.S. Unified Precipitation (1948–1998, 0.25o x 0.25o) CPC realtime U.S. Daily Precipitation Analysis (1999–2007)
SST NOAA Extended Reconstructed SST (ERSST v3, 2o x 2o)
Wind and height NCEP/NCAR Reanalysis (850 and 200 hPa, 2.5o x 2.5o)
Period JJA 1948–2007, 60 years
Data: 1984–2007
CPC
JJA Rainfall & Variability
Southeast:Large mean rainfallHigh variability
Part 1
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) 4 11Contribution to
Precipitation Variance
32% 68%
Change in SE Rainfall Variability
SE SPI (3-month) SE SPI (3-month)
SE SPI (9-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
Severely to extremely wet/dry
1st 30 yrs2nd 30 yrs
60 yrs 1st 30 yrs 2nd 30 yrsClimatology Wet (5) Wet (5)
Dry (5) Dry (5)
Shift of Rainfall PDF
2nd 1st clm 1st 2nd 2nd 1st clm 1st 2nd
2nd 1st clm 1st 2nd
Fre
qu
ency
Intensity
2nd 30 years:Decrease of frequency and intensity in dry summers.Increase of frequency and intensity in wet summers.
5 1 812 15 11
Difference at 11% significance level
Difference at 5% significance level
Bootstrap test
Midwest floodsMidwest floods
Warm Season Rainfall
Southeast and Midwest: out of phase
Composite: Drought-related Large-scale CirculationRegression vs. SE precip indexJJA 1948–2007
Contour: u200Shading: z200
Shading: prcpVector: wind 850Contour: div 925
LLJLLJ
1st 30 yrs
2nd 30 yrs
Change in Zonal Wind Variability at the Jet-Steam Level
ENSO
ATL Zonal Mode
Warming Trend
Co-variability of SE Rainfall with SSTHomogeneous correlation map
PCF SSTSVD Mode 1
ATL SSTSVD Mode 2
ATL SST SVD Mode 1
Part 2
200-hPa Zonal Wind Variance: 1st 30 yrs vs. 2nd 30 yrs
1st 30yrs 2nd 30yrsENSO SST
ATL Zonal Mode
Warming Trend
SST US precip
R = 0.58
R = 0.60
R = 0.63
ENSO1st
30yrs2nd
30yrs
SST 51% 49%
US P 35% 65%
Zonal mode
1st 30yrs
2nd 30yrs
SST 39% 61%
US P 37% 63%
Warm-ing
1st 30yrs
2nd 30yrs
SST 35% 65%
US P 39% 61%
Variance
Variance
Variance
ENSO
ATL zonal Mode
Warming
SVD Time Series
Total (R=0.92)ATL ZM (R=0.87)Warming (0.63)ENSO (R=0.39)
ENSO dominant
Reconstruction of SE Precipitation Index
Multilinear regressionbased on SVD precipitation time series.
Correlation with OBS
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-ValidationAnomaly Correlation: Hindcast vs. OBS
Hindcasts: JJA 1948–2007 rainfall based on observed SST
ENSO 2 ATL modes
“Forecast” of Precipitation Anomaly for JJA 2008Based on Observed SST
“Forecast” Observation
DryDry
Wet Wet
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.
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