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.