Relationship of U.S. Summer Droughts with SST and Soil Moisture: Distinguishing the Time Scale of Droughts Renguang Wu Center for Ocean-Land-Atmosphere Studies CTB Joint Seminar Series February 24, 2009, NCEP Co-author: James L. Kinter III 1

Why distinguish the time scale?. Difference in impacts. Difference in preferred regions. Difference in mechanisms/factors 2

Mo&Schemm 2008 Wet Spell:PDSI>=2Dry Spell:PDSI<=-2 Ratio of the total number of months ( ) 1-5 months Eastern US 6-11 months > 1 year Central-western Difference in preferred regions 3

Difference in mechanisms Roles of soil moisture for persistence of droughts: Entekhabi et al Findell and Eltahir 1997 Pal and Eltahir 2001 Schubert et al Seager et al soil moisture-precipitation feedback --- 4

P > n-month mean >transform >SPI * 1-month meanspi01 ** 2-month meantransformspi02 *** 3-month meanfrom Pearsonspi03 ****** 6-month meanIII distributionspi06 ********* 9-month meanto normalspi09 ************ 12-month meandistributionspi12 ***……….*** 24-month meanspi24 How to distinguish the time scale? Standardized Precipitation Index (SPI): defined for different time scales 5

Drought indices PDSI (Palmer 1965): based on the water balance between supply and demand SPI (McKee et al. 1993): based on precipitation only In the present study, SPI03: short-term (<= 3months) SPI09: medium-term (6-12 months) SPI24: long-term (> 1year) 6 Both PDSI and SPI are used in the drought monitoring

7 PDSISPI09 SPI03SPI24 Drought indicesSeptember 2008

Questions (1)Which region SST has the most significant relationship with the U.S. summer droughts in Great Plains/Southwest? (2) Which region summer droughts are mostly influenced by remote SST forcing (ENSO, Tropical IO, North Atlantic)? (3) What is the role of soil moisture in droughts? (4) How is the long-term change in the relationship between droughts and SST forcing? 8 What are the relative roles of remote SST forcing and regional soil moisture in the droughts at different time scales? Focus on summer droughts in this talk

Datasets a. Drought indices (PDSI and SPI): 344 US Climate Divisions, , NCDC b. Soil moisture (/Evaporation) 344 US Climate Divisions, CPC bucket model, stations in Illinois, Grid (1/8 degree), Variable Infiltration Capacity (VIC) model of University of Washington, c. SST ERSST Version 2, Hadley Center SST1, Looking for statistically significant (robust) relationship

PDSI vs SPI: local correlation 10 SPI01 SPI03 SPI02 SPI12 SPI09 SPI06 SPI24 Short-term: higher correlation in eastern part Medium-term and long-term: higher correlation in central- western part

PDSI vs SPI: standard deviation 11 SPI01SPI06 SPI02SPI09 SPI12 SPI24 SPI03 PDSI/3 PDSI: larger std in central-western part SPI: relatively uniform

PDSI vs SPI: pattern 12 EOF1: Continental PDSI: larger loading in central-western SPI: larger loading in eastern

PDSI vs SPI: pattern 13 EOF2: East- west contrast A trend: Drying in the central-western Wetting in the eastern Similar patterns

Example: 1930s 14 SPI01 SPI02 SPI03 PDSI/3 SPI06 SPI09 SPI12 SPI24 JJA

Example: 1950s 15 SPI01 SPI02 SPI03 PDSI/3 SPI06 SPI09 SPI12 SPI24 JJA

Drought-SST relationship Schubert et al C20C run NSIPP model OBS SST GP Precipitation (>= 6yr) OBSMODEL SST Correlation (Annual Mean) 16 ?? ? ??

Drought-SST relationship DJF SSTJJA SST PDSI SPI03 SPI24 SPI09 17 JJA Great Plains cor with SST

Drought-SST relationship DJF SSTJJA SST PDSI SPI03 SPI09 SPI24 18 JJA Southwest cor with SST

Drought-soil moisture relationship Findell & Eltahir’97 Initial Soil Saturation vs P(Initial-August23) Soil Saturation(June25) vs P(June25-August23) 19 cc=0.55 Soil saturation = moisture content/porosity

Validation: bucket model vs obs Illinois all-months 20 bucket model observation

Comparison: bucket model vs VIC GP JJA 21 bucket model VIC

Drought-soil moisture relationship 22 Great Plains JJA

Drought-soil moisture relationship 23 Southwest JJA

Piechota & Dracup’96 ENSO-drought relationship PDSI Composite 24 El NinoLa Nina

Mo&Schemm 2008 ENSO-drought relationship PDSI Composite: Warm-Cold ENSO OND AMJ JFM JJA JAS ASO 25 JFM vs JJA

ENSO-drought relationship DJF NINO3.4 SST JJA NINO3.4 SST PDSI SPI03 SPI24 SPI09 26 JJA cor with NINO3.4 SST

ENSO-drought connection: Roles of soil moisture 27 cor with DJF NINO3.4 SST Soil moistureEvaporation

ENSO-drought connection: Roles of soil moisture 28 ENSO  DJF P  DJF-MAM Soil  MAM-JJA Evap  JJA drought Great Plains Southwest

SST-drought relationship Subtropical North Atlantic (SNA) PDSI SPI03 SPI09 SPI24 29 JJA cor with SNA SST DJF SNA SSTJJA SNA SST

SST-drought relationship PDSI SPI03 SPI09 SPI24 30 JJA cor with TIO SST DJF TIO SSTJJA TIO SST

Types of SST-drought relationship Psimultaneous (JJA) SST SSTleading to summer P P --- persistent --- Ppersisting SST SST --- persisting ---SSTleading to persistent P P SM Ppreceding (DJF) SST SSTleading to summer P via SM P --- persistent --- P persisting SST leading to persistent P SST --- persisting ---SST preceding SST leading to JJA P via SM SM DJF JJA 31 short-term drought persistent drought I II I+II

Auto-persistence of SPI and roles of soil moisture and evaporation SM(DJF-MAM) P(DJF) 32 How to distinguish them? SPI(DJF) (1)Auto-persistence larger than cor (SM-JJA SPI) Possibilities: SM(DJF-MAM) SPI(JJA) P(DJF) SPI(DJF) (2) Cor (SM-JJA SPI) larger than auto-persistence (3): (1) + (2)

Findell & Eltahir’97 Precipitation persistence vs soil moisture on precipitation 33

The larger correlation between MAM soil moisture and JJA drought than the auto-correlation of drought from DJF to JJA indicates the contribution of soil moisture to the persistence of droughts from DJF to JJA. SPI persistence vs soil moisture on drought 34

Rajagopalan et al ENSO-drought relationship: Long-term Change Cor (Summer PDSI-Winter NINO3)

Cole and Cook’98 Cor (DJF SOI-Annual PDSI) 36 ENSO-drought relationship: Long-term Change

DJF NINO3.4 SST 37 strong weak strong SW SPI GP SPILarger fluctuations in GP than in SW

JJA NINO3.4 SST 38 SW SPI GP SPI weak/negative strong/positive

DJF TIO SST 39 SW SPI GP SPI

JJA TIO SST 40 weak strong √ √ √ SW SPI GP SPI weak strong

Summary The relationship of boreal summer U.S. droughts with SST and soil moisture differs significantly between short-term and long-term droughts. The short-term droughts ( = 6 months) are influenced by both preceding and simultaneous SST forcing. The soil moisture change shows obvious leading for medium-term and long-term droughts. A dominant remote forcing for U.S. droughts is tropical Pacific SST. Tropical Indian Ocean SST forcing has notable influence on medium- term and long-term droughts. Additional impacts for short-term and medium-term droughts are from the North Atlantic SST forcing. 41

Summary (continued) The most notable impacts of the tropical Pacific SST forcing on medium-term and long-term droughts are found in the Southwest with extension to the Great Plains. Anomalous soil moisture induced by remote ENSO forcing contributes to the persistence of droughts from winter to summer through anomalous evaporation during late spring to summer. The relationship between tropical Pacific SST and boreal summer U.S. droughts show obvious long-term changes. In comparison, the long-term change is more pronounced for the GP droughts than for the SW droughts. Obvious long-term changes are also found in the correlation of U.S. droughts with tropical Indian Ocean SST, especially for JJA SST. 42

THANK YOU! 43