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Renguang Wu Center for Ocean-Land-Atmosphere Studies

CTB Joint Seminar Series. 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. February 24, 2009, NCEP. Co-author: James L. Kinter III. 1. Why distinguish the time scale?.

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Renguang Wu Center for Ocean-Land-Atmosphere Studies

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  1. CTB Joint Seminar Series 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 February 24, 2009, NCEP Co-author: James L. Kinter III 1

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

  3. Difference in preferred regions Mo&Schemm 2008 Wet Spell:PDSI>=2 Dry Spell:PDSI<=-2 Ratio of the total number of months (1900-2004) 1-5 months Eastern US 6-11 months > 1 year Central-western 3

  4. Difference in mechanisms Roles of soil moisture for persistence of droughts: Entekhabi et al. 1992 Findell and Eltahir 1997 Pal and Eltahir 2001 Schubert et al. 2004 Seager et al. 2005 --- soil moisture-precipitation feedback --- 4

  5. How to distinguish the time scale? Standardized Precipitation Index (SPI):defined for different time scales P > n-month mean > transform > SPI * 1-month mean spi01 ** 2-month mean transform spi02 *** 3-month mean from Pearson spi03 ****** 6-month mean III distribution spi06 ********* 9-month mean to normal spi09 ************ 12-month mean distribution spi12 ***……….*** 24-month mean spi24 5

  6. 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) Both PDSI and SPI are used in the drought monitoring 6

  7. Drought indices September 2008 PDSI SPI09 SPI03 SPI24 7

  8. Questions 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? 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 8

  9. Datasets a. Drought indices (PDSI and SPI): 344 US Climate Divisions, 1895-2007, NCDC b. Soil moisture (/Evaporation) 344 US Climate Divisions, CPC bucket model, 1932-2005 19 stations in Illinois, 1981-2004 Grid (1/8 degree), Variable Infiltration Capacity (VIC) model of University of Washington, 1950-2000 c. SST ERSST Version 2, 1854-2005 Hadley Center SST1, 1870-2006 Looking for statistically significant (robust) relationship 9

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

  11. PDSI vs SPI: standard deviation SPI01 SPI06 SPI02 SPI09 PDSI: larger std in central-western part SPI: relatively uniform SPI03 SPI12 PDSI/3 SPI24 11

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

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

  14. Example: 1930s JJA 1933-1939 SPI01 SPI06 SPI02 SPI09 SPI03 SPI12 PDSI/3 SPI24 14

  15. Example: 1950s JJA 1952-1956 SPI01 SPI06 SPI02 SPI09 SPI03 SPI12 PDSI/3 SPI24 15

  16. Drought-SST relationship GP Precipitation (>= 6yr) ? ? Schubert et al. 2008 C20C run NSIPP model OBS SST SST Correlation (Annual Mean) OBS MODEL ? ? ? 16

  17. Drought-SST relationship DJF SST JJA SST JJA PDSI Great Plainscor with SST SPI03 SPI09 SPI24 17

  18. Drought-SST relationship DJF SST JJA SST JJA PDSI Southwestcor with SST SPI03 SPI09 SPI24 18

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

  20. Validation:bucket model vs obsIllinois all-months bucket model observation 20

  21. Comparison:bucket model vs VICGP JJA bucket model VIC 21

  22. Drought-soil moisture relationship Great Plains JJA 22

  23. Drought-soil moisture relationship Southwest JJA 23

  24. ENSO-drought relationship Piechota & Dracup’96 PDSI Composite El Nino La Nina 24

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

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

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

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

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

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

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

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

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

  34. SPI persistence vs soil moisture on drought 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. 34

  35. ENSO-drought relationship:Long-term Change Rajagopalan et al. 2000 Cor (Summer PDSI-Winter NINO3) 1895-1928 1929-1962 1963-1995 35

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

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

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

  39. DJF TIO SST SW SPI GP SPI 39

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

  41. 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 (<= 3 months) are mostly influenced by simultaneous SST forcing. The medium-term and long-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

  42. 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

  43. THANK YOU! 43

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