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29 th Annual Climate Diagnostics & Prediction Workshop (October 2004, Madison WI)

Understanding the Sensitivity of North American Drought in the Present and Past Climate to the Tropical SSTs. 29 th Annual Climate Diagnostics & Prediction Workshop (October 2004, Madison WI) Sang-Ik Shin, Robert S. Webb, Prashant D. Sardeshmukh, Robert J. Oglesby, Joshep J. Barsugli

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29 th Annual Climate Diagnostics & Prediction Workshop (October 2004, Madison WI)

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  1. Understanding the Sensitivity of North American Drought in the Present and Past Climate to the Tropical SSTs 29th Annual Climate Diagnostics & Prediction Workshop (October 2004, Madison WI) Sang-Ik Shin, Robert S. Webb, Prashant D. Sardeshmukh, Robert J. Oglesby, Joshep J. Barsugli NOAA-CIRES Climate Diagnostics Center, NASA/Marshall Space Flight Center

  2. Questions • Do the tropical Pacific SST changes explain the paleoclimate records of North American Drought at the mid-Holocene (about 6,000 years ago) ? • Was sensitivity of the North American drought to the tropical Pacific SSTs changed at 6ka?

  3. Outline • Paleoclimate Evidence • Model & Experimental Setup • 6ka Changes in Hydroclimate • Sensitivity Issue • Summary & Conclusions

  4. Paleoclimate Records 6ka Present-day Climate Dai’s Leading EOF of the PDSI (Dai et al., Geophys. Res. Lett. [1998]) wet dry Leading EOF of the PDSI SOI (r=0.61) Darwin SLP index (r=0.67)

  5. Overpeck and Webb (PANS, 2000) Warmer Colder Cooler SSTs (~-2°C) at 6ka Koutavas et al., (Science,2002) Paleoclimate Records (Tropical Pacific) Tudhope et al., (Science, 2001) 6ka Variability Weaker ENSO Mean State La Niña-like condition 0ka

  6. Model & Experimental Setup 50-year Composite • NCAR-CCM v3.10 (T42) / Slab Mixed Layer Ocean / Prescribed Tropical Pacific SST Conditions (by using the 50-year composite of present-day observation) • Sensitivity Experiments orbital forcing (A-O coupling) amplified (weakened) seasonal cycle in Northern (Southern) Hemisphere reduced greenhouse gases e.g., CO2 0ka : 355 ppmv (year 1950) 6ka : 280 ppmv tropical Pacific conditions Average (10S - 10N)

  7. 6ka-0ka  Precip. (mmday-1) Month Monsoon Changes Present-day North America Precip. (mmday-1) North Africa Asia Month

  8. 1. 6ka Changes in HydroclimeteAnnually-Averaged ChangesSeasonal Cycle of Changes

  9. Uncoupled (PMIP-I type) Coupled Temp. (C) Precip. (mmday-1) V (m/s) PSL (hPa) WES feedback  ITCZ shift land/sea contrast Orbital Forcing (Enhanced Seasonal Cycle)

  10. Temp. (C) Precip. (mmday-1) V (m/s) PSL (hPa) Greenhouse Gases Radiative Forcing by Reduced Greenhouse Gases = -2.12 Wm-2 CO2 -1.30 Wm2 CH4 -0.47 Wm2 N2O -0.12 Wm2 CFCs -0.23 Wm2 Opposite to the Reconstructed Precipitation Changes general cooling plus reversed WES feedback

  11. 500 hPa Height (m) Temp. (C) Precip. (mmday-1) V (m/s) PSL (hPa) CI = 5 m La Niña-Like Tropical Pacific Condition

  12. Other Tropical Pacific SST Conditions Temp. (C) Precip. (mmday-1) V (m/s) and PSL (hPa) La Niña plus 1C La Niña Times -1 El Niño

  13. 50N Temp. (C) 20N 120W 70W Precip. (mmday-1) Month Seasonal Cycle over N. America

  14. 2. Sensitivity IssueWas sensitivity of the North American drought to the tropical Pacific SSTs changed at 6ka?  Sensitivity Experiments by using “CCM-MLM-Prescribed Tropical Pacific SSTs” - two model runs with prescribed tropical Pacific SSTs observed present-day climatology observed 50-year La Niña composite - in three different climate settings present-day climate climate under the 6ka orbital forcing climate under the 6ka forcings (orbital plus greenhouse gases)

  15. Effect of La Niña-Like Condition on the N. American Drought Temp. (C) Precip. (mmday-1) Present-day Climate Orbitally-Forced (6ka) Climate 6ka Climate

  16. 50N 20N 120W 70W Springtime (MAM) Precipitation Sensitivity Map Target Patch Experiments 1C Drought Area-averaged Precipitation over Target Sensitivity = Area-weighted  SST over Patch Spline Smoothing (SNR) Sensitivity Map

  17. Springtime (MAM) Combined EOFs 1st EOF (30.06%) 2nd EOF (15.96%) 500 hPa Height and Surface Temp. (CI = 1 m) and Surface Temp. (C) Precip. Precip. (mmday-1) PCs Sensitivity

  18. Fraction of Local Variability in EOFs (Springtime) 1st EOF (30.06%) 2nd EOF (15.96%) 500 hPa Height Temp. Precip. %

  19. Conclusions • Similar sensitivity of the springtime North American drought at 6ka and the present-day climate • Cooler Tropical Pacific SSTs (La Niña)  Springtime Drought

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