1. Statistical corrections of Central Southwest Asia winter precipitation simulations (and forecasts!) Michael K. Tippett, IRI Matt Barlow, AER Brad Lyon, IRI

2. Introduction CSW Asia drought (1998-2002). –Winter (DJFM) precipitation is key. –“May Kabul be without gold but not without snow” Tropical forcing –Recent drought associated with La Nina. –“Perfect Ocean for drought” GCMs have poor simulation skill. Is statistical correction possible?

3. ClimatologyVariability Elevation Precipitation related to elevation. NE Focus on NE region.

4. Is there any predictability? Extended GCM simulations forced by observed SSTs show little skill. Observations = CRU (New et al 2002) Period = ~1965 through winter 1997-98

5. Approach Identify basis for predictability –Physical mechanisms. –Possible predictors. Linear regression between model output and observations (MOS). –Use model variables to predict observed anomalies. CCA.

6. Yang et al. 2002 Negative correlation between CSW Asia precipitation and EAJS strength. Positive correlation between EAJS strength and Maritime Continent precipitation. East Asia Jet Stream

7. Correlation: Wind EOF1/Precip EOF1 =.66 Wind EOF1/NE =.58 First EOF of DJFM precip First EOF of DJFM 200 hPa reanalysis winds Model winds (ECHAM4.5) do not have the same variability or relation with observed precipitation.

8. Precipitation anomaly patterns associated with NE precipitation anomalies.

9. Physical connections Positive MC precipitation anomalies. Upper level convergence Southerly flow + Coriolis effect = westerly flow. Strengthened EAJS. Negative CSW Asia precipitation anomalies. Idea: Use Western Pacific model precip to predict CSW Asia precip.

10. MOS Details 4-month DJFM seasonal averages. Training –GCM simulations (observed SST) ~1965-98. Western Pacific precipitation. –UEA/CRU New precipitation data set interpolated to T42 grid 1950-1998. Corrected simulation skill estimate –Leave one year out cross-validation. Forecasts 1998-2002 –GCM forecasts made in Oct. –Forecast SST (NCEP Pacific + climatology + Stat. Atl.).

11. Western Pacific model precip (simulation) predicting CSW Asia precip. Leading CCA modes

12. Leave-one-out cross-validated anomaly correlation of MOS corrections Corrected simulation skill

13. GCM simulation skill

14. MOS Corrected Forecast DJFM 1998-99 Observations GCM Forecast Oct 1998

15. GCM Forecast Oct 1999 DJFM 1999-2000 Observations MOS Corrected Forecast

16. GCM Forecast 2000 DJFM 2000-2001 Observations MOS Corrected Forecast

17. DJFM 2001-2002 Observations GCM Forecasts Oct 2001 (More models now!)

18. DJFM 2001-2002 Observations MOS Corrected Forecasts

19. GCM Forecasts Oct 2002 Observations 2001-2002 Reports of snow.

20. MOS Corrected Forecasts Observations 2001-2002 Reports of snow.

21. Summary GCMs show poor simulation skill for CSW Asia winter precipitation. CSW Asia and western Pacific precipitation are associated via EAJS. –Observed winds are a good predictor. –Model winds are not. –Western Pacific model precip is a usable predictor. MOS corrections improve simulation skill. Some promising forecast results (1998-2003).