1. The effect of doubled CO 2 and model basic state biases on the monsoon- ENSO system Andrew Turner, Pete Inness, Julia Slingo Walker Institute / NCAS-Climate University of Reading, UK NCAS Advisory Group 30 January 2007

2. How will the Asian summer monsoon, crucial to the lives of more than 2 billion people, change with increased greenhouse gas forcing? Motivation #1

3. The model & datasets HadCM3 -atmosphere 3.75°lon x 2.5°lat -ocean 1.25° x 1.25° L30 used rather than L19 - more realistic intraseasonal tropical convection (Inness et al. 2001) and better convective response to high SSTs (Spencer & Slingo 2003). 100 year integrations at pre-industrial CO 2 (control) and 2xCO 2. ERA-40 Reanalysis (1958-1997). All –India Rainfall (AIR) gauge dataset; Parthasarathy et al. (1994).

4. The effect of climate change HadCM3 summer (JJAS) surface temperature differences: 2xCO 2 -1xCO 2

5. summer (JJAS) 850hPa wind differences: 2xCO 2 -1xCO 2 The effect of climate change HadCM3

6. summer (JJAS) precipitation differences: 2xCO 2 -1xCO 2 The effect of climate change HadCM3

7. Correct simulation of the basic state in the tropics essential for accurate seasonal prediction of precipitation variability (Sperber & Palmer 1996). Systematic biases could have an enormous influence on predictions of the future climate (Federov & Philander 2000). Test the effect of systematic biases at 2xCO 2 using limited area heat-flux adjustments. Motivation #2

8. Heat flux adjustments Traditionally used in older models (e.g. HadCM2) to prevent climate drift; HadCM3 does not have this problem. Used here to counteract biases in the mean state. Devised by Inness et al. (2003) to investigate the role of systematic low-level zonal wind and SST errors on the MJO. Coupled model run for 20 years, Indian and Pacific SSTs within 10 S- 10 N relaxed back towards climatology. Anomalous heat fluxes generate a mean annual cycle which is applied to a new 100 year integration (HadCM3FA).

9. Heat flux adjustments Large fluxes (up to 186Wm -2 at 120 W) into the cold tongue. Much smaller (~30 W.m -2 ) over Maritime Continent and Indian Ocean. Annual Mean Amplitude of annual cycle Small annual cycle apart from upwelling region off African coast, and central Pacific.

10. Improvements to the mean state HadCM3FA mean summer (JJAS) surface temperature differences with HadCM3 HadCM3 differences with ERA-40

11. Heat flux adjustments Same heat flux adjustments used as in 1xCO 2 experiment (Turner et al. 2005). Assume that systematic model biases will remain consistent (there is no dataset for comparison). 100 year integrations of HadCM3FA compared at 1xCO 2, 2xCO 2.

12. The effect of climate change summer (JJAS) surface temperature differences: 2xCO 2 -1xCO 2 HadCM3 HadCM3FA

13. The effect of climate change summer (JJAS) 850hPa wind differences: 2xCO 2 -1xCO 2 HadCM3FA HadCM3

14. The effect of climate change summer (JJAS) precipitation differences: 2xCO 2 -1xCO 2 HadCM3 HadCM3FA

15. Monsoon & ENSO variability HadCM3 HadCM3FA 1.22 1.51 2.05 2.17 0.94 1.05 1.21 1.32

16. The teleconnection Lag-correlation of summer (JJAS) Indian rainfall with Nino-3 SSTs

17. Instantaneous correlation of summer (JJAS) Indian rainfall with Nino-3 SST (in 21-year moving window). Turner et al. (2007a) submitted, Q. J. R. Meteorol. Soc. Decadal variations

18. The ability to change regimes? See Turner et al. (2007b), submitted Q. J. R. Meteorol. Soc.

19. Summary Future monsoon simulation: Tendency to stronger monsoons in future climate scenario, irrespective of flux correction. Increased interannual variability using both dynamic and rainfall indices. Increased climate change signal when biases are removed.

20. Summary Future monsoon-ENSO relationship: Monsoon-ENSO teleconnection more susceptible to bias removal than greenhouse warming. Stronger biennial character to flux-adjusted future ENSO. Large amplitude variations across decadal timescales under fixed CO 2 forcing suggest recent changes in the observed record may not be due to climate change.