1. The effect of climate change and systematic model bias on the monsoon-ENSO system: the TBO and changing ENSO regimes Andrew Turner (a.g.turner@rdg.ac.uk) Pete Inness, Julia Slingo

2. Outline Basis for study & heat flux adjustments. The effect of systematic bias in the future. Changing ENSO regimes and their properties. Why the eastward tendency? TBO in the model. Monsoon predictability and the observed record.

3. Background GCMs which generate mean climatologies close to observations are more likely to correctly represent interannual variability of precipitation. (Sperber & Palmer 1996). Equatorial Pacific SSTs important predictor of monsoon (Charney & Shukla 1981). Heat flux adjustments used at 1xCO 2 to correct systematic biases (Turner et al. 2005). Systematic model errors likely to have large impact on future predictions. Same adjustments used at 2xCO 2.

4. Annual mean heat flux into ocean Peak-to-peak amplitude

7. Nino-3 region SSTA in the Flux adjusted model at 2xCO 2

8. Nino-3 region SST annual cycle Nino-3 variance cycle

10. DJF Nino-3 vs equatorial band SSTs

11. Lag-correlation of TransNinoIndex with Nino3 SSTA, 12yr moving window

13. Why the tendency to thermocline mode (eastward) El Nino? Zonal advection (Local) Federov & Philander (2001) interpretation of Zebiak-Cane SST equation Vertical movements of themocline (governed by winds further west, remote) Linear damping Temperature diffusion Entrainment across thermocline (local)

14. Possibilities for tendency to T-modes Weakening of trades allows deepening of thermocline in east Pacific (Federov & Philander 2001) dominance of vertical movements over zonal advection. Wang & An 2002 see similar pattern to 2xCO2 or FA response in 1976 climate shift, and with increased mean upwelling in east, decreased zonal temperature gradient.

15. Webster Yang South Asia DMI (JJAS)

16. MAM(0) JJA(+1)MAM(+1) DJF(+1) SON(0) JJA(0)MAM(0)JJA(0) DJF(+1)SON(0) MAM(+1)JJA(+1)

19. Tendency to biennial period Subsurface Kelvin wave propagation more prevalent during biennial regime. Meridional confinement of wind stress response to ENSO (Kirtman 1997; Guilyardi et al. 2004) shortens period.

20. JJAS DMI vs AIR: Irregular cc=0.31 Biennial cc=0.71

23. Summary Tendency to T-mode ENSO (together with associated Kelvin-wave behaviour at depth) when FA applied / CO 2 doubled. Lack of broad meridional wind stress response in HadCM3 shortens period (only comes to light during strong T-mode behaviour).

24. Issues More understanding needs to be gained into the impact of different ENSO types on the monsoon system. Is modelled response to T-mode related to bienniality and/or incorrect representation in GCM?