Changing ENSO regimes and the Asian-Australian monsoon system in a future climate scenario

1. AAMP8, Honolulu, 19-21 February 2007 Changing ENSO regimes and the Asian-Australian monsoon system in a future climate scenario Andrew Turner Pete Inness & Julia Slingo

2. Outline • Introduction • Model set-up • Climate change response • Monsoon-ENSO teleconnection • Regimes and the TBO • Conclusions • Future studies

3. Introduction • ENSO behaviour in HadCM3FA (a version of HadCM3 with equatorial Indo-Pacific flux adjustments). • Distinct ENSO regimes are present.

4. Model set-up • HadCM3 run at higher vertical resolution (L30) due to better representation of intraseasonal tropical convection (Inness et al. 2001) and more realistic atmospheric response to high SST such as El Niño (Spencer & Slingo 2003). • Pre-industrial (1xCO2) and future climate (2xCO2, stabilized) integrations to test the impact of increased GHG forcing. • Further integration of each climate scenario using applied flux adjustments to test the role of basic state errors.

5. Systematic biases in HadCM3

6. Flux adjustments #1 • Flux adjustments calculated by relaxing Indo-Pacific SSTs back toward climatology in a control integration. • The heat fluxes required for the relaxation (Haney forcing) are saved and meaned to form an annual cycle. • Annual cycle applied to equatorial band of the Indo-Pacific (after Inness et al. 2003).

7. Flux adjustments #2 Annual Mean • Large fluxes (up to 186Wm-2 at 120°W) into the cold tongue. • Much smaller (~30Wm-2) over Maritime Continent and Indian Ocean. Amplitude of annual cycle • Small annual cycle outside of central Pacific and Somali upwelling region.

8. Systematic biases in HadCM3 & their reduction in HadCM3FA

9. Improved behaviour of the Indo-Pacific 1 • Less confinement of the warm pool. • Increased SSTs in the cold tongue region. • Less convection over the Maritime Continent. • Coupled response: reduced Pacific trade winds. • Reduced precipitation errors over the Indian Ocean. • Reduction in the monsoon jet. • Stronger Indo-Pacific variability. • Better timed and stronger monsoon-ENSO teleconnection (Turner et al. 2005).

10. Flux adjustments at 2xCO2 • Assume that the adjustments necessary to correct biases will be the same in a future climate scenario. • Same annual cycle of flux adjustments used at 2xCO2 (in common with previous studies where adjustments were necessary to combat drift, eg. Collins 2000).

11. Response to 2xCO2: HadCM3

12. Response to 2xCO2: HadCM3FA

13. Monsoon precipitation response to 2xCO2

14. Monsoon-ENSO teleconnection: lag-correlations • Flux adjustments have dramatic impact on the teleconnection, particularly when measured by Indian rainfall. • The impact of increased GHG forcing is less clear.

15. Monsoon-ENSO teleconnection: moving correlations HadISST vs. All-India gauge data • Amplitude of variations in correlation strength in model runs of similar order to those seen in observations despite fixed CO2 forcing. • See also Annamalai et al. (2007). DMI rainfall

16. Climatic differences between the regimes

17. ENSO behaviour in the regimes

18. The TBO in monsoon dynamics Strong (weak) monsoons defined as those strong (weak) relative to previous and subsequent years (Meehl & Arblaster 2002; Loschnigg et al. 2003; Meehl et al. 2003 etc)

19. The TBO

20. The TBO and biennial ENSO

21. The TBO and irregular ENSO

22. ENSO regimes and the monsoon-ENSO relationship DMI rainfall

23. Explanation for the overall biennial tendency of HadCM3FA • The tendency cannot simply be related to differences in the structure of ENSO in the Pacific. • After Capotondi et al. (2006), measurements of the meridonal extent of the zonal windstress response to ENSO SST forcing show little change between HadCM3 and HadCM3FA. • Additionally, FA move the centre of action of ENSO further east

24. Explanation for the overall biennial tendency of HadCM3FA • Key mechanism to ENSO is monsoon wind forcing in West Pacific (Kim & Lau 2001), eg strong monsoon forcing adjusting the WPA (Lau & Wu 2001). • Chung & Nigam (1999) included ASM heating anomalies in the Zebiak Cane model and found increased feedbacks between the Indo-Pacific sectors. • More recently, Kug & Kang (2006) extend Jin’s recharge oscillator to the Indian sector (now 3 eqns). Increased coupling between the two basins significantly shortens the period of oscillation. Kug et al. (2006) show El Niño events coupled more strongly to the Indian Ocean terminate more rapidly than uncoupled events (SINTEX CGCM). • Boreal fall dipole decays basinwide (Annamalai et al. 2005)

25. Explanation for the overall biennial tendency of HadCM3FA Biennial minus irregular SST during ENSO onset years (SON) • Strong Indo-Pacific coupling is implicated in the biennial tendency. • Note that Merryfield (2006) notes overall shortening of median period of ENSO PC1 and Nanjundiah et al. (2005) show an increased number of models with Asian-Australian monsoon rainfall in TBO as their primary mode.

26. How are the regime changes occuring? • A further integration of HadCM3FA 2xCO2 confirms the overall biennial tendency of the model. • The causes of the bifurcation between the ENSO regimes are not yet understood

27. Summary • Projections of the future climate show numerous changes to the Indo-Pacific also noted in other modelling studies. • The flux adjusted experiment suggests that systematic model biases may be masking the true impact of increased GHG forcing. • An obvious tendency of HadCM3FA 2xCO2 to oscillate on biennial timescales is noted, related to strong Indo-Pacific coupling. • The direct causes of the regime changes are not yet understood.