Biennial and interdecadal variations in the monsoon-ENSO system of a coupled GCM under doubled CO 2 conditions Andrew Turner, P.M. Inness & J.M. Slingo IUGG meeting: JPS001 Interannual and interdecadal climate variability

Introduction Notable tendency for biennial oscillation of the monsoon-ENSO system in this coupled GCM. Dynamical monsoon index* Niño-3 SSTA * P.J. Webster & S. Yang (1992). QJRMS 118: 877—926.

Outline Introduction Model framework Scientific questions Characteristics of each regime Reasons for the overall biennial tendency The regimes as part of the TBO Future work

Model set-up Hadley Centre coupled model HadCM3 run at high vertical resolution (L30) which better represents intraseasonal tropical convection 1 and has an improved atmospheric response to El Niño 2. 1 P.M. Inness, J.M. Slingo, S. Woolnough, R. Neale, V. Pope (2001). Clim. Dyn. 17: H. Spencer, J.M. Slingo (2003). J. Climate 16: A.G. Turner, P.M. Inness, J.M. Slingo (2005). QJRMS 131: A.G. Turner, P.M. Inness, J.M. Slingo (2007a). QJRMS, accepted Integration shown is 95-year run using equatorial Indo-Pacific flux adjustments (HadCM3FA 3,4 ) under 2xCO 2.

Why use flux adjustments? Systematic biases in HadCM3 Summer climate of HadCM3 1xCO 2 HadCM3 minus observations

Flux adjustments at 1xCO 2 Flux adjustments are calculated by relaxing Indo- Pacific SSTs back toward climatology in a control integration. The heat fluxes required for the relaxation are saved and meaned to form an annual cycle. Annual cycle applied to the equatorial band of a new integration*. Annual Mean Amplitude of annual cycle * After: P.M. Inness, J.M. Slingo, E. Guilyardi, J. Cole (2003). J. Climate 16:

Systematic biases in HadCM3 & their reduction in HadCM3FA Maritime Continent cooled; cold tongue warmed Coupled response: reduced trade wind errors and monsoon jet Reduced convection over Maritime Continent & other precip errors opposed HadCM3 minus observations HadCM3FA minus HadCM3 Results from A.G. Turner, P.M. Inness, J. M. Slingo (2005) QJRMS 131:

Flux adjustments at 2xCO 2 Assume systematic biases will still be present in the future climate. Assume that the adjustments necessary to correct these biases will be the same. Same annual cycle of flux adjustments used at 2xCO 2 (in common with previous studies where adjustments were necessary to combat drift, eg in HadCM2*). *M. Collins (2000). J. Climate 13:

HadCM3FA 2xCO 2 ENSO ENSO at 2xCO 2 in HadCM3FA Why the overall biennial tendency? Why are there distinct regime shifts?

Regime climatic differences Biennial minus irregular annual means SST 850wind precip Central Pacific cooling and east Pacific warming, especially in boreal winter. Slackened zonal temperature gradient beyond the dateline. Gill response to diabatic heating over Maritime Continent, associated with increased precipitation.

Mean climate of the regimes Biennial regime features: Cooler central region consistent with surface conditions. Deeper east Pacific thermocline, consistent with warming at surface and reduced upwelling. Annual mean equatorial thermocline irregular biennial Biennial minus irregular

ENSO characteristics Biennial regime features large amplitude events strongly phase locked to the seasonal cycle. Biennial power exceeds annual cycle. Niño-3 power spectra (normalized to annual cycle) Phase-lockingNiño-3 anomaly index

ENSO propagation Irregular regime shows signature of longer duration El Niño events based in the central Pacific. Biennial regime shows more evidence of basinwide, eastward propagation at depth, consistent with thermocline mode events. irregularbiennial Anomalous depth of equatorial 20 ° C isotherm

ENSO propagation #2 Lag correlations of the Trans-Niño Index 1 with Niño-3 show strong eastward propagation of SST anomalies during biennial regime, consistent with thermocline mode events. Tendency towards eastward propagation occurs both with 2xCO 2 2 and with flux adjustments. 1 K.E. Trenberth, D.P. Stepaniak (2001). J. Climate 14: E. Guilyardi (2006). Clim. Dyn. 26: HadCM3 1xCO 2 HadCM3FA 1xCO 2 HadCM3 2xCO 2 HadCM3FA 2xCO 2

Summary of regime characteristics Low amplitude, irregular ENSO, annual cycle dominates. ENSO more central, consistent with S-mode. Large amplitude, periodic, strong phase-locking, ENSO dominant mode. ENSO peaks in east, with eastward propagation, consistent with T-mode. Consistent with irregular and self-excited modes in Jin’s recharge oscillator* as coupling strength is increased. Short biennial period in contrast to observed T-mode ENSO (4-5 years) and at odds with longer period as air-sea coupling is increased in Zebiak-Cane models. Irregular regimeBiennial regime *F-F. Jin (1997). J. Atmos. Sci. 54:

Explanation for the overall biennial tendency of HadCM3FA The tendency cannot simply be related to differences in the structure of ENSO in the Pacific. Capotondi et al. (2006) relate ENSO period in coupled GCMs to two measurements: 1.the meridional extent of the zonal windstress response to ENSO SST variations 2.The longitudinal position of the centre of action of ENSO Meridional width of zonal average taux regressed onto Niño-3 – little change in HadCM3FA. EOF1 of SSTA at 2xCO 2 – FA moves this further east. HadCM3 HadCM3FAdifference

Explanation for the overall biennial tendency of HadCM3FA #2 A key mechanism for biennial ENSO is monsoon wind forcing in West Pacific 1, eg, strong monsoon forcing adjusting the WPA 2. Inclusion of ASM heating anomalies in the Zebiak-Cane model leads to increased feedbacks between the Indo- Pacific 3. Extension of Jin’s recharge oscillator 4 to the Indian Ocean shows that increased coupling between the two basins significantly shortens the period of oscillation. Strongly coupled El Niño events terminate more rapidly than uncoupled events 5 (SINTEX CGCM). 1 K-M. Kim, K-M. Lau (2001). GRL 28: K-M. Lau, H.T. Wu (2001). J. Climate 14: C. Chung, S. Nigam (1999). J. Climate 12: J-S. Kug, I-S. Kang (2006). J. Climate 19: J-S. Kug, T. Li, S-I. An, I-S. Kang, J-J. Luo, S. Masson, T. Yamagata (2006). GRL 33.

Strong Indo-Pacific coupling is implicated in the biennial tendency. Dynamical monsoon index used to generate composite evolution of strong minus weak events. Explanation for the overall biennial tendency of HadCM3FA #3 Biennial minus irregular SST during ENSO onset years (SON)

The TBO

The TBO and biennial ENSO

The TBO and irregular ENSO

Explanation for the overall biennial tendency in HadCM3FA Strong Indo-Pacific coupling is implicated, relating to increased variability of the Asian-Australian monsoon on interannual timescales. Indian Ocean dipole central to the mechanism, its decay to a basinwide SST anomaly instrumental in causing ENSO phase change. Coupling between monsoon, IOD and ENSO is strengthened by both 2xCO 2 and flux adjustments.

Summary ENSO behaviour in HadCM3FA 2xCO 2 features distinct irregular and biennial regimes, with notable biennial tendency. Some consistency with ENSO modes based on air-sea interaction and those dependent on basinwide ocean wave coupling. Increased Indo-Pacific coupling and monsoon- IOD-ENSO interactions implicated in biennial tendency.

The monsoon-ENSO teleconnection rainfall DMI ENSO regimes have dramatic impact on teleconnection. Much greater monsoon predictability during the biennial regime.

Further questions Realism of regime changes uncertain, but they have potential to have dramatic impacts on remote teleconnections. Reasons for changes between regimes not yet elucidated, possibly: –Interactions with the annual cycle in east Pacific. –Changes to meridional circulations in the subtropical Pacific.

Thank you!